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Chapter 3Chapter 3Alkanes and Cycloalkanes: Alkanes and Cycloalkanes:
Conformations and cis-trans Conformations and cis-trans StereoisomersStereoisomers
3.13.1
Conformational Analysis of EthaneConformational Analysis of Ethane
Conformations are different spatial Conformations are different spatial arrangements of a molecule that are arrangements of a molecule that are generated by rotation about single bonds.generated by rotation about single bonds.
eclipsed conformation
Ethane
Ethane
eclipsed conformation
Ethane
staggered conformation
Ethane
staggered conformation
Projection Formulas of the Staggered Conformation of Ethane
Newman Sawhorse
H
H
H H
H H
H
H HH
H
H
H
H
H H
H H
H
H HH
H
H180°
Anti Relationships
Two bonds are anti when the angle between them is 180°.
H
H
H H
H H
H
H HH
H
H
60°
Gauche Relationships
Two bonds are gauche when the angle between them is 60°.
An Important Point:
The terms anti and gauche apply only to bonds (or groups) on adjacent carbons, and only to staggered conformations.
The terms anti and gauche apply only to bonds (or groups) on adjacent carbons, and only to staggered conformations.
0° 60° 120° 180° 240° 300° 360°
12 kJ/mol12 kJ/mol
The eclipsed conformation of ethane is 12 kJ/mol less stable than the staggered.
The eclipsed conformation is destabilized bytorsional strain.
Torsional strain is the destabilization that resultsfrom eclipsed or partially eclipsed bonds.
Torsional Strain
3.23.2Conformational Analysis of ButaneConformational Analysis of Butane
0° 60° 120° 180° 240° 300° 360°
3 kJ/mol3 kJ/mol
14 kJ/mol14 kJ/mol
The gauche conformation of butane is 3 kJ/molless stable than the anti.
The gauche conformation is destabilized byvan der Waals strain (also called steric strain).
van der Waals strain is the destabilization that results from atoms being too close together.
van der Waals Strain
The conformation of butane in which the twomethyl groups are eclipsed with each other isthe least stable of all the conformations.
It is destabilized by both torsional strain(eclipsed bonds) and van der Waals strain.
van der Waals Strain
3.33.3
Conformations of Higher AlkanesConformations of Higher Alkanes
The most stable conformation of unbranchedalkanes has anti relationships between carbons.
Hexane
Unbranched Alkanes
3.4The Shapes of Cycloalkanes:
Planar or Nonplanar?
Baeyer assumed cycloalkanes are planar polygons,
and that distortion of bond angles from 109.5° givesangle strain to cycloalkanes with rings eithersmaller or larger than cyclopentane.
Baeyer deserves credit for advancing the ideaof angle strain as a destabilizing factor.
But Baeyer was incorrect in his belief that cycloalkanes were planar.
Adolf von Baeyer (19th century)
• Torsional strain strain that results from eclipsed bonds
• van der Waals strain (steric strain)strain that results from atoms being too close together
• angle strainstrain that results from distortion of bondangles from normal values
Types of Strain
Measuring Strain in Cycloalkanes
Heats of combustion can be used to comparestabilities of isomers.
But cyclopropane, cyclobutane, etc. are not isomers.
All heats of combustion increase as the numberof carbon atoms increase.
Measuring Strain in Cycloalkanes
Therefore, divide heats of combustion by number of carbons and compare heats of combustion on a "per CH2 group" basis.
Cycloalkane kJ/mol Per CH2
Cyclopropane 2,091 697
Cyclobutane 2,721 681
Cyclopentane 3,291 658
Cyclohexane 3,920 653
Cycloheptane 4,599 657
Cyclooctane 5,267 658
Cyclononane 5,933 659
Cyclodecane 6,587 659
Heats of Combustion in Cycloalkanes
Cycloalkane kJ/mol Per CH2
According to Baeyer, cyclopentane should
have less angle strain than cyclohexane.
Cyclopentane 3,291 658
Cyclohexane 3,920 653
The heat of combustion per CH2 group is
less for cyclohexane than for cyclopentane.
Therefore, cyclohexane has less strain than
cyclopentane.
Heats of Combustion in Cycloalkanes
Adolf von Baeyer (19th century)
assumed cycloalkanes are planar polygons
distortion of bond angles from 109.5° givesangle strain to cycloalkanes with rings eithersmaller or larger than cyclopentane
Baeyer deserves credit for advancing the ideaof angle strain as a destabilizing factor.
