146
Chapter 4 Alkanes: Nomenclature, Conformational Analysis, and an Introduction to Synthesis
Chapter 4Alkanes: Nomenclature, Conformational
Analysis, and an Introduction to Synthesis
Chapter 4Alkanes and Cycloalkanes: Conformations of Molecules
Alkanes and Cycloalkanes
Alkanes are saturated hydrocarbons meaning they have the maximum number of hydrogens for the carbon number. Alkanes have the general formula CnH2n+2.
Cycloalkanes are cyclic structures with the carbon atoms in a ring. Although cycloalkanes are saturated hydrocarbons, the general formula is CnH2n because two hydrogens are lost when a C-C bond is made to form a ring.
hexane cyclohexaneC6H14 C6H12
12
34
56
Sources of Alkanes: Petroleum and Natural Gas
.
Petroleum is the source of alkanes. It is a complex mixture of mostly alkanes and aromatic hydrocarbons with smaller amounts of oxygen-, nitrogen-, and sulfur-containing compounds
Natural gas is a gaseous mixture of hydrocarbons recovered from natural sources. It is mostly methane (CH4, BP -162oC) with small amounts of ethane (C2H6, BP -88oC) and propane (C3H8, BP -42o).
Petroleum Refining
Liquid petroleum and natural gas are usually separated at the wellhead and shipped independently to processing (refining) plants. The liquid petroleum (crude) is separated by distillation according to the volatility (BPs) of the different size hydrocarbons. The fractions collected by refining are still mixtures of hydrocarbons that have commercial value.
Hydrocarbon Fractions from Petroleum
boiling rangeof fraction (oC)
size range name and use
below 20 C1 to C4natural gas, bottledgas, petrochemicals
20 to 60 C5 to C6 petroleum "ether". solvents
60 to 100 C6 to C7 ligroin, solvents
40 to 200 C5 to C10 straight-run gasoline
175 to 325 C12 to C18 kerosene and jet fuel
250 to 400 C12 and higher gas oil, fuel oil and diesel oil
nonvolatileliquids C20 and higher mineral oil,
lubricating oilnonvolatile solids
C20 and higher paraffin wax, asphalt, tar
Petroleum Technologies
.Technologies exist to interconvert the various hydrocarbons using catalysts
Cracking is a process for breaking down larger alkanes into smaller alkanes by heating. The mixture of larger alkanes is heated in the absence of oxygen at high temperatures (~500oC) for only a few minutes in the presence of catalysts. By this method, alkanes of size C12 and larger can be turned into gasoline-size alkanes (C5 to C10).Isomerization
Since the 1920s, it has been known that highly branched alkanes perform better in the internal combustion engine than straight-chain alkanes. Catalytic isomerization changes straight-chain alkanes into the more useful branched alkanes.
hexane
acid catalyst+
branched alkanes
CH3CH2CH2CH2CH2CH3CH3CH2CH2CHCH3
CH3
CH3CH2CHCH2CH3CH3
Catalytic Reforming
Alkanes are transformed into cycloalkanes and aromatic hydrocarbons by catalytic reforming.
heptane
silica-aluminacatalyst, 500oC20 atm H2
+ 4H2CH3CH2CH2CH2CH2CH2CH3
CH3
The aromatic hydrocarbons produced by catalytic reforming are used as additives in gasoline and as starting materials for the petrochemical industry. Production of these aromatics is in the billions of pounds per year in the United States.
Crude Petroleum Refining
straight- chain alkanes of different sizes
Cracking Isomerization Reforming
smaller alkanes
branched alkanes aromatics
Petroleum Products
Daily consumption of petroleum in the United States is over 17 million barrels which amounts to close to one billion tons per year. Of this total, approximately 43% goes into gasoline, another 25% into fuel oil, and approximately 7.5% into jet fuel. Thus, about 75% of all the petroleum consumed is burned as a source of energy. The remainder is used as "feedstock" for polymers (~4%) and the chemical industry (~3%), and the many other products used in our society such as oils, lubricants and asphalt.
An Overview of Petroleum Refining
Combustion
All hydrocarbons undergo combustion, the reaction with oxygen that liberates energy. Thus, all hydrocarbons are potential "fuels", materials that burn in oxygen releasing energy.
Heat of Combustion
The heat of combustion (Hcomb) is the amount of heat liberated when one mole of material undergoes combustion at 1 atm pressure to produce gaseous CO2 and liquid water.
CH4 + 2O2 CO2(g) + 2H2O(l)methane
Hcomb = -882 kJ/mol (or -55.1 kJ/g)
For a gasoline-size hydrocarbon::
2C8H18 + 25O2 16CO2(g) + 18H2O(l)
Hcomb = -5452 kJ/mol (or -47.8 kJ/g)
Note, the total amount of heat liberated increases with the size of the hydrocarbon, but that doesn't make it a better fuel. On a per weight basis, methane is a better fuel than the octane.
Gasoline Performance: The Octane Rating
The combustion of alkanes is a complicated reaction probably involving free radicals. Much attention has been directed towards the combustion of hydrocarbons in the internal combustion engine. Since the 1920s, it has been known that some hydrocarbons tend to give better performance during combustion. Some fuels cause "knocking", the premature ignition of the fuel before the piston is in the firing position for a power stroke. Knocking causes loss of power.
Branched hydrocarbons were found to perform better than straight-chain alkanes in the internal combustion engine. In 1927, an arbitrary engineering performance standard was developed called "the octane rating." The performance of the branched alkane "isooctane" (actually 2,2,4-trimethylpentane) in a specific internal combustion engine was given a rating of 100. Heptane, which causes severe knocking, was given a rating of 0.
A fuel with a performance equivalent to a mixture of 75% isooctane and 25% heptane is given an octane rating of 75.
"isooctane"100
heptane0
CH3CCH2CHCH3CH3
CH3 CH3CH3CH2CH2CH2CH2CH2CH3
Octane Boosters
.
It is common practice to add octane boosters to gasoline to improve the performance of the fuel. Many years ago, tetraethyllead, (C2H5)4Pb, was an important additive for this purpose. It is now illegal to use "leaded" gasoline in an automobile in the United States. Aromatics and "oxygenated fuels" are blended into gasoline to raise the octane rating
Some Octane Ratings ofHydrocarbons and Additives
Octane Ratingheptane 01-pentene 912,2,4-trimethylpentane 100benzene 106methanol 107ethanol 108methyl t-butyl ether 116toluene 118
Methyl t-butyl ether (MTBE) is an oxygenated fuel blended into gasoline to improve performance and reduce air pollution. Production of MTBE increased over the past 10 years to many billions of pounds per year in the United States. However, MTBE is being phased out because of environmental and health concerns.
Shapes of Alkanes• “Straight-chain” alkanes have a zig-zag orientation
when they are in their most straight orientation– Straight chain alkanes are also called unbranched alkanes
11
• Branched alkanes have at least one carbon which is attached to more than two other carbons
12
• Constitutional isomers have different physical properties (melting point, boiling point, densities etc.)– Constitutional isomers have the same molecular formula
but different connectivity of atoms
13
• The number of constitutional isomers possible for a given molecular formula increases rapidly with the number of carbons
14
A Systematic Naming System
.
Because of the dramatic increase in C-C connectivity possibilities with increasing carbon number, it is necessary to have a systematic naming system that unambiguously identifies the carbon structure. The IUPAC Nomenclature System (described below) serves this need
Common Names
Simpler organic compounds (C4 or smaller) are often named by common or trivial names that go back hundreds of years. Because these names specifically refer to widely used compounds, there is no ambiguity. Larger structures require a systematic name.
IUPAC Names
In 1892, the International Union of Pure and Applied Chemistry proposed the systematic nomenclature system still in use today. Over the years, it has been revised. The IUPAC naming method is closely linked to structure, and is able to give a unique name to each of the known 7 million or more organic compounds.
