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Chapter 4
Introduction toHydrocabons
Carbon Backbone, Nomenclature, Physical &
Chemical Properties
HYDROCARBONS• Compounds composed of only carbon and hydrogen atoms
(C, H). Each carbon has 4 bonds.
• They represent a “backbone” when other “heteroatoms” (O, N, S, .....) are substituted for H. (The heteroatoms give function to the molecule.)
• Acyclic (without rings); Cyclic (with rings); Saturated: only carbon-carbon single bonds; Unsaturated: contains one or more carbon-carbon double and/or triple bond
HYDROCARBONS• Alkanes contain only single ( ) bonds and have the
generic molecular formula: [CnH2n+2]
• Alkenes also contain double ( + ) bonds and have the generic molecular formula: [CnH2n]
• Alkynes contain triple ( + 2) bonds and have the generic molecular formula: [CnH2n-2]
• Aromatics are planar, ring structures with alternating single and double bonds: eg. C6H6
Types of Hydrocarbons
Each C atom is trigonal planar with sp2 hybridized orbitals.There is no rotation about the C=C bond in alkenes.
Each C atom is tetrahedral with sp3 hybridized orbitals. They only have single bonds.
Question 4.1
• What is the hybridization of the starred carbon in humulene (shown)?
• A) sp• B) sp2
• C) sp3
• D) 1s2 2s2 2p2
Question 4.2
• What is the hybridization of the starred carbon of geraniol?
• A) sp• B) sp2
• C) sp3
• D) 1s2 2s2 2p2
Types of Hydrocarbons
Each C atom is linear with sp hybridized orbitals.
Each C--C bond is the same length; shorter than a C-C bond: longer than a C=C bond.The concept of resonance is used to explain this phenomena.
Propane
It is easy to rotate about the C-C bond in alkanes.
Naming AlkanesC1 - C10 : the number of C atoms present in the chain.
Each member C3 - C10 differs by one CH2 unit. This is called a homologous series.
Methane to butane are gases at normal pressures.Pentane to decane are liquids at normal pressures.
Nomenclature of Alkyl Substituents
Examples of Alkyl Substituents
Constitutional or structural isomers have the same molecular formula, but their atoms are linked differently. Naming has to account for them.
Question 4.3
• How many hydrogens are in a molecule of isobutane?
• A) 6• B) 8• C) 10• D) 12
A compound can have more than one name, but a name must unambiguously specify only one compound
C7H16 can be any one of the following:
Question 4.4
• How many isomeric hexanes exist?• A) 2• B) 3• C) 5• D) 6
Question 4.5
• The carbon skeleton shown at the bottom right accounts for 9 carbon atoms. How many other isomers of C10H22 that have 7 carbons in their longest continuous chain can be generated by adding a single carbon to various positions on this skeleton?
• A) 2• B) 3• C) 4• D) 5
Alkanes (Different types of sp3 carbon atoms)
• Primary, 1o, a carbon atom with 3 hydrogen atoms:
[R-CH3]
• Secondary, 2o, a carbon atom with 2 hydrogen atoms:
[R-CH2-R]
• Tertiary, 3o, a carbon atom with 1 hydrogen atom:
• [R-CH-R] R
• Quaternary, 4o, a carbon atom with 0 hydrogen atoms: CR4
Different Kinds of sp3 Carbons and Hydrogens
Question 4.6
• In 3-ethyl-2-methylpentane, carbon #3 (marked by a star) is classified as:
• A) primary (1°)• B) secondary (2°)• C) tertiary (3°)• D) quaternary (4°)
Question 4.7
• How many primary carbons are in the molecule shown at the bottom right?