But Baeyer was incorrect in his belief that cycloalkanes were planar.
Cyclopropane Cyclopropane
Cyclobutane Cyclobutane
3.53.5Small RingsSmall Rings
sources of strain
torsional strain
angle strain
Cyclopropane
nonplanar conformation relieves some torsional strain
angle strain present
Cyclobutane
3.63.6CyclopentaneCyclopentane
all bonds are eclipsed in planar conformation
planar conformation destabilizedby torsional strain
Cyclopentane
Envelope Half-chair
Relieve some, but not all, of the torsional strain.
Envelope and half-chair are of similar stabilityand interconvert rapidly.
Nonplanar Conformations of Cyclopentane
heat of combustion suggests that angle strain is unimportant in cyclohexane
tetrahedral bond angles require nonplanar geometries
3.73.7Conformations of CyclohexaneConformations of Cyclohexane
All of the bonds are staggered and the bond angles at carbon are close to tetrahedral.
Chair is the most stable conformation of cyclohexane
All of the bond angles are close to tetrahedralbut close contact between flagpole hydrogenscauses van der Waals strain in boat.
180 pm
Boat conformation is less stable than the chair
Eclipsed bonds bonds gives torsional strain toboat.
Boat conformation is less stable than the chair
Less van der Waals strain and less torsional strain in skew boat.
Boat Skew boat
Skew boat is slightly more stable than boat
The chair conformation of cyclohexane is themost stable conformation and derivativesof cyclohexane almost always exist in the chair conformation.
Generalization
3.83.8Axial and Equatorial Bonds in Axial and Equatorial Bonds in
CyclohexaneCyclohexane
The 12 bonds to the ring can be divided into two sets of 6.
Axial bonds point "north” and “south"
6 Bonds are axial
The 12 bonds to the ring can be divided into two sets of 6.
Equatorial bonds lie along the “equator.”
6 Bonds are equatorial
3.93.9Conformational InversionConformational Inversion
in Cyclohexane in Cyclohexane
chair-chair interconversion (ring-flipping)
rapid process (activation energy = 45 kJ/mol)
all axial bonds become equatorial and vice versa
Conformational Inversion
Half-chair
Skewboat
45 45 kJ/molkJ/mol
45 45 kJ/molkJ/mol
23 kJ/mol
most stable conformation is chairmost stable conformation is chair
substituent is more stable when equatorialsubstituent is more stable when equatorial
3.103.10Conformational Analysis of Conformational Analysis of
Monosubstituted CyclohexanesMonosubstituted Cyclohexanes
5% 95%
Chair chair interconversion occurs, but at any instant 95% of the molecules have their methyl group equatorial.
Axial methyl group is more crowded than an equatorial one.
Methylcyclohexane
CH3 CH3
5% 95%
Source of crowding is close approach to axial hydrogens on same side of ring.
Crowding is called a "1,3-diaxial repulsion" and is a type of van der Waals strain.
Methylcyclohexane
40% 60%
Crowding is less pronounced with a "small" substituent such as fluorine.
Size of substituent is related to its branching.
F
F
Fluorocyclohexane
Less than 0.01% Greater than 99.99%
Crowding is more pronounced with a "bulky" substituent such as tert-butyl.
tert-Butyl is highly branched.
C(CH3)3 C(CH3)3
tert-Butylcyclohexane
van der Waalsstrain due to1,3-diaxialrepulsions
tert-Butylcyclohexane
3.113.11Disubstituted Cycloalkanes:Disubstituted Cycloalkanes:
cis-trans Stereoisomerscis-trans Stereoisomers
Stereoisomers are isomers that have Stereoisomers are isomers that have same constitution but different same constitution but different arrangement of atoms in spacearrangement of atoms in space
Isomers
Constitutional isomersConstitutional isomers StereoisomersStereoisomers
1,2-Dimethylcyclopropane
There are two stereoisomers of 1,2-dimethylcyclopropane.
They differ in spatial arrangement of atoms.
1,2-Dimethylcyclopropane
cis-1,2-Dimethylcyclopropane has methyl groupson same side of ring.
trans-1,2-Dimethylcyclopropane has methyl groupson opposite sides.
Relative stabilities of stereoisomers may bedetermined from heats of combustion.
3371 kJ/mol
3366 kJ/mol
Van der Waals strain makes cis stereoisomer less stable than trans.