• Nomenclature of Unbranched Alkanes
16
• Nomenclature of Unbranched Alkyl groups • The unbranched alkyl groups are obtained by
removing one hydrogen from the alkane and named by replacing the -ane of the corresponding alkane with -yl
17
Alkyl Group Names
.
The alkyl group names are used in common names and when the alkyl groups are substituents along a "parent" carbon chain in the IUPAC naming system
IUPAC Names of Alkanes
.(1) Select the longest continuous carbon chain as the parent name of the alkane
CH3CH2CH2CHCH3
CH3
or CH3CH2CH2CH-CH3CH3
a pentane either way
CH3CH2CH2CH2CH-CH3CH2CH3
a heptane
.(2) Number the carbons of the longest chain beginning at the end that gives lower numbers to positions of substituents
CH3CH2CH2CH2CHCH3CH3
6 5 4 3 2 1
2-methylhexane
CH3CH2CHCH3CH2CH2CH3
1 2 3
4 5 6
3-methylhexane
CH3CHCH2CHCH2CH3CH3 CH3
1 2 3 4 5 6
2,4-dimethylhexane
.(3) If the same substituent appears more than once, use the prefixes di, tri, tetra...
(4) When two or more different substituents are present, give each a positional number and list them alphabetically. Disregard prefixes in determining alphabetical order.
CH3CH2CHCH2CHCH3CH2CH3
CH3CH3CH2CHCHCHCH3
CH2
CH3
CH3CH3
4-ethyl-2-methylhexane 4-ethyl-2,3-dimethylhexane
(5) When two substituents are on the same carbon, use the positional number twice.
CH3CH2CCH2CH2CH3
CH3
CH2CH3
3-ethyl-3-methylhexane
These rules lead to unambiguous names for the alkanes and solve the problem of numerous constitutional isomers.
6 5 4 3 2 1 6 5 4 3 2 1
Quiz Chapter 4 Section 3
Provide the IUPAC names of the alkanes below.
CH3CH2CHCH2CHCH3
CH3
CH2CH3
(CH3)2CHCH2CH(CH3)2
3,5-dimethylheptane 2,4-dimethylpentane
• When two or more substituents are identical, use the prefixes di-, tri-, tetra- etc.– Commas are used to separate numbers from each other– The prefixes are used in alphabetical prioritization
• When two chains of equal length compete to be parent, choose the chain with the greatest number of substituents
22
• When branching first occurs at an equal distance from either end of the parent chain, choose the name that gives the lower number at the first point of difference
23
• Nomenclature of Branched Alkyl Chains• Two alkyl groups can be derived from propane
24
• Nomenclature of Branched Alkyl Chains• Four groups can be derived from the butane isomers
25
• The neopentyl group is a common branched alkyl group
• Examples
26
Classification of Hydrogen Atoms• Hydrogens take their classification from the carbon
they are attached to
27
Quiz Chapter 4 Section 3D
Name the alkane below and identify as 1o, 2o and 3o , all groups of equivalent H.
CH3CH2CHCH2CH3CH3
1o
2o 3o
1o
3-methylpentane
Nomenclature of Alkyl Halides
.The halogen substituted alkanes are named as haloalkanes in the IUPAC system. All the rules presented earlier apply
CH3CHClCH3 CH3CH2CH2CH2Br2-chloropropane 1-bromobutane
When both halo and alkyl substituents are present, number from the end nearer the first substituent, whether halo or alkyl, and list the substituents in alphabetical order.
CH3CHCHCH2CH3Br
CH3
2-bromo-3-methylpentane
.If the two substituents are equal distance from an end of the chain, number from the end that gives alphabetical preference
CH3CHCH2CHCH3I CH3
2-iodo-4-methylpentane
Common Names: Alkyl Halides
.Common names of alkyl halides are based on the alkyl group name and the name of the halide
CH3CH2Brethyl bromide
CH3CHCH3
Cl
isopropyl chlorideCH3CCH3
CH3
Cl
tert-butyl chloride
• Nomenclature of Alkyl Halides• In IUPAC nomenclature halides are named as
substituents on the parent chain– Halo and alkyl substituents are considered to be of equal
ranking
31
• Nomenclature of Alkyl Halides• In common nomenclature the simple haloalkanes
are named as alkyl halides– Common nomenclature of simple alkyl halides is accepted
by IUPAC and still used
32
• IUPAC Substitutive Nomenclature• An IUPAC name may have up to 4 features: locants, prefixes, parent
compound and suffixes• Numbering generally starts from the end of the chain which is closest
to the group named in the suffix
• IUPAC Nomenclature of Alcohols• Select the longest chain containing the hydroxyl and change the suffix
name of the corresponding parent alkane from -ane to -ol• Number the parent to give the hydroxyl the lowest possible number• The other substituents take their locations accordingly
33
• Examples
• Common Names of simple alcohols are still often used and are approved by IUPAC
34
Common Names of Alcohols
Alkyl group names are approved by IUPAC for naming alcohols:"alkyl group + alcohol."
CH3CH2OH CH3CHCH3
OHCH3CCH2OH
CH3
CH3
ethyl alcohol isopropyl alcohol neopentyl alcohol
"Glycol" is a common name for compounds containing two hydroxyl groups. In the IUPAC system, they are diols.
HOCH2CH2OH CH3CHCH2OHOH
ethylene glycol(1,2-ethanediol)
propylene glycol(1,2-propanediol)
.
Note: The glycol name uses the common name of the alkene that yields the diol upon hydroxylation
• Alcohols with two hydroxyls are called diols in IUPAC nomenclature and glycols in common nomenclature
36
Quiz Chapter 4 Section 3F
Name the following compound.
CH3CHClCH2CHOHCH3
4-chloro-2-pentanol
Nomenclature of Cycloalkanes
Cyclic alkanes are named with the "cyclo" prefix followed by the alkane name indicating the number of carbon atoms in the ring.
CH2
CH2CH2
CH2 CH2
CH2CH2
cyclopropane cyclobutane
Nomenclature of Cycloalkanes• The prefix cyclo- is added to the name of the
alkane with the same number of carbons• When one substituent is present it is assumed to be at
position one and is not numbered• When two alkyl substituents are present the one with
alphabetical priority is given position 1• Numbering continues to give the other substituent the
lowest number• Hydroxyl has higher priority than alkyl and is given
position 1• If a long chain is attached to a ring with fewer carbons,
the cycloalkane is considered the substituent
39
Substituted Cycloalkanes
When there are two or more substituents, the positions around the ring are numbered beginning with the substituent first in the alphabet.
.The name of a substitutent is added as a prefix to the cycloalkane name. Alkyl group names are used for simple alkyl group substituents
eethylcyclohexan 1,3-dimethylcyclohexane 1-chloro-2-methylcyclopentane
The cycloalkane also can be named as a substituent on a long chain, which is sometimes more convenient.
CH2CH2CH2OH
3-cyclohexyl-1-propanol
3 2 1
CH2CH3 CH3
CH3
Cl
CH3
41
Bicyclic compounds• Bicyloalkanes contain 2 fused or bridged rings• The alkane with the same number of total carbons
is used as the parent and the prefix bicyclo- is used
42
Nomenclature of Bicyclic Compounds
Bicyclic compounds are named by first counting the total number of carbons in the bicyclic ring. The base name is the alkane name for the total number of carbons with the prefix bicyclo (i.e. ten carbonsbecomes bicyclodecane).
