2
Organic
Chemistry
William H. Brown &
Christopher S. Foote
2-1
2
Alkanes
and
Cycloalkanes
Chapter Two
2-2
2 Structure
 Hydrocarbon:
a compound composed only of
carbon and hydrogen
 Saturated hydrocarbon: a hydrocarbon
containing only single bonds
 Alkane: a saturated hydrocarbon whose carbons
are arranged in a open chain
 Aliphatic hydrocarbon: another name for an
alkane
2-3
2 Structure
 Shape
• tetrahedral about carbon
• all bond angles are approximately 109.5°
2-4
2 Nomenclature
 Alkanes
have the general formula CnH2n+2
Name
methane
ethane
propane
Molecular
Formula
Condensed
Structural Formula
CH4
C2 H6
C3 H8
C4 H10
CH4
CH3 CH3
CH3 CH2 CH3
CH3 (CH 2 ) 2 CH3
C5 H12
CH3 (CH 2 ) 3 CH3
hexane
C6 H14
CH3 (CH 2 ) 4 CH3
heptane
C7 H16
CH3 (CH 2 ) 5 CH3
butane
pentane
2-5
2 Nomenclature
 Alkanes
(contd.)
octane
nonane
C8 H18
C9 H20
C10 H22
CH3 (CH 2 ) 6 CH3
tetradecane
C12 H26
C14 H30
hexadecane
octadecane
eicosane
C16 H34
C18 H38
C20 H42
CH3 (CH 2 ) 10 CH3
CH3 (CH 2 ) 12 CH3
CH3 (CH 2 ) 14 CH3
CH3 (CH 2 ) 16 CH3
decane
dodecane
CH3 (CH 2 ) 7 CH3
CH3 (CH 2 ) 8 CH3
CH3 (CH 2 ) 18 CH3
2-6
2 Constitutional Isomerism
 Constitutional
isomers: compounds with the
same molecular formula but a different
connectivity (order of attachment of their atoms)
• example: C4H10
CH3
CH3 CH2 CH2 CH3
Butane
CH3 CHCH3
2-Methylpropane
2-7
2 Constitutional Isomerism
Molecular
Formula
CH4
Constitutional
Isomers
C5 H1 2
1
3
C1 0 H2 2
75
C1 5 H3 2
4,347
C2 5 H5 2
36,797,588
C3 0 H6 2
4,111,846,763
World population
is about
6,000,000,000
2-8
2 Nomenclature - IUPAC
 Suffix
-ane specifies an alkane
 Prefix tells the number of carbon atoms
Prefix Carbons
meth1
eth2
prop3
but4
pent5
hex6
7
heptoct8
non9
dec10
Carbons
Prefix
undec11
dodec12
tridec13
tetradec14
pentadec- 15
hexadec16
heptadec- 17
octadec18
nonadec19
eicos20
2-9
2 Nomenclature - IUPAC
 Parent
name: the longest carbon chain
 Substituent: a group attached to the parent chain
• alkyl group: a substituent derived by removal of a
hydrogen from an alkane; given the symbol RAlkane
Name
Alkyl group Name
CH4
Methane
CH3 -
Methyl group
CH3 CH3
Ethane
CH3 CH2 -
Ethyl group
2-10
2 Nomenclature
1.The name of a saturated hydrocarbon with an
unbranched chain consists of a prefix and suffix
2. For branched alkanes, the parent chain is the longest
chain of carbon atoms
3. Each substituent is given a name and a number
CH 3
CH 3 CHCH 3
2-Methylpropane
2
1
3
4. If there is one substituent, number the chain from the
end that gives it the lower number
CH3
CH3 CH2 CH2 CHCH3
2-Methylpentane
5
4 3
2
1
2-11
2 Nomenclature
6. If there are two or more different substituents,
• list them in alphabetical order
• number from the end of the chain that gives the
substituent encountered first the lower number
CH3
CH3 CH2 CHCH2 CHCH2 CH3
2
1
3
4
5
6
7
CH2 CH3
3-Ethyl-5-methylheptane
2-12
2 Nomenclature
7. The prefixes di-, tri-, tetra-, etc. are not
included in alphabetization
CH3 CH2 CH3
CH3 CCH2 CH CH2 CH3
CH3
1
2 3
4
5
6
4-Ethyl-2,2-dimethylhexane
8. Substituents are named by the same set of
rules.