• A) 2• B) 3• C) 4• D) 5
Nomenclature of Alkanes
1. Determine the number of carbons in the parent hydrocarbon
CH3CH2CH2CH2CHCH2CH2CH3
CH3
12345678
CH3CH2CH2CH2CHCH2CH3
CH2CH2CH3
45678
123
CH3CH2CH2CHCH2CH2CH3
CH2CH2CH2CH3
1234
5 6 7 8
2. Number the chain so that the substituent gets the lowest possible number
CH3CHCH2CH2CH3
CH3
1 2 3 4 5
2-methylpentane
CH3CH2CH2CHCH2CH2CH2CH3
CHCH3
CH3
1 2 3 4 5 6 7 8
4-isopropyloctane
CH3CHCH2CH2CH3
CH3
common name: isohexanesystematic name: 2-methylpentane
3. Number the substituents to yield the lowest possible number in the number of the compound
CH3CH2CHCH2CHCH2CH2CH3
CH3 CH2CH3 5-ehtyl-3-methyloctanenot
4-ethyl-6-methyloctanebecause 3<4
(substituents are listed in alphabetical order)
4. Assign the lowest possible numbers to all of the substituents
CH3CH2CHCH2CHCH3
CH3CH3
2,4-dimethylhexane
CH3CH2CH2C
CH3
CH3
CCH 2CH 3
CH3
CH3
3,3,4,4-tetramethylheptane
CH3CH2CHCH2CH2CHCHCH2CH2CH3
CH2CH3
CH2CH3 CH2CH3
CH3
3,3,6-triethyl-7-methyldecane
5. When both directions lead to the same lowest number for oneof the substituents, the direction is chosen that gives the lowest possible number to one of the remaining substituents
CH3CHCH2CHCH3
CH3
CH3 CH3
2,2,4-trimethylpentanenot
2,4,4-trimethylpentanebecause 2<4
CH3CH2CHCHCH2CHCH2CH3
CH3
CH3 CH2CH3
6-ethyl-3,4-dimethyloctanenot
3-ethyl-5,6-dimethyloctanebecause 4<5
6. If the same number is obtained in both directions, the firstgroup receives the lowest number
CH3CH2CHCH2CHCH2CH3
CH3
CH2CH3
3-ethyl-5-methylheptanenot
5-ethyl-3-methylheptane
CH3CH2CHCH3
Cl
Br
2-bromo-3-chlorobutanenot
3-bromo-2-chlorobutane
7. In the case of two hydrocarbon chains with the same number ofcarbons, choose the one with the most substituents
CH3CH2CHCH2CH2CH3
CHCH3
CH31
2
3 4 5 6
3-ethyl-2-methylhexane (two substituents)
CH3CH2CHCH2CH2CH3
CHCH3
CH3
1 2 3 4 5 6
3-isopropylhexane (one substituent)
8. Certain common nomenclatures are used in the IUPAC system
CH3CH2CH2CH2CHCH2CH2CH3
CHCH3
CH3
4-isopropyloctaneor
4-(1-methylethyl)octane
CH3CH2CH2CH2CHCH2CH2CH2CH2CH3
CH2CHCH3
CH3
5-isobutyldecaneor
5-(2-methylpropyl)decane
Question 4.7
• The correct structure of 3-ethyl-2-methylpentane is:
• A) B)
• C) D)
CnH2n
Cycloalkane Nomenclature
Cycloalkanes• Cycloalkanes are alkanes that contain a
ring of three or more carbons.• Count the number of carbons in the ring,
and add the prefix cyclo to the IUPAC name of the unbranched alkane that has that number of carbons.
Cyclopentane Cyclohexane
Ethylcyclopentane
CH2CH3
• Name any alkyl groups on the ring in the usual way. A number is not needed for a single substituent.
Cycloalkanes
• Name any alkyl groups on the ring in the usual way. A number is not needed for a single substituent.
• List substituents in alphabetical order and count in the direction that gives the lowest numerical locant at the first point of difference.
3-Ethyl-1,1-dimethylcyclohexane
CH2CH3
H3C CH3
Cycloalkanes
For more than two substituents,
CH3CH2CH2
H3C CH2CH3
4-ethyl-2-methyl-1-propylcyclohexanenot
1-ethyl-3-methyl-4-propylcyclohexanebecause2<3
not 5-ethyl-1-methyl-2-propylcyclohexane
because 4<5
CH3
CH3
CH3
1,1,2-trimethylcyclopentanenot
1,2,2-trimethylcyclopentanebecause1<2
not1,1,5-trimethylcyclopentane
because 2<5
Question 4.8
• Which one contains the greatest number of tertiary carbons?
• A) 2,2-dimethylpropane• B) 3-ethylpentane• C) sec-butylcyclohexane• D) 2,2,5-trimethylhexane
Physical Properties of Alkanes
and Cycloalkanes
Crude oil
Refinery gas
C1-C4
Light gasoline(bp: 25-95 °C)
C5-C12
Naphtha(bp 95-150 °C)
Kerosene(bp: 150-230 °C)
C12-C15
Gas oil(bp: 230-340 °C)
C15-C25
Residue
Question 4.9Arrange octane, 2,2,3,3-tetramethylbutane and
2-methylheptane in order of increasing boiling point.
• A) 2,2,3,3-tetramethylbutane < octane < 2-methylheptane
• B) octane < 2-methylheptane < 2,2,3,3-tetramethylbutane
• C) 2,2,3,3-tetramethylbutane < 2-methylheptane < octane
• D) 2-methylheptane < 2,2,3,3-tetramethylbutane < octane
• The gasoline fraction of crude oil only makes up about 19%, which is not enough to meet demand.