Next, the number of carbons in each of the three bridges is put in brackets in decreasing order and then inserted between the prefix and the alkane name (i.e. if a bicyclodecane contains bridges with four carbons, three carbons and one carbon, the correct IUPAC name would be bicyclo[4.3.1]decane). The numbers are seperated by periods not commas. Note the numbers in the bracket always add to two less than the total number of carbons (decane = ten total carbons; 4 + 3 + 1 = 8; 10 - 8 = 2). Where are the missing two carbons? They are the twobridgehead carbons.
H2C
H2CCH
CH2
CH2
HC
CH2
1
2
3
7
5
4
6
Two-carbon bridge
Two-carbon bridge
One-carbon bridge
bicyclo heptane[2.2.1]
Bicyclic compounds• The number of carbons in each bridge is included in
the middle of the name in square brackets
44
Nomenclature of Branched Bicyclic Compounds
If substituents are present, the bridgehead carbon will always be the number one carbon. Numbering precedes through the longest bridge to the other bridgehead carbon. The next longest bridge is numberednext and finally the smallest bridge is numbered last.
176
5
8
42
3
[3.2.1]octanebicyclo
7
6 5
1
4
3
2
[3.2.0]heptanebicyclo
2
11
56
1
10
79
34
8
[4.3.2]undecanebicyclo
After numbering the chain, the subsituents are named by standardIUPAC rules.
176
5
8
42
3bicyclo4-methyl [3.2.1]octane
7
6 5
1
4
3
2Br
7-bromo-3,6-dimethyl [3.2.0]heptanebicyclo
10-chloro-1-methyl [4.3.2]undecan-7-olbicyclo
2
11
56
1
10
79
34
8
Cl
OH
Nomenclature of Alkenes and Cycloalkenes• Alkenes are named by finding the longest chain containing
the double bond and changing the name of the corresponding parent alkane from -ane to -ene
• The compound is numbered to give one of the alkene carbons the lowest number
• The double bond of a cylcoalkene must be in position 1 and 2
46
• Compounds with double bonds and alcohol hydroxyl groups are called alkenols • The hydroxyl is the group with higher priority and must be given the
lowest possible number
• Two groups which contain double bonds are the vinyl and the allyl groups
47
• If two identical groups occur on the same side of the double bond the compound is cis• If they are on opposite sides the compound is trans
• Several alkenes have common names which are recognized by IUPAC
48
Quiz Chapter 4, Section 5
Name the compound below.
OH
2,2-dimethylcyclopent-3-en-ol
2,2-dimethyl-3-cyclopenten-1-olor
IUPAC Names of Alkynes
Alkynes are named in a similar way as alkenes.
(1) The name of the parent alkane is modified by dropping the "ane" ending and adding "yne."
(3) The location of the triple bond is given by the lower positional number of the alkynyl carbons.
CH3CH2CH2C CH5 4 3 2 1
1-pentyne
CH3C CCH3
2-butyne
(2) The parent chain is numbered to give the carbons of the alkyne lower numbers.
CH3C CCH2Cl4 3 2 1
e1-chloro-2-butyn
CH3CH2CHC CHCH3
5 4 3 2 1
e3-methyl-1-pentyn
.(5) In an alkynol, the alcohol has priority in numbering
.(4) Positions of substituents are determined by the usual rules
HC CCH2CH2OH4 3 2 1
3-butyn-1-ol
C CH
1-Alkynes are also called terminal alkynes.
pKa = 25R
Physical Properties of Alkanes and Cycloalkanes• Boiling points of unbranched alkanes increase smoothly with
number of carbons
• Melting points increase in an alternating pattern according to whether the number of carbon atoms in the chain is even or odd
52
Physical Properties of Alkanes and Cycloalkanes
Homologous Series
.
The unbranched alkanes (CH4, CH3CH3, CH3CH2CH3, CH3CH2CH2CH3, etc.)form a regular series where each member differs from the next in order by -CH2
-. Such a regular series is called a homologous series
Boiling Points and Melting Points of the Unbranched Alkanes
pentane hexane heptane octanebp (oC) 36 69 98 126
.
The boiling points of the unbranched alkanes increase more or less regularly with increasing size reflecting the increasing van der Waals attractions
The Importance of Packing Forces in the Solid State
.
The melting points of the unbranched alkanes increase with increasing size but with some notable features. Propane (mp = -188oC) melts lower than methane (mp = -182oC) or ethane (mp = -183oC). Also, the incremental increase in melting point in going from an odd numbered alkane to the next even numbered alkane is large, relative to going from an even to the next odd numbered alkane
pentane hexane heptane octanemp (oC) -130 -95 -91 -57
= 35 = 4 = 34
It has been suggested that these melting point anomalies are due to variable packing forces in the solid state. Methane and ethane have higher melting points than propane because they are more compact and pack better in the solid state. Alkane chains with an even number of carbons pack more closely in the solid state. The resulting stronger attractive forces lead to higher melting points.
Density and Solubility
.
All alkanes and cycloalkanes have densities less than the density of water (1.00 g/mL at 4oC), and are insoluble in water. Since they are nonpolar materials, they dissolve well in other nonpolar or low polarity organic solvents
Sigma Bonds and Bond Rotation
Rotation is possible around single bonds (sigma bonds). The orientations of atoms and groups that result from rotation are called conformations.
Different conformations may have different energies. An analysis of the energy changes with rotation around a bond is called conformational analysis.
Conformational Analysis of Ethane: H3C-CH3
An energy barrier of close to 12.6 kJ/molis observed during rotation around the C-C bond in ethane. This energy barrieris attributed to torsional strain.
C
H
HH
H
H
H
Sigma Bonds and Bond Rotation• Ethane has relatively free rotation around the carbon-
carbon bond• The staggered conformation has C-H bonds on adjacent
carbons as far apart from each other as possible– The drawing to the right is called a Newman projection
• The eclipsed conformation has all C-H bonds on adjacent carbons directly on top of each other
57
.
An analysis of the rotation around the C-C bond in ethane shows there are two extreme conformations. These two conformations called eclipsed and staggered are shown below. These two conformations interconvert by simple rotation around the C-C bond
The Conformations of Ethane
C C
H
HH
H
H
H
eclipsed
rotation
staggered
C C
H
HH
H
HH
rotationH
H
H
HH
HHH
HH HH
The intersection of the three bonds represents the orientation of the three H around the front carbon, and the lines to the circle represent the orientation of the three H around the back carbon.
The relative orientations of the hydrogens around the two carbons are easier to see in a Newman projection formula, wherein the structure is viewed along the carbon-carbon bond.
• The potential energy diagram of the conformations of ethane shows that the staggered conformation is more stable than eclipsed by 12 kJ mol-1
59
The Conformations in Propane: CH3-CH2-CH3
There are two equivalent C-C bonds in propane:
HC
CC
H
H
H
H H
H
HThe conformationalfeatures are the samefor the two C-C bonds.
It is easier to see these conformational features by examining propane as a substituted ethane where a methyl group has replaced an H.
eclipsed
rotation
staggered
rotationH
HH HH
CH3
HH
CH3H
H
H
HC C
CH3
H
HH
HC C
H
HH
H
HH
The barrier to rotation in propane is ~13.8 kJ/mol, slightly higher than the torsional barrier in ethane. Again there are three equal barriers in one complete rotation, each occurring at an eclipsed conformation. In propane, the eclipsing of a CH3 group with an H does not significantly increase thebarrier.
