Chapter 3
Structure and Stereochemistry
of Alkanes
Organic Chemistry,5th Edition
L,G,Wade,Jr.
Jo Blackburn
Richland College,Dallas,TX
Dallas County Community College District
2003,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
B u t a n e,C 4 H 10 =>
CH
H
C C
H
H
H
C H
H
H
H
H
I s o b u t a n e,C4 H 10
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 - p e n t a n e,C5 H 12
CH
H
C C
HH
C H
H
H
H
H
C
H
H
H
i s o p e n t a n e,C5 H 12
=>
C
C H 3
C H 3H 3 C
C H 3
n e o p e n t a n e,C5 H 12
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
C H 3
C H 2
C H 3
C H C H 2 C H 2 C H 3
C H C H 2 C H 3
H 3 C
H 3 C
=>
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 7
C HH 3 C
C H 3
C H
C H 2 C H 3
C H 2 C H 2 C H
C H 3
C H 3
=>
Chapter 3 8
Name Alkyl Groups
CH3-,methyl
CH3CH2-,ethyl
CH3CH2CH2-,n-propyl
CH3CH2CH2CH2-,n-butyl
C H 3 C H C H 2 C H 3
s e c - b u ty l
C H 3 C H
C H 3
C H 2
is o b u t y l
C H 3 C H C H 3
is o p r o p y l
CH 3 C
C H 3
C H 3
t e r t- b u t y l
=>
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
C
H H
C
H
H
H H
CH
H
H
C
C
H H
C H
H
H
H
H
H
A primary carbon A tertiary carbon
=>
isobutyl tert-butyl
Chapter 3 12
Alphabetize
Alphabetize substituents by name.
Ignore di-,tri-,etc,for alphabetizing.
C HH 3 C
C H 3
C H
C H 2 C H 3
C H 2 C H 2 C H
C H 3
C H 3
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 odd-
number 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:
H
CH3 CH
CH3
CH2 CH2 CH3
bp 60° C
mp -154° C
CH3 CH
CH3
CH CH3CH
3
bp 58° C
mp -135° C
=>
bp 50° C
mp -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
Combustion
C H 3 C H 2 C H 2 C H 3 + O 2 C O 2 + H 2 Oheat 8 10132
l o n g - c h a i n a l k a n e s c a t a l y s t s h o rt e r- c h a i n a l k a n e s
C H 4 + C l 2 C H 3 C l + C H 2 C l 2 C H C l 3 C C l 4+ +h e a t o r l i g h t
=>
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
H
H
H H
Newman
projection
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 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.
C H 3 C H 2 C H 2 C H 2 C H 3
C
H C
C
C
C
H H H H
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.
C H 2 C H 3
C H 2 C H 3
C H 3 =>
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
166.6 164.0
158.7 158.6
=>
158.3
Chapter 3 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
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 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 axial-
equatorial around the ring.
C H 3
C H 3
=>
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.
b i c y c l o [ 3,1,0 ] h e x a n ebicyclo[3.1.0]hexane b i c y c l o [ 2,2,1 ] h e p t a n e
=>
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
Chapter 3 52
End of Chapter 3
Structure and Stereochemistry
of Alkanes
Organic Chemistry,5th Edition
L,G,Wade,Jr.
Jo Blackburn
Richland College,Dallas,TX
Dallas County Community College District
2003,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
B u t a n e,C 4 H 10 =>
CH
H
C C
H
H
H
C H
H
H
H
H
I s o b u t a n e,C4 H 10
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 - p e n t a n e,C5 H 12
CH
H
C C
HH
C H
H
H
H
H
C
H
H
H
i s o p e n t a n e,C5 H 12
=>
C
C H 3
C H 3H 3 C
C H 3
n e o p e n t a n e,C5 H 12
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
C H 3
C H 2
C H 3
C H C H 2 C H 2 C H 3
C H C H 2 C H 3
H 3 C
H 3 C
=>
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 7
C HH 3 C
C H 3
C H
C H 2 C H 3
C H 2 C H 2 C H
C H 3
C H 3
=>
Chapter 3 8
Name Alkyl Groups
CH3-,methyl
CH3CH2-,ethyl
CH3CH2CH2-,n-propyl
CH3CH2CH2CH2-,n-butyl
C H 3 C H C H 2 C H 3
s e c - b u ty l
C H 3 C H
C H 3
C H 2
is o b u t y l
C H 3 C H C H 3
is o p r o p y l
CH 3 C
C H 3
C H 3
t e r t- b u t y l
=>
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
C
H H
C
H
H
H H
CH
H
H
C
C
H H
C H
H
H
H
H
H
A primary carbon A tertiary carbon
=>
isobutyl tert-butyl
Chapter 3 12
Alphabetize
Alphabetize substituents by name.
Ignore di-,tri-,etc,for alphabetizing.
C HH 3 C
C H 3
C H
C H 2 C H 3
C H 2 C H 2 C H
C H 3
C H 3
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 odd-
number 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:
H
CH3 CH
CH3
CH2 CH2 CH3
bp 60° C
mp -154° C
CH3 CH
CH3
CH CH3CH
3
bp 58° C
mp -135° C
=>
bp 50° C
mp -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
Combustion
C H 3 C H 2 C H 2 C H 3 + O 2 C O 2 + H 2 Oheat 8 10132
l o n g - c h a i n a l k a n e s c a t a l y s t s h o rt e r- c h a i n a l k a n e s
C H 4 + C l 2 C H 3 C l + C H 2 C l 2 C H C l 3 C C l 4+ +h e a t o r l i g h t
=>
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
H
H
H H
Newman
projection
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 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.
C H 3 C H 2 C H 2 C H 2 C H 3
C
H C
C
C
C
H H H H
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.
C H 2 C H 3
C H 2 C H 3
C H 3 =>
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
166.6 164.0
158.7 158.6
=>
158.3
Chapter 3 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
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 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 axial-
equatorial around the ring.
C H 3
C H 3
=>
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.
b i c y c l o [ 3,1,0 ] h e x a n ebicyclo[3.1.0]hexane b i c y c l o [ 2,2,1 ] h e p t a n e
=>
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
Chapter 3 52
End of Chapter 3