Crude Oil and Uses of Alkanes
van der Waals ForcesWeak Intermolecular Attractive Forces
The boiling point of a compound increases with the increase in van der Waals force…and a
Gecko uses them to walk!
Gecko: toe, setae, spatulae6000x Magnification
http://micro.magnet.fsu.edu/primer/java/electronmicroscopy/magnify1/index.html
Geim, Nature Materials (2003) Glue-free Adhesive100 x 10 6 hairs/cm2
Full et. al., Nature (2000)5,000 setae / mm2
600x frictional force; 10-7 Newtons per seta
Ion-Dipole Forces (40-600 kJ/mol)• Interaction between an ion and a dipole (e.g. NaOH and
water = 44 kJ/mol)• Strongest of all intermolecular forces.
Intermolecular Forces
Ion-Dipole & Dipole-Dipole Interactions: like dissolves like
• Polar compounds dissolve in polar solvents & non-polar in non-polar
Dipole-Dipole Forces
(permanent dipoles)
Intermolecular Forces
5-25 kJ/mol
Dipole-Dipole Forces
Intermolecular Forces
Boiling Points &
Hydrogen Bonding
Hydrogen Bonding
• Hydrogen bonds, a unique dipole-dipole (10-40 kJ/mol).
London or Dispersion Forces• An instantaneous dipole can induce another dipole in an
adjacent molecule (or atom).• The forces between instantaneous dipoles are called
London or Dispersion forces ( 0.05-40 kJ/mol).
Intermolecular Forces
Boiling Points of Alkanes
• governed by strength of intermolecular attractive forces
• alkanes are nonpolar, so dipole-dipole and dipole-induced dipole forces are absent
• only forces of intermolecular attraction are induced dipole-induced dipole forces
Boiling Points
• Increase with increasing number of carbons
• more atoms, more electrons, more opportunities for induced dipole-induceddipole forces
• Decrease with chain branching
• branched molecules are more compact withsmaller surface area—fewer points of contactwith other molecules
London Dispersion Forces
Intermolecular Forces
Which has the higherattractive force?
Question 4.10
• Which alkane has the highest boiling point?
• A) hexane• B) 2,2-dimethylbutane• C) 2-methylpentane• D) 2,3-dimethylbutane
•Increase with increasing number of carbons
• more atoms, more electrons, more opportunities for induced dipole-induceddipole forces
Heptanebp 98°C
Octanebp 125°C
Nonanebp 150°C
Boiling Points
•Decrease with chain branching
• branched molecules are more compact withsmaller surface area—fewer points of
contactwith other molecules
Octane: bp 125°C
2-Methylheptane: bp 118°C
2,2,3,3-Tetramethylbutane: bp 107°C
Boiling Points
• Gasoline is a mixture of straight, branched, and aromatic hydrocarbons (5–12 carbons in size).
– Large alkanes can be broken down into smaller molecules by CRACKING.
– Straight chain alkanes can be converted into branched alkanes and aromatic compounds through REFORMING.
– After using these processes, the yield of gasoline is about 47% rather than 19%.
Sources and Uses of Alkanes
•All alkanes burn in air to givecarbon dioxide and water.
Chemical Properties:Combustion of Alkanes
4817 kJ/mol
5471 kJ/mol
6125 kJ/mol
654 kJ/mol
654 kJ/mol
Heptane
Octane
Nonane
Heats of Combustion
What pattern is noticed in this case?
•Increase with increasing number of carbons
• more moles of O2 consumed, more moles
of CO2 and H2O formed
Heats of Combustion
5471 kJ/mol
5466 kJ/mol
5458 kJ/mol
5452 kJ/mol
5 kJ/mol
8 kJ/mol
6 kJ/mol
Heats of Combustion
What pattern is noticed in this case?
8CO2 + 9H2O
5452 kJ/mol5458 kJ/mol
5471 kJ/mol
5466 kJ/molO2+ 25
2
O2+ 25
2 O2+ 25
2 O2+ 25
2
ENERGY Diagrams /
Reaction Coordinate Diagrams
•Increase with increasing number of carbons
• more moles of O2 consumed, more moles
of CO2 and H2O formed
•Decrease with chain branching
• branched molecules are more stable(have less potential energy) than theirunbranched isomers
Heat of CombustionPatterns
•Isomers can differ in respect to their stability.
•Equivalent statement:
–Isomers differ in respect to their potential energy.
Important Point
Differences in potential energy can be measured by comparing heats of combustion. (Worksheet problems)