Summary of the Conformational Propertiesof Ethane and Propane
ethane
three equivalent barriers of 11.72 kJ/mol
propane
two equivalent C-C bonds,each with three equivalent barriers of 13.8 kJ/mol
H
C C
CH3
H
HH
HH
C C
H
H
HH
H
Conformational Features of the Butanes
There are two constitutional isomers of C4H10, butane and isobutane, with different conformational features.
butane isobutane
CH3CH2CH2CH3 CH3CHCH3CH3
Butane
.There are two different C-C bonds in butane, two "terminal" bonds (1) and one "internal" bond (2)
C C
H3C
CH3H
H
H
H
121
Conformational Features of the Terminal Bonds in Butane
The two equivalent terminal bonds in butane have the conformational features observed in propane, except that the energy barrier to rotationis slightly higher (15.1 kJ/mol compared with 13.8 kJ/mol).
propaneH
C C
CH3
H
HH
H
butane (terminal bond)H
C C
CH2CH3
H
HH
H
two equivalent C-C bonds,each with three equivalentbarriers of 13.8 kJ/mol
two equivalent terminal C-C bonds, each with three equivalent barriers of 15.1 kJ/mol
The Conformational Features of Isobutane
All three C-C bonds in isobutane are equivalent.
isobutane
CH3-CHCH3
CH3
.The conformational features may be more easily seen when isobutane is analyzed as a disubstituted ethane
staggered eclipsed
rotationC C
H
HH
HCH3
CH3
C CH
HH
H
CH3CH3
During a complete rotation around the C-C bond, there are three equivalent staggered and three equivalent eclipsed conformations. In the eclipsed conformation, there are two alignments of CH3~H resulting in an energy barrier close to 16.7 kJ/mol.
Overview of the Conformational Features of CH3-CX3 Systems
.
In a complete rotation around the C-C bond, there are three equivalent energy barriers. In simple ethane, the barrier is assigned to torsional strain. As CH3 or other alkyl groups replace H, the barrier increases as elements of steric strain (nonbonded repulsive interactions) are introduced
energy barrier(kJ/mol) 11.72 13.8 15.1 16.7
increasing steric strain
ethane propane butane isobutane
H
C C
H
H
HH
HH
C C
CH3
H
HH
HH
C C
CH2CH3
H
HH
HH
C C
CH3
CH3
HH
H
Conformational Features of Butane
The Internal Bond in CH3CH2-CH2CH3
Rotation around the internal bond in butane introduces conformational features different from the CH3-CX3 systems. Structures of the general type XCH2-CH2X, where X is larger than H, have rotational barriers with significant steric strain in the eclipsed conformation where the X groups are aligned.
These conformational features are more easily noted when the several extreme conformations (staggered and eclipsed) are analyzed as substituted ethanes.
C CHH
H
CH3
CH3 H
Analysis of the Extreme Conformations
During one complete rotation around the internal bond in butane, the three staggered conformations include two called "gauche" and one called "anti."
120o
rotation
120o
rotationC CHH
H
CH3
CH3 HC CH
HH
CH3CH3
HC C
H
HH
CH3
CH3
H
The Newman projection formulas as viewed from the left are:
120o
rotation
120o
rotation
anti gauche gauche
CH3
HH
HH
CH3
CH3
H
H
HH
CH3
CH3
H
H
HH
CH3stericstrain
.
The staggered anti is more stable than the two equivalent staggered gauche conformations. In the anti conformation, the two CH3 groups are on opposite sides of the structure. In the gauche conformations, the two groups are within van der Waals repulsive interaction distance, and 3.8 kJ/mol of steric strain energy is introduced
Analysis of the Eclipsed Conformations
.All of the staggered conformations are more stable than any of the eclipsed conformations shown below
120o
rotation
120o
rotationC CHH
H
CH3CH3
H
C CHH
HCH3
CH3 HC CH
H HCH3
CH3 H
:The Newman projection formulas for the above structures viewed from the left are
120o
rotation
120o
rotation
CH3
HHH H
CH3
CH3
HHH
H
CH3
CH3
HH
H
HCH3
.
This eclipsed conformation is the highest energy point during rotation around the C2-C3 bond because of severe steric strain from the aligned CH3 groups
These eclipsed conformations have the features observed in propane and isobutane.
Conformational Analysis of Butane• Rotation around C2-C3 of butane gives six important
conformations• The gauche conformation is less stable than the anti
conformation by 3.8 kJ mol-1 because of repulsive van der Waals forces between the two methyls
68
Some Observations
.The staggered anti conformation is the energy minimum. It is free of both torsional and steric strain energy
A 60o rotation yields an eclipsed conformation with torsional and some steric strain, similar to what is observed in propane.
An additional 60o rotation yields the first staggered gauche conformation that has 3.8 kJ/mol of steric strain energy. There is no torsional strain in the staggered conformation.
An additional 60o rotation yields another eclipsed conformation that is the highest energy point in the complete rotation. This conformation has aligned CH3 groups. The 18.8 kJ/mol barrier includes torsional strain and perhaps 7.1 kJ/mol of steric strain energy.
Fast Rotation
.
Even with a rotational barrier of 18.8 kJ/mol for the internal bond in butane, rotation is very fast (~1010 rotations per second at room temperature). Sometimes the term "free rotation" is used to describe the rotation around C-C bonds. The "free" really means fast. There are rotational barriers, but usually they are just a few kJ/mol
The Anti-Gauche Equilibrium
.
Overall, chemical structures rapidly survey all available conformations, spending more time in the energetically favored ones. A fast equilibrium exists among the staggered conformations associated with rotation around the internal bond in butane: the anti and the two gauche conformations. A snapshot picture would show that at room temperature, the anti conformation is favored about 3:1 over the two gauche conformations
Quiz Chapter 4 Section 9
Provide the IUPAC name of each alkane shown below as a Newman projection formula. For II, draw the Newman projection formula of the conformation at the top of the energy barrier for rotation around the C-C bond.
CH3 CH3
H
HH
H
CH3 CH3
H
HCH3CH3
I II
2-methylpropane
isobutaneor
2,3-dimethylbutane
CH3CH3 CH3
CH3
HH
Measurements of Relative Stabilities
Heats of Combustion
.
The heat of combustion of a compound is the enthalpy change for complete combustion of one mole of the compound. For a hydrocarbon the products are CO2(g) and H2O(l). The amount of heat evolved is measured in a calorimeter. In giving the heat of combustion, it is important to note the physical state of the reactant: liquid, gas or solid. If the state is liquid or solid, the heat of vaporization must be taken into account
An ExampleCH3CH2CH2CH2CH3(g) + 8O2 5CO2(g) + 6H2O(l)
Hcomb = -3536 kJ/mol
The difference of 26 kJ/mol is the heat required to change one mole of liquid pentane to one mole of gaseous pentane, the heatof vaporization.
but for liquid pentane, Hcomb = -3510 kJ/mol
Stability of Isomers
.The relative stability of isomeric hydrocarbons may be determined by measuring their heats of combustion under identical conditions
An Example: The Isomeric Butanes
:The heats of combustion of the isomeric butanes are
CH3CH2CH2CH3(g) + 6.5 O2(g) 4CO2(g) + 5H2O(l)
Hcomb = -2876.5 kJ/mol
CH3CHCH3(g) + 6.5 O2(g) 4CO2(g) + 5H2O(l)
Hcomb = -2868.1 kJ/molCH3
Analysis
.
The analysis of the relative stabilities of the isomeric butanes from the above data is easiest to see in the energy state diagram below. Since the combustion reactions are exothermic, the product states are lower in energy than the reactant states. Since the same product state is produced in each combustion reaction, the levels of the two reactant states automatically are set by the amount of heat released. The difference in energy levels reflects the difference in energies of the isomeric butanes since O2 is common to both reactant states
4CO2(g) + 5H2O(l)
CH3CH2CH2CH3(g) + 6.5 O2(g) CH3CHCH3(g) + 6.5 O2(g)
CH3
Hcomb =
-2868.1 kJ/mol
Hcomb =
-2876.5 kJ/mol
8.4 kJ/molIsobutane is more stable than butaneby 8.4 kJ/mol.
Ent
halp
y
Heats of Combustion of the Cycloalkanes:A Measure of their Relative Stabilities
The cycloalkanes form a homologous series (CH2)n with n > 3.