3.123.12Conformational AnalysisConformational Analysis
of Disubstituted Cyclohexanesof Disubstituted Cyclohexanes
cis trans
CH3
5219 kJ/mol 5212 kJ/mol
less stable more stable
Trans stereoisomer is more stable than cis, but methyl groups are too far apart to crowd each other.
H3C
H H
H3C
CH3H
H
1,4-Dimethylcyclohexane Stereoisomers
CH3H3C
H H
Two equivalent conformations; each has one axial methyl group and one equatorial methyl group
H
CH3
HCH3
H
H3C
H
CH3
Conformational analysis ofcis-1,4-
dimethylcyclohexane
CH3
H3C
H
H
Two conformations are not equivalent; most stableconformation has both methyl groups equatorial.
H
H3C
H
CH3
H
H3C
H
CH3
Conformational analysis oftrans-1,4-
dimethylcyclohexane
cis trans
5223 kJ/mol 5217 kJ/mol
less stable more stable
Analogous to 1,4 in that trans is more stablethan cis.
CH3
CH3H
HH3C
CH3
H
H
1,2-Dimethylcyclohexane Stereoisomers
CH3
CH3H
H
Two equivalent conformations; each has one axial methyl group and one equatorial methyl group
HCH3
H
CH3 H
CH3
H
CH3
Conformational analysis ofcis-1,2-
dimethylcyclohexane
CH3
H3C H
H
Two conformations are not equivalent; most stableconformation has both methyl groups equatorial.
H
CH3
H
CH3
H
H3C
H
CH3
Conformational analysis oftrans-1,2-
dimethylcyclohexane
cis trans
5212 kJ/mol 5219 kJ/mol
more stable less stable
Unlike 1,2 and 1,4; cis-1,3 is more stable than trans.
H3C
CH3
H
H
CH3
H3C
H H
1,3-Dimethylcyclohexane Stereoisomers
CH3
H3C
H H
Two conformations are not equivalent; most stableconformation has both methyl groups equatorial.
H3C
HH
CH3
H
CH3
H
CH3
Conformational analysis ofcis-1,3-
dimethylcyclohexane
Two equivalent conformations; each has one axialand one equatorial methyl group.
H3C H
H CH3
H
H3C
HCH3
H3C
CH3
H
H
Conformational analysis oftrans-1,3-
dimethylcyclohexane
Compound Orientation -H° (kJ/mol)
cis-1,2-dimethyl ax-eq 5223trans-1,2-dimethyl eq-eq 5217*
cis-1,3-dimethyl eq-eq 5212*trans-1,3-dimethyl ax-eq 5219
cis-1,4-dimethyl ax-eq 5219trans-1,4-dimethyl eq-eq 5212*
*more stable stereoisomer of pair
Table 3.2 Heats of Combustion ofIsomeric Dimethylcyclohexanes
3.133.13Medium and Large RingsMedium and Large Rings
More complicated than cyclohexane.
Common for several conformations to be of similar energy.
Principles are the same, however:minimize total strain.
Cycloheptane and Larger Rings
contain more than one ring
bicyclic
tricyclic
tetracyclic
etc
3.143.14
Polycyclic Ring SystemsPolycyclic Ring Systems
spirocyclic
fused ring
bridged ring
Types of Ring Systems
Adamantane: A Tricyclic Compound
Three bond cleavages are needed to create an open-chain structure.
one atom common to two rings Spirocyclic
adjacent atoms common to two rings
two rings share a common side Fused Ring
nonadjacent atoms common to two rings
Bridged Ring
equals minimum number of bond disconnectionsrequired to give a noncyclic species
Number of Rings
requires one bond disconnection Monocyclic
requires two bond disconnections Bicyclic
requires two bond disconnections Bridged Bicyclic
carbon skeleton is tetracyclic Steroids
3.153.15
Heterocyclic CompoundsHeterocyclic Compounds
a cyclic compound that contains an atom other
than carbon in the ring
(such atoms are called heteroatoms)
typical heteroatoms are N, O, and S
Heterocyclic Compound
Ethylene oxide
Tetrahydrofuran
Tetrahydropyran
Oxygen-containing Heterocycles O
O
O
O
Pyrrolidine Piperidine
Nitrogen-containing Heterocycles N H
N
H
Lipoic acid
Lenthionine
CH2CH2CH2CH2COH
O
SS SS
SS
S
Sulfur-containing Heterocycles