Organic Chemistry, 6th Edition
L. G. Wade, Jr.
Chapter 3
Structure and
Stereochemistry
of Alkanes
2006, Prentice Hall
Classification Review
Chapter 3
2
Alkane Formulas
•
•
•
•
•
All C-C single bonds
Saturated with hydrogens
Ratio: CnH2n+2
Alkane homologs: CH3(CH2)nCH3
Same ratio for branched alkanes
H
H H H H
H C C C C H
H C H
H
H
H H H H
H C C C H
Butane, C 4H10
H H H
Chapter 3
Isobutane, C 4H10
3
Common Names
• Isobutane, “isomer of butane”
• Isopentane, isohexane, etc., methyl
branch on next-to-last carbon in chain.
• Neopentane, most highly branched
• Five possible isomers of hexane,
18 isomers of octane and 75 for
decane!
Chapter 3
4
Alkane Examples
Chapter 3
5
IUPAC Names
• Find the longest continuous carbon
chain.
• Number the carbons, starting closest to
the first branch.
• Name the groups attached to the chain,
using the carbon number as the locator.
• Alphabetize substituents.
• Use di-, tri-, etc., for multiples of same
substituent.
Chapter 3
6
Longest Chain
• The number of carbons in the longest
chain determines the base name:
ethane, hexane. (Listed in Table 3.2,
page 82.)
• If there are two possible chains with the
same number of carbons, use the chain
with the most substituents.
H3C
CH CH2
CH3
CH3
H3C CH2
C
CH
CH2
CH2
CH3
CH3
Chapter 3
7
Number the Carbons
• Start at the end closest to the first
attached group.
• If two substituents are equidistant, look
for the next closest group.
1
CH3
3
4
H3C CH CH CH2
2
CH2CH3
Chapter 3
5
CH2
CH3
CH CH3
6
7
8
Name Alkyl Groups
•
•
•
•
CH3-, methyl
CH3CH2-, ethyl
CH3CH2CH2-, n-propyl
CH3CH2CH2CH2-, n-butyl
CH3 CH
CH3
isopropyl
CH3
CH CH2
CH3
CH3
isobutyl
CH3
CH3
sec-butyl
Chapter 3
CH CH2
H3C C CH3
tert-butyl
9
Propyl Groups
H H H
H C C C
H H H
H
H C C C H
H H H
H H H
n-propyl
isopropyl
A primary carbon
A secondary carbon
Chapter 3
10
Butyl Groups
H H H H
H C C C C
H H H H
H
H C C C C H
H H H H
H
n-butyl
A primary carbon
H
H H
sec-butyl
A secondary carbon
Chapter 3
11
Isobutyl Groups
H
H
C H
H
H
C H
H
H
H C
C
C
H
H
H
H C
C
C H
H H H
H H H
isobutyl
tert-butyl
A primary carbon
A tertiary carbon
Chapter 3
12
Alphabetize
• Alphabetize substituents by name.
• Ignore di-, tri-, etc. for alphabetizing.
CH3
CH3
H3C CH CH CH2
CH2
CH CH3
CH2CH3
3-ethyl-2,6-dimethylheptane
Chapter 3
13
Complex Substituents
• If the branch has a branch, number the
carbons from the point of attachment.
• Name the branch off the branch using a
locator number.
• Parentheses are used around the
complex branch name.
1
2
3
1-methyl-3-(1,2-dimethylpropyl)cyclohexane
Chapter 3
14
Physical Properties
• Solubility: hydrophobic
• Density: less than 1 g/mL
• Boiling points increase with
increasing carbons (little less for
branched chains).
• Melting points increase with
increasing carbons (less for oddnumber of carbons).
Chapter 3
15
Boiling Points of Alkanes
Branched alkanes have less surface area contact,
so weaker intermolecular forces.
Chapter 3
16
Melting Points of Alkanes
Branched alkanes pack more efficiently into
a crystalline structure, so have higher m.p.
Chapter 3
17
Branched Alkanes
• Lower b.p. with increased branching
• Higher m.p. with increased branching
• Examples:
CH3
CH3
CH3
CH CH2 CH2 CH3
bp 60°C
mp -154°C
CH3
CH3
CH
CH
CH3
CH3
bp 58°C
mp -135°C
Chapter 3
CH3 C CH2 CH3
CH3
bp 50°C
mp -98°C
18
Major Uses of Alkanes
•
•
•
•
•
•
C1-C2: gases (natural gas)
C3-C4: liquified petroleum (LPG)
C5-C8: gasoline
C9-C16: diesel, kerosene, jet fuel
C17-up: lubricating oils, heating oil
Origin: petroleum refining
Chapter 3
19
Reactions of Alkanes
• Combustion
2 CH3CH2CH2CH3
heat
+ 13 O2
8 CO2
+ 10 H2O
• Cracking and hydrocracking (industrial)
long-chain alkanes
catalyst
shorter-chain alkanes
• Halogenation
CH4 + Cl2
heat or light
CH3Cl + CH2Cl2 + CHCl3 + CCl4
Chapter 3
20
Conformers of Alkanes
• Structures resulting from the free
rotation of a C-C single bond
• May differ in energy. The lowest-energy
conformer is most prevalent.