The general reaction for the combustion of a cycloalkane is:
(CH2)n + 1.5n O2 nCO2 + nH2O + heat
As n increases, more heat is evolved. In order to use the heats of combustion to determine the relative stabilities of the cycloalkane structures, the amount of heat evolved must be adjusted for the number of CH2 groups. The table that follows provides this information.
Heats of Combustion of Cycloalkanes
cycloalkane (CH2)n n Hcomb(kJ/mol)
heat evolvedper CH2 group (kJ/mol)
cyclopropane 3 2091 697cyclobutane 4 2744 686
cyclopentane 5 3320 664
cyclohexane 6 3952 659cycloheptane 7 4637 662cyclooctane 8 5310 664cyclononane 9 5981 664cyclodecane 10 6636 664unbranched alkanes (659)
.
Note: The total amount of heat evolved increases with the size of the cycloalkane, as expected. However, the amount of heat evolved per CH2 group is highest for the smallest cycloalkanes, and is lowest for cyclohexane, where the amount is consistent with that evolved in the combustion of unbranched alkanes
Ring Strain in Cycloalkanes
"
Because the amount of heat evolved in the combustion of cyclohexane is consistent with the value expected from the combustion of unbranched (and unstrained) alkanes, it is assumed that cyclohexane is free of any "strain energy.
The greater amounts of heat evolved per CH2 group in the other cycloalkanes are assumed to be due to elements of "ring strain" that lead to higher energies. The total amount of ring strain is calculated by multiplying 659 kJ/mol x n, where n is the number of CH2 groups, and subtracting this value from the measured heat of combustion.
The Relative Stabilities of Cycloalkanes: Ring Strain• Heats of combustion per CH2 unit reveal
cyclohexane has no ring strain and other cycloalkanes have some ring strain
78
The Origin of Ring Strain in Cyclopropane and Cyclobutane : Angle Strain and Tortional Strain
• Angle strain is caused by bond angles different from 109.5o
• Tortional strain is caused by eclipsing C-H bonds on adjacent carbons• Cyclopropane has both high angle and tortional strain
79
The Origin of Ring Strain in Cyclopropane and Cyclobutane :• Cyclobutane has considerable angle strain
– It bends to relieve some tortional strain
• Cyclopentane has little angle strain in the planar form but bends to relieve some tortional strain
80
Cyclobutane
.Cyclobutane also has both angle strain and torsional strain
But, cyclobutane is not planar.
90o
a planar structure
The Newman projections along two bonds reveal eclipsed H.
severe torsional strain
HH
HH
HH
HH H
H
H
H
H
HH
H
As in cyclopropane, the angle strain arises from the difference between the internal internuclear angle (close to 90o), and the idealized bond angle of 109.5o. If cyclobutane were planar, there would be severe torsional strain as well.
The Conformation of Cyclobutane
.
Cyclobutane has a bent geometry. This conformation is formed by a slight rotation around the C-C bonds. This rotation reduces the severe torsional strain in the planar geometry
Clockwise and counterclockwise rotations around the C-C bond give the bent geometry.
H
H
H
H
H
HH
H
There is reduced torsional strain in the bent geometry.
HH
H H
H
HH
H 88o
There is a slight closing of the internalangle increasing angle strain in the bent structure.
C
C
CC
H
H
H
H
H
HH
H
The bent cyclobutane structure is not static. The ring rapidly changes from one bent form to another with partial rotations around the C-C bonds.
fastH
HH
H
HH
H
H
H
HH
H
H H
HH
Inversion of the Bent Geometry
Cyclopentane
.
The internal angles of a regular pentagon are 108o, close to the idealized tetrahedral bond angles. Thus, a planar cyclopentane would have very little angle strain
108o
But a planar geometry would have very severe torsional strain (10 eclipsed H). Consequently, the geometry of cyclopentane is bent.
bent or puckered geometry
H
HH
HH
H
H
HH
H
The torsional strain is reduced inthe bent structure. Four H are stilleclipsed, but 6 are staggered.
Quiz Chapter 4 Section 11
From an analysis of the heats of combustion of the following cycloalkanes, which ones contain elements of strain energy? From a conformational analysis, what are the significant types of strain energy in each cycloalkane?
significantstrain energy
significantstrain energy
small amountof strain energy
no strain energy
torsional andangle strain
torsional andangle strain
torsionalstrain energy
The Conformations of Cyclohexane
Baeyer Strain Theory
Cyclohexane
Baeyer's theory predicts considerable strain in cyclohexane, which he assumed had the geometry of a hexagon with an internal angle of 120o.
120o
severe angle strainin planar cyclohexane
The prediction is wrong because cyclohexane is not planar. Cyclohexane exists in nonplanar geometries with the most stable being the "chair" with internal bond angles of 109.5o. In addition, there is no torsional strain in this conformation.
In 1885 Adolf von Baeyer (Univ. of Munich) proposed a theory to explain aspects of the chemistry of cycloalkanes. He assumed the structures were planar, and qualitatively estimated the amount of strain in each according to the difference between the internal angle of the regular polygon, and the idealized bond angle of the recently deduced tetrahedral carbon. This theory became known as the Baeyer strain theory.
Conformations of Cyclohexane• The chair conformation has no ring strain
– All bond angles are 109.5o and all C-H bonds are perfectly staggered
87
There are two types of H in the chair conformation of cyclohexane: 6 axial (up and down), and 6 equatorial (close to the plane of the ring).
1 2
3
45
6 HH
HH
HH
H
HH
H
HH
Two Types of Hydrogens: Axial and Equatorial
Two Types of Hydrogens: Axial and Equatorial
Newman projections alongthe C1-C6 and C3-C4 bondsshow the different orientationsof the axial and equatorial H. 5
2
1 3CH2
CH2
H
H
H
H
H
H
H
Haxialeq
Chair-Chair Interconversion
.The chair conformation is not fixed. Partial rotations around C-C bonds lead to chair to chair conversions
1 2
3
4
65
chair 1
YX
HH
HH
H
HXY
HH
2
1 3
46
5
View chair 1 along the C1-C6 and the C3-C4 bonds for the Newman projection shown above.
CH2
CH2
H
Y
Y
H
X
H X
H
.In the Newman projection, imagine rotating carbons 1 and 4 clockwise, while rotating carbons 3 and 6 counterclockwise
.These rotations push carbon 2 down and carbon 5 up producing chair 2 and its Newman projection structure
1
2 34
6 5
chair 2
H
HH
H
H
H
HH
XY
Y
X
213
465
CH2
CH2X
Y
H
H
Y
X
H
H
Axial-Equatorial Positional Exchange during Chair-Chair Interconversion
.
When one cyclohexane chair conformation transforms into the other chair conformation, the axial groups become equatorial groups and the equatorial groups become axial groups
1 2
3
45
61
2 34
6 5
chair 1 chair 2
HH
HH
HH
H
HHH
HH H
HH
H
H
H
HH
HH
H
H
As will be discussed in detail later, there is a fast equilibrium between the two chair conformations of cyclohexane. If there are no substituents on cyclohexane, the two chairs are equivalent structures. But if one or more substituents are present, one of the chair conformations may dominate the equilibrium.
The Boat Conformation
Another conformation of cyclohexane that is free of angle strain is the boat. H
H
H
H
H
H
HH
H H
H
H
The "pure" boat conformation (above) has torsional strain from eclipsed H as revealed by the Newman projections along the C1-C2 and C5-C4 bonds.
1
2
5
4
"pure" boat
H
H
H
HH
H
H
H
1 245
1.83 Å
.
In addition, there is steric strain from nonbonded repulsive interaction between the two "flagpole" H that are closer than the 2.5 Å minimum distance apart for two H
• The boat conformation is less stable because of flagpole interactions and tortional strain along the bottom of the boat
93
The Twist Boat Conformation
.
