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Chapter 3Structure and Stereochemistry
of Alkanes
Organic Chemistry, 5th EditionL. G. Wade, Jr.
Jo BlackburnRichland College, Dallas, TX
Dallas County Community College District2003,Prentice Hall
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
CH
H
C
H
H
C
H
H
C
H
H
HH
Butane, C4H10=>
CHH
C CH
HH
C H
H
H
HH
Isobutane, C4H10
Chapter 3 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
Pentanes
CH
H
C
H
H
C
H
H
C
H
C
HH
H
H
H
n-pentane, C5H12
CHH
C CHH
C H
H
H
HH
CH
HH
isopentane, C5H12
=>
C
CH3
CH3H3C
CH3
neopentane, C5H12
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 81.)
• If there are two possible chains with the same number of carbons, use the chain with the most substituents.
C
CH3
CH2
CH3
CH CH2 CH2 CH3
CH CH2 CH3H3C
H3C
=>
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
2
3 4 5
6 7CHH3C
CH3
CH
CH2CH3
CH2 CH2 CH
CH3
CH3
=>
Chapter 3 8
Name Alkyl Groups
• CH3-, methyl
• CH3CH2-, ethyl
• CH3CH2CH2-, n-propyl
• CH3CH2CH2CH2-, n-butylCH3 CH CH2 CH3
sec-butyl
CH3 CH
CH3
CH2
isobutyl
CH3 CH CH3
isopropyl
CH3C
CH3
CH3
tert-butyl
=>
Chapter 3 9
Propyl Groups
C
H
H
H
C
H
H
C
H
H
H
n-propyl
C
H
H
H
C
H
C
H
H
H
isopropyl
H
A primary carbon A secondary carbon
=>
Chapter 3 10
Butyl Groups
C
H
H
H
C
H
C
H
H
C
H
H
H
C
H
H
H
C
H
C
H
HH
C
H
H
n-butyl sec-butyl
H
H
A primary carbon A secondary carbon
=>
Chapter 3 11
Isobutyl Groups
CH
H
H
C
CH H
C
HH
H H
CH
H
H
C
CH H
C H
HH
HH
H
A primary carbon A tertiary carbon
=>
isobutyl tert-butyl
Chapter 3 12
Alphabetize
• Alphabetize substituents by name.• Ignore di-, tri-, etc. for alphabetizing.
CHH3C
CH3
CH
CH2CH3
CH2 CH2 CH
CH3
CH3
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
31-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 odd- number of carbons).
•
Chapter 3 15
Boiling Points of AlkanesBranched alkanes have less surface area contact,so weaker intermolecular forces.
=>
Chapter 3 16
Melting Points of AlkanesBranched alkanes pack more efficiently intoa 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:
H
CH3CH
CH3
CH2 CH2 CH3
bp 60°Cmp -154°C
CH3CH
CH3
CHCH3
CH3 bp 58°Cmp -135°C
=>
bp 50°Cmp -98°C
CH3 C
C 3
CH3
CH2 CH3
Chapter 3 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
• CombustionCH3CH2CH2CH3 + O2 CO2 + H2O
heat8 10132
long-chain alkanes catalystshorter-chain alkanes
CH4 + Cl2 CH3Cl + CH2Cl2 CHCl3 CCl4+ +heat or light
=>
• Cracking and hydrocracking (industrial)
• Halogenation
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
HH
H H
Newmanprojection
sawhorse
=>
model
Chapter 3 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 3.0 kcal/mol
=>
Chapter 3 24
Propane ConformersNote slight increase in torsional straindue 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 30
Higher Alkanes• Anti conformation is lowest in energy.• “Straight chain” actually is zigzag.
CH3CH2CH2CH2CH3
CH C
CC
CH H H H
H H
H H
HH 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 Alkane + O2 CO2 + H2O
Long-chain
157.4 157.4
166.6 164.0158.7 158.6
=>
158.3
Chapter 3 38
Cyclobutane• Angle strain due to compression• Torsional strain partially relieved by
ring-puckering
=>
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 48
Cis-Trans Isomers
Bonds that are cis, alternate axial-equatorial around the ring.
CH3
CH3
=>
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[3.1.0]hexane
=>
bicyclo[2.2.1]heptane
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
cis-decalin
H
H=>
trans-decalin
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