• Molecules constantly rotate through all
the possible conformations.
Chapter 3
21
Ethane Conformers
• Staggered conformer has lowest energy.
• Dihedral angle = 60 degrees
H
H
H
H
H
H
model
Newman
projection
Chapter 3
sawhorse
22
Ethane Conformers (2)
• Eclipsed conformer has highest energy
• Dihedral angle = 0 degrees
Chapter 3
23
Conformational Analysis
• Torsional strain: resistance to rotation.
• For ethane, only 12.6 kJ/mol
Chapter 3
24
Propane Conformers
Note slight increase in torsional strain
due to the more bulky methyl group.
Chapter 3
25
Butane Conformers C2-C3
• Highest energy has methyl groups eclipsed.
• Steric hindrance
• Dihedral angle = 0 degrees
totally eclipsed
Chapter 3
26
Butane Conformers (2)
• Lowest energy has methyl groups anti.
• Dihedral angle = 180 degrees
anti
Chapter 3
27
Butane Conformers (3)
• Methyl groups eclipsed with hydrogens
• Higher energy than staggered
conformer
• Dihedral angle = 120 degrees
eclipsed
Chapter 3
28
Butane Conformers (4)
• Gauche, staggered conformer
• Methyls closer than in anti conformer
• Dihedral angle = 60 degrees
gauche
Chapter 3
29
Conformational Analysis
Chapter 3
30
Higher Alkanes
• Anti conformation is lowest in energy.
• “Straight chain” actually is zigzag.
CH3CH2CH2CH2CH3
H H H H H
C
C
C
C
C
H
H
H H H H
H
Chapter 3
31
Cycloalkanes
•
•
•
•
•
Rings of carbon atoms (-CH2- groups)
Formula: CnH2n
Nonpolar, insoluble in water
Compact shape
Melting and boiling points similar to
branched alkanes with same number of
carbons
Chapter 3
32
Naming Cycloalkanes
•
•
•
•
Cycloalkane usually base compound
Number carbons in ring if >1 substituent.
First in alphabet gets lowest number.
May be cycloalkyl attachment to chain.
CH2CH3
CH2CH3
CH3
Chapter 3
33
Cis-Trans Isomerism
• Cis: like groups on same side of ring
• Trans: like groups on opposite sides of ring
Chapter 3
34
Cycloalkane Stability
•
•
•
•
5- and 6-membered rings most stable
Bond angle closest to 109.5
Angle (Baeyer) strain
Measured by heats of combustion
per -CH2 -
Chapter 3
35
Heats of Combustion/CH2
Alkane + O2  CO2 + H2O
697.1 686.1
658.6 kJ
Long-chain
664.0
Chapter 3
663.6 kJ/mol
662.4
658.6
36
Cyclopropane
• Large ring strain due to angle compression
• Very reactive, weak bonds
Chapter 3
37
Cyclopropane (2)
Torsional strain because of eclipsed
hydrogens
Chapter 3
38
Cyclobutane
• Angle strain due to compression
• Torsional strain partially relieved by ringpuckering
Chapter 3
39
Cyclopentane
• If planar, angles would be 108, but all
hydrogens would be eclipsed.
• Puckered conformer reduces torsional strain.
Chapter 3
40
Cyclohexane
• Combustion data shows it’s unstrained.
• Angles would be 120, if planar.
• The chair conformer has 109.5 bond
angles and all hydrogens are staggered.
• No angle strain and no torsional strain.
Chapter 3
41
Chair Conformer
Chapter 3
42
Boat Conformer
Chapter 3
43
Conformational Energy
Chapter 3
44
Axial and Equatorial
Positions
Chapter 3
45
Monosubstituted Cyclohexanes
Chapter 3
46
1,3-Diaxial Interactions
Chapter 3
47
Disubstituted Cyclohexanes
Chapter 3
48
Cis-Trans Isomers
Bonds that are cis, alternate axialequatorial around the ring.
CH3
CH3
One axial, one equatorial
Chapter 3
49
Bulky Groups
• Groups like t-butyl cause a large energy
difference between the axial and equatorial
conformer.
• Most stable conformer puts t-butyl
equatorial regardless of other substituents.
Chapter 3
50
Bicyclic Alkanes
• Fused rings share two adjacent carbons.
• Bridged rings share two nonadjacent C’s.
bicyclo[2.2.1]heptane
bicyclo[2.2.1]heptane
bicyclo[3.1.0]hexane
bicyclo[3.1.0]hexane
Chapter 3
51
Cis- and Trans-Decalin
• Fused cyclohexane chair conformers
• Bridgehead H’s cis, structure more flexible
• Bridgehead H’s trans, no ring flip possible.
H
H
H
H
cis-decalin
trans-decalin
Chapter 3
52
Bicyclo[4.4.0]decane
Chapter 3
53
End of Chapter 3
Chapter 3
54