Because of torsional and steric strain, the "pure" boat conformation of cyclohexane is estimated to be 29.7 kJ/mol less stable than the chair conformation
1
2
5
4
"pure" boat
1 245
H
H
H
HH
H
H
H
H
H
H
H
H
H
HH
H H
H
H
HH
.
Some of the strain energy can be eliminated by a slight twisting (rotation) around the C1-C2 and C5-C4 bonds. The resulting conformation is called the "twist" boat
"twist" boat
1
254
5
4
1
2
reduced torsional and steric strain
H
HH
H
H
HH
H
H
HH
H
H H
HH
H H
HH
H H
.
The twist boat is less stable than the chair conformation by 23 kJ/mol
The Half-Chair Conformation
The half-chair conformation is not a stable conformation for cyclohexane. It actually is an energy barrier (45.2 kJ/mol) that must be overcome during the chair to chair conversion. An alternative path that passes through a completely planar cyclohexane would be considerably higher in energy.
163
4
2
5
chair(no strain energy)
HH
HH
HH
H
HH
H
HH
Partial rotations around the C1-C2 and C4-C5 bonds push C6 into a plane containing C1, C2, C4, and C5.
bond
rotations
H
H
H
H
H
H
HH
HH
H
H
half-chair
The half-chair conformation has angle strain and torsionalstrain that place it 45.2 kJ/mol above the chair conformation.
Conformational Energy Diagram for Cyclohexane
chair
half-chair
boat
energetically preferredconformation is the twist boat
half-chair
chair
45.2 kJ/mol
Rel
ativ
e Po
tent
ial E
nerg
y
Conformational Change
0
42 kJ/mol
Some Key Observations
chair 1 chair 2boat
(1) The energy barrier of 45.2 kJ/mol leads to a rate of ~105 chair-chair interconversions per second at room temperature.
.
(2) The difference in energy of 23 kJ/mol between the chair and twist-boat conformations means that, at room temperature, more than 99% of the cyclohexane molecules are in the more stable chair conformations. However, because of the rapid equilibrium, some cyclohexane molecules are always passing through the less stable twist-boat conformation
Quiz Chapter 4 Section 12
Name the following conformations of cyclohexane. Rank them in order from most to least stable. Indicate the type of strain energy present in each conformation.
I II III IV
Stability order (most to least): > > >
Types of strain energy:
Name: planar half-chair chair boat
III IV II I
severe angleand torsionalstrain
angle andtorsionalstrain
torsionaland stericstrain
no torsionalangle strain or steric strain
• The twist conformation is intermediate in stability between the boat and the chair conformation
99
Substituted Cyclohexanes: Axial and Equatorial Hydrogen Atoms
• Axial hydrogens are perpendicular to the average plane of the ring• Equatorial hydrogens lie around the perimeter of the ring
• The C-C bonds and equatorial C-H bonds are all drawn in sets of parallel lines– The axial hydrogens are drawn straight up and down
100
Substituted Cyclohexanes
In the chair conformation of cyclohexane, there are axial and equatorial positions that interchange during chair-chair interconversion. For simple cyclohexane, the two chair conformations are identical.
fast
H
HH
H
HH
H
HH
H
H
H
H
HH
H
H
H
H
HH
H
HH
Stereoisomeric Forms of Monosubstituted Cyclohexanes
In monosubstituted cyclohexanes, the substituent may be either in an equatorial or axial position which are different stereoisomers.
Example: Methylcyclohexane
axial-methylcyclohexane
chair-chair interconversion
equatorial-methylcyclohexane
fast
fast
H
H
H H
H
H
HH
H
H
HCH3H
HH
HH
HHH
H
HH CH3
.
The axial- and equatorial-methylcyclohexane are stereoisomers. The equatorial isomer is more stable by 7.5 kJ/mol and therefore dominates in the fast equilibrium
Why is equatorial-methylcyclohexane more stable?
Relative Stabilities of the Methylcyclohexanes
The stability difference between the equatorial and axial stereoisomers of methylcyclohexane is due to steric strain associated with any group larger than H in the axial position.
The axial-methylcyclohexane hassteric strain energy that can berecognized by the juxtapositionsof the axial-methyl and the two axial-hydrogens.
H
H
HH
HH
HH
CH3
H H
H
The origin of the steric strain are gauche interactions as found in butane.
anti gauche
3.8 kJ/mol ofstrain energy
(strain-free)
CH3
CH3
CH3CH3HH
HH H
H
HH
Two Gauche Interactions in Axial-Methylcyclohexane
.In axial-methylcyclohexane there are two gauche interactions
1
23
axial-methylcyclohexane
4
56
H
H
HH
HH
HH
CH3
H H
H 1
view along the C1-C2 bond
26
3
gauche CH3
H
H
H
CH2
CH2
view along the C3-C2 bond
1
24
3
gaucheCH3
H
H
H
CH2
CH2
Each gauche interaction introduces3.75 kJ/mol of steric strain. There is a total of 2 x 3.75 = 7.5 kJ/mol of steric strain in the axial isomer.
Equatorial-Methylcyclohexane: Strain-Free
.Equatorial-methylcyclohexane has no steric strain due to gauche interactions because the stereochemical relationships are anti
1 2
3
equatorial-methylcyclohexane
5
6
4H
H
H H
H
H
H
H
H
H
HCH3
view along the C1-C2 bond
anti12
3
6
H
HCH2
CH3CH2
H
3anti
view along the C3-C2 bond
2
4
1H
HH
CH3
CH2
CH2
There are no gauche stereochemicalrelationships as in the axial isomer.The equatorial isomer is free ofsteric strain. Equatorial-methylcyclohexane ismore stable than the axial isomerby 7.5 kJ/mol.
The Axial-Equatorial Isomer Equilibrium
.
Chair-chair interconversion in methylcyclohexane is fast as in cyclohexane. In methylcyclohexane, this interconversion represents an equilibrium between the axial and equatorial stereoisomers. The energy difference of 7.5 kJ/mol means that at room temperature, the equatorial stereoisomer is favored over the axial stereoisomer by 95:5
(5 parts) (95 parts)
fast
fast
CH3H
CH3H
Effect of Substituent Size: G
axial equatorial
GG
fast
fast
H
H
When G is the large tert-butyl group, the equatorial isomer dominates the equilibrium to the extent of >99.9%.
As the size of G increases, the steric strain due to the two gauche interactions in the axial stereoisomer increases, and the equilibrium shifts more towards the equatorial stereoisomer. However, chair-chair interconversion still occurs.
.When there are two substituents in a cycloalkane at different positions, cis and trans isomers, as found in alkenes, are possible
Disubstituted Cycloalkanes: Cis-Trans Isomers
1,2-Dimethylcyclopentanes(shown as planar structures to illustrate stereoisomerism)
CH3 CH3
H H
H CH3
CH3 H
cis-1,2-dimethylcyclopentane trans-1,2-dimethylcyclopentanemethyl groups are onthe same side of ring
methyl groups are onopposite sides of ring
BP 99.5oC BP 91.9oC
.
The cis and trans 1,2-dimethylcyclopentanes are stereoisomers, they differ in the arrangement of atoms in space but have the same basic bond connectivities. These stereoisomers cannot be interconverted by simple bond rotations---they are not conformational isomers. These stereoisomers are called diastereomers and are separable by physical methods
Disubstitued Cycloalkanes• Can exist as pairs of cis-trans stereoisomers
– Cis: groups on same side of ring– Trans: groups on opposite side of ring
108
Test One: Examine the Bonds to the CH3 Groups
At each carbon where the substituent is located, identify each bond as "upper" or "lower."
CH3
CH3
H
H
Since the two bonds to theCH3 groups are "upper", meaning same side, they are "cis."
CH3
CH3
H
H
Test Two: Flatten the Ring
CH3 CH3
H H
Since the CH3 groups areon the same side, they are"cis."
upper bond
upper bond
lower bond
lower bond
SQUEEZE IT
• Trans-1,4-dimethylcylohexane prefers a trans-diequatorial conformation
110
Note: You cannot assign cis or trans isomers according to whether the bonds are equatorial or axial.
equatorial-axial axial-equatorial
CH3
CH3
H
HCH3
CH3
H
H
In cis-1,4-dimethylcyclohexane, the methyl groups are in equatorial and axial positions in both chair conformations, which are equivalent structures.
1,2-Dimethylcyclohexane
CH3CH3
H
H
equatorial
equatorial
?Is this structure the cis or trans isomer
:Draw the diequatorial-1,2-dimethylcyclohexane structure
Analysis
.
First, draw the second chair conformation remembering that chair-chair interconversion exchanges equatorial and axial positions
1,2-diequatorial 1,2-axial
CH3CH3
H
H CH3
CH3HH
1,2-diequatorial 1,2-axial
CH3CH3
H
H CH3
CH3
HH
.Application of Tests 1 and 2 to each chair conformation reveals that this isomer is trans-1,2-dimethylcyclohexane
Examples
CH3
CH3HH
cis-1,3-dimethylcyclohexaneCH3
CH3
H
H
cis-1,2-dimethylcyclohexane
CH3
CH3HH
trans-1,3-dimethylcyclohexane
(a,a) (e,e)
CH3
CH3
CH3H3C
HH
H
H
(e,a) (a,e)
CH3
CH3 CH3
CH3
H
H H
H
Quiz Chapter 4 Section 14
Draw both chair conformations of the dimethylcyclohexanes listed below. Indicate whether the methyl groups are axial or equatorial in each conformation.
trans-1,2-dimethylcyclohexane
cis-1,4-dimethylcyclohexane
Relative Stabilities of Chair Conformations
As with methylcyclohexane, the conformations are analyzed to identify gauche interactions. The conformation with thefewer such interactions is more stable and will dominate in the chair-chair equilibrium.
Which chair conformation of trans-1,4-dimethylcyclohexaneis more stable?
diequatorial diaxial
CH3CH3
H
H
A
CH3
CH3
H
H
B
H
H
H
H
no gauche interactions 2x2 = 4 gauche interactions
Chair A, the diequatorial conformation, is more stable by 4 gauche interactions or 4 x 3.75 = 15 kJ/mol and will dominate in the chair-chair equilibrium.
Which is the more stable chair conformation of trans-1-tertiary-butyl-3-methylcyclohexane?
.First draw one chair conformation with the substituents in the 1 and 3 positions and in the trans stereochemistry
CH3-CCH3
CH3
CH3HH
CH3-CCH3
CH3
H
CH3
H
A B
.Then, draw the second chair conformation
.Evaluate the elements of steric strain in each chair conformation
In A, the larger tert-butyl group is equatorial while the smaller methyl group is axial (2 gauche interactions).
In B, the larger tert-butyl group is axial (2 gauche interactions) while the smaller methyl group is equatorial.
There is more steric strain associated with the larger tert-butyl group in the axial position. Therefore, chair A is more stable and will dominate in the chair-chair equilibrium.
trans-1,2-dimethylcyclohexane: Watch out for the curve ball!
1 2
1,2-diaxial 1,2-diequatorial
1
2CH3
CH3
HH CH3CH3
H
H
Each axial methyl groupgenerates 2 x 3.75 = 7.5 kJ/molof strain energy, for a totalof 15 kJ/mol.
But the energy difference between the 1,2-axial and the 1,2-diequatorial conformations is only 3 gauche interactions or 3 x 3.75 = 11.25 kJ/mol.
:
There is one gauche interaction from the interacting CH3 groups
12 CH3
CH3
H
H
CH2
CH2
gauche
Quiz Chapter 4 Section 14 part II
Draw the two chair conformations of each dialkylcyclohexane below. Circle the more stable chair conformation.
cis-1,3-dimethylcyclohexane
cis-1-tert-butyl-4-methylcyclohexane
CH3
CH3CH3
H3CH
H H H
CH3
CH3
H
H(CH3)3C
H
H(CH3)3C
• A very large tert-butyl group is required to be in the more stable equatorial position
119
Polycyclic Alkanes
.
Polycyclic organic molecules are common in nature. For example, the steroid hormones (progesterone, testosterone, etc.) are polycyclic alkane derivatives
The steroid hormones are based on a polycyclic structure of 4 cycloalkanes referred to as the A,B,C and D rings.
A B
C D
O
CH3
CH3CCH3
O=
HH
H
progesterone(pregnancy hormone)
OH
testosterone(male hormone)
O
CH3
CH3
HH
H
The A and B rings have stereochemical features of bicyclo[4.4.0]decane, commonly called "decalin." decalin
bicyclo[4.4.0]decane
Some Stereochemical Features of Fused Cycloalkanes
The Decalins: Models for the A/B Ring Fusion
.
The fusion of the A and B rings in decalin shows cis-trans isomerism. This isomerism is understandable as an extension of the isomerism in 1,2-dimethylcyclohexane
trans-1,2-diequatorial cis-1-equatorial-2-axial
CH3CH3
CH3CH3
H
H
H
H
Bicyclic and Polycyclic Alkanes• The bicyclic decalin system exists in non-interconvertible
cis and trans forms
122
Chemical Reactions of Alkanes
The Limited Chemical Reactivity of Alkanes
.
As nonpolar organic structures, alkanes have limited chemical reactivity compared with other classes of organic compounds. Neither acids nor bases react readily with alkanes. One notable and important reaction of alkanes is combustion, the combination with oxygen, discussed earlier
Synthesis of Alkanes and Cycloalkanes
It is important to distinguish between large scale industrial production of some alkanes, often carried out under carefully controlled catalytic methods, and standard laboratory procedures.
These standard procedures are the synthetic methods covered in detail in this course. The study of these general syntheses is closely connected with, if not inseparable from, the investigation of reaction mechanism theory in organic chemistry.
Synthesis of Alkanes and Cycloalkanes• Hydrogenation of Alkenes and Alkynes
124
• Reduction of Alkyl Halides
125
• Alkylation of Terminal Alkynes • Alkynes can be subsequently hydrogenated to
alkanes
126
An Example of a Synthesis
3-methyl-1-butyne
CH3CHC C-HCH3 Na+ NH2
-
(-NH3) C:- Na+CH3CHCCH3
alkylation reactionCH3Br
4-methyl-2-pentyne
CH3CHC CCH3
CH3H2 (excess)
hydrogenation reaction
Pt2-methylpentane
CH3CHCH2CH2CH3
CH3
.
In a multi-step synthesis, the products often are isolated and purified after each step. In the above example, the alkynide anion would not be isolated because of its reactivity, and because of the "completeness" of the acid-base reaction. It would be prepared and used immediately in the alkylation step. The alkyne product would be isolated and purified before the hydrogenation step
Some Terminology
The alkynide anion has a nonbonding electron pair on carbon that reacts readily with electron-deficient species.The carbanion is called a nucleophile.
The alkyl halide reacts via the electropositive carbon. This electron-deficient atom seeks electron density and is called an electrophile.
.This pattern of combining opposite charges in chemical reactions is widely found in organic chemistry
R-C-
a sodium alkynide
"nucleophile"
C: +alkyl halide
"electrophile"
C X R-C + X-C C
A compound's molecular formula allows a chemist to calculate the Index of Hydrogen Deficiency (IHD) of that compound.
The IHD is calculated by converting the molecular formula to a formula that contains carbons and hydrogens only and then comparing it to the formula for a saturated hydrocarbon (CnH2n+2).
If the molecular compound has the same number of hydrogens as its saturated hydrocarbon counterpart then it has an IHD = 0. The IHD increases by one for every missing two hydrogens (i.e. a saturated octane has 18 hydrogens, if the target compound has 16 hydrogens (2 less than 18), it will have an IHD = 1; if it has 14 hydrogens, it willhave an IHD = 2...).
Index of Hydrogen Deficiency
1-Hexene (C6H12)
Cyclohexene (C6H12)
Saturated Hydrocarbon C6H14
1-Hexene or Cyclohexne C6H12
Hydrogens missing 2 which means IHD = 1
Each IHD is equal to one ring or one multiple bond.
A double bond is equal to 1 IHD.
A triple bond is equal to 2 IHD.
Saturated Hydrocarbons have the general molecular formula CnH2n + 2
Therefore, with 6 carbons, 1-Hexene and Cyclohexene should bothhave 14 hydrogens (C6H2(6) + 2 , C6H12 + 2 , C6H14).
Compounds containing halogen atoms
For compounds containing halogen atoms, each halogen atom is equal to one hydrogen atom.
Molecular Formula Converts to Compare to IHD
C6H10Cl2
C8H12ClBr3
C8H8Br4Cl2
C6H12 C6H14 1
C8H16 C8H18 1
C8H14 C8H18 2
Compounds containing oxygen atoms
For compounds containing oxygen atoms, each oxygen atom is equal to zero hydrogen atoms.
Molecular Formula Converts to Compare to IHD
C6H12O2
C7H12O3
C10H18O2
C6H12 C6H14 1
C7H12 C7H16 2
C10H18 C10H22 2
For compounds containing nitrogen atoms, each nitrogen atom is equal to minus one hydrogen atoms.
Compounds containing nitrogen atoms
Molecular Formula Converts to Compare to IHD
C6H13N
C7H16N2
C10H26N4
C6H12 C6H14 1
C7H14 C7H16 1
C10H22 C10H22 0
Quiz Chapter 4 Section 18
What is the Index of Hydrogen Deficiency for each of the following?
C6H8Br2N2O
C12H21Cl2NO3
C15H27Cl2Br3N2O2
First, convert each of the formulas to carbons and hydrogens only.
C6H8
C12H22
C15H30
Write down the formula of the comparable saturated hydrocarbon.
C15H32
C12H26
C6H14
How many hydrogens are missing from each one?
6
4
2
What is the IHD?
3
2
1
The Index of Hydrogen Deficiency
The general formula of an alkane is CnH2n + 2
The general formula of an alkene is CnH2n
The presence of a double bond is revealed by a deficiency of 2H in the molecular formula.
Useful structural information is available from the molecular formula of a hydrocarbon.
CH3CH2CH2CH3 CH3CH2CH=CH2
C4H10 C4H8
The missing 2H also is calledone degree of unsaturation or 1U.
A cycloalkane has one degree of unsaturation.
CnH2n
CH3CH2CH2CH3
C4H10
CH2-CH2
CH2-CH2C4H8
Degrees of Unsaturation
Each double bond or ring in a structure adds one degree of unsaturation or 1 U.
CH2=CH-CH=CH2C4H6
Since the formula C4H10 is"saturated," C4H6 has 2U.
(2U)
cyclohexene(C6H10)
CH=CH2
1-vinylcyclopentene(C7H10)
propyneCH3C CH
(C3H4)2 U 3 U 2 U
Index of Hydrogen Deficiency
Rings and multiple bonds may be distinguished by the number of moles of H2 taken up in catalytic hydrogenation:
Pt, 25o CC6H10 2 U
H2
C6H12 1 U
UThe remaining 1 indicates the presence of one ring.
Quiz Chapter 7 Section 15
Provide the index of hydrogen deficiency for each compound below.
Cl
ClO
SolutionDetermine the molecular formula of each compound and follow the rules for determining the U-number.
C5H6O C5H8 C7H10Cl23 U 2 U 2 U
Carbon-13 NMR (CMR) Spectroscopy
The carbon-13 nucleus (13C), like 1H, has a nuclear spin quantum number of 1/2. However, this isotope occurs in a natural abundance of only 1.1%. The major isotope of carbon, 12C, is not "nmr active" since it has a nuclear spin quantum number of I =0.
6
6
1
However, commercial instruments have been available for more than 20 years that allow the recording of quality cmr spectra using the naturally occurring minor isotope of carbon. The low level of 13C simplifies the cmr spectra.
Some Key Points about CMR Spectroscopy
(1) The number of resonances is a measure of how many magnetically different carbons are in the structure.
(2) The splitting of a resonance indicates how many hydrogens are attached to that carbon. (The splittings may be eliminated by running proton-decoupled spectra. See below.)
(3) The chemical shift reflects the hybridization of the carbon: sp3, sp2, sp.
(4) The chemical shift is also affected by the electronic environment of the carbon (inductive influences) and other shielding and deshielding effects.
(5) Note: the signal intensities in cmr spectroscopy are not related to the relative numbers of the carbons.
Proton Splittings: Decoupling Strategies
13C-13C Coupling
.
Spin-spin interaction between two 13C is not generally observed because of the low natural abundance of this minor isotope (1.1%). It is very unlikely that two of these "nmr active" nuclei will be close enough in a structure to spin-spin couple
13C-1H Coupling
. .
Spin-spin interactions between 13C and 1H are observed and may complicate the cmr spectrum. Typical J values for 13C-1Ha are 100-320 Hz. Long range splittings due to 13C-1Hb and even 13C-1Hg are also significant. CMR spectra often show complex and overlapping multiplets that may be difficult to interpret
A comparison of proton decoupled (top) and off-resonance proton decoupled (bottom) 13C-NMR spectra of diethyl phthalate.
From the splittings in the off-resonance proton decoupled spectrum, the number of H attached to each carbon (n+1 rule) may be determined. If the resonance is a singlet, there are no H attached.
==
diethyl phthalate
1 5 62
43
orCOCH2CH3
O
COCH2CH3O
C-1 (0 H)
C-2 (0 H)
C-3 (1 H)
C-4 (1 H)and
C-5 (2 H)C-6 (3 H)
CMR Chemical Shifts
The 13C chemical shifts are much larger than 1H chemical shifts. The scale is typically from 0 to 200.
.The biggest influence on the chemical shift is the hybridization of carbon: sp3, sp2 or sp
Examples
CH3-CH2-CH2-CH2-CH2-CH2-CH=CH2
1-octene
.
sp2sp3
14.1 22.9 32.1 29.3 29.1 34.1 139 114
2,4,4-trimethyl-1-pentene
30.4
31.6 52.2
25.4
143.7
114.4CH3-C-CH2-C=CH2
CH3
CH3
CH3
ethylbenzene
29.1 15.6
144.2127.9
128.4
125.7
CH2-CH3
CMR Chemical Shifts
.
The influence of substituents and functional groups on the cmr chemical shift is similar in some ways to 1H nmr. The carbon of the carbonyl group is strongly shifted downfield
020406080100120140160180200
alkanes CC=C
aromatic CC C
C-O C-N
C=Oacids, esters,amides
C=O
aldehydes andketones
The off-resonance proton decoupled spectrum gives the following results: 22.1 (2H), 24.5 (2H) and 127.3 (1H). Which structure is consistent with this result?
Quiz Chapter 4 Section 19
The 13C NMR spectrum of a hydrocarbon, C6H10, shows three resonances at 22.1, 24.5 and 127.3. Which structures below are consistent with this result?
CH3
H3C
H3C
CH3
CH3
Retrosynthetic Analysis-Planning Organic Synthesis• The synthetic scheme is formulated working
backward from the target molecule to a simple starting material• Often several schemes are possible
146