2-13
2 Nomenclature
 Alkyl
groups
Name
Condensed
Structural Formula
Name
butyl
- CH 2 CH2 CH2 CH 3
methyl
Condensed
Structural Formula
- CH 3
2-methylpropyl
(isobutyl)
- CH 2 CHCH 3
ethyl
- CH 2 CH3
propyl
- CH 2 CH2 CH3
1-methylpropyl
(sec-butyl)
- CH CH 2 CH3
CH3
CH3
1-methylethyl - CH CH 3
(isopropyl)
CH3
CH3
1,1-dimethylethyl - CCH3
(tert-butyl)
CH3
2-14
2 Classification of C & H
 Primary
(1°) C: a carbon bonded to one other
carbon
• 1° H: a hydrogen bonded to a 1° carbon
 Secondary
(2°) C: a carbon bonded to two other
carbons
• 2° H: a hydrogen bonded to a 2° carbon
 Tertiary
(3°) C: a carbon bonded to three other
carbons
• 3° H: a hydrogen bonded to a 3° carbon
 Quaternary
(4°) C: a carbon bonded to four other
carbons
2-15
2 Cycloalkanes
 General
formula CnH2n
• five- and six-membered rings are the most common
 Structure
and nomenclature
• to name, prefix the name of the corresponding openchain alkane with cyclo-, and name each substituent
on the ring
• if only one substituent, no need to give it a number
• if two substituents, number from the substituent of
lower alphabetical order
• if three or more substituents, number to give them the
lowest set of numbers and then list substituents in
alphabetical order
2-16
2 Cycloalkanes
 Line-angle
drawings
• each line represents a C-C bond
• each angle represents a C
C C
C C
C
C C
C
H2 C
H2 C
CH2
CH3
CH CH
CH2
CH3
C8 H1 6
2-17
2 Cycloalkanes
Example: name these cycloalkanes
(a)
(b)
(c)
(d)
2-18
2 Bicycloalkanes
 Bicycloalkane:
an alkane that contains two rings
that share two carbons
Bicyclo[4.4.0]decane
(Decalin)
Bicyclo[4.3.0]nonane
(Hydrindane)
Bicyclo[2.2.1]heptane
(Norbornane)
2-19
2 Bicycloalkanes
 Nomenclature
• parent is the alkane of the same number of carbons as
are in the rings
• number from a bridgehead, along longest bridge back
to the bridgehead, then along the next longest bridge,
etc.
• show the lengths of bridges in brackets, from longest
to shortest
bicyclo[2.2.1]heptane
2-20
2 IUPAC - General
 prefix-infix-suffix
• prefix tells the number of carbon atoms in the parent
• infix tells the nature of the carbon-carbon bonds
• suffix tells the class of compound
Infix
Nature of Carbon-Carbon
Bonds in the Parent Chain
Suffix
Class
-e
hydrocarbon
-an-en-
all single bonds
-ol
one or more double bonds
-yn-
one or more triple bonds
-al
alcohol
aldehyde
-amine
-one
-oic acid
amine
ketone
carboxylic acid
2-21
2 IUPAC - General
CH3 CH=CH 2
prop-en-e = propene
OH
eth-an-ol = ethanol
HC CH
but-an-one = butanone
but-an-al = butanal
pent-an-oic acid = pentanoic acid CH3 CH2 NH 2
cyclohex-an-ol = cyclohexanol
O
eth-yn-e = ethyne
CH3 CH2 CH2 CH
eth-an-amine = ethanamine
O
O
CH3 CCH2 CH 3
CH3 CH2 OH
CH3 CH2 CH2 CH2 COH
2-22
2 Conformations
 Conformation:
any three-dimensional
arrangement of atoms in a molecule that results
from rotation about a single bond
 Newman projection: a way to view a molecule by
looking along a carbon-carbon bond
2-23
2 Conformations
 Staggered
conformation: a conformation about a
carbon-carbon single bond in which the atoms
on one carbon are as far apart as possible from
the atoms on an adjacent carbon
H
H
H
H
H
H
2-24
2 Conformations
 Eclipsed
conformation: a conformation about a
carbon-carbon single bond in which the atoms
on one carbon are as close as possible to the
atoms on an adjacent carbon
H
H
H
H
HH
2-25
2 Conformations
 Torsional
strain: the force that opposes the
rotation of one part of a molecule about a bond
while the other part of the molecule is held fixed
• the torsional strain between eclipsed and staggered
ethane is approximately 12.6 kJ (3.0 kcal)/mol
+12.6 kJ/mol
2-26
2 Conformations
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2-27
2 Conformations
 anti
conformation
• a conformation about a single bond in which the
groups lie at a dihedral angle of 180°
CH 3
H
H
H
H
CH 3
2-28
2 Conformations
 Nonbonded
interaction strain:
• the strain that arises when atoms not bonded to each
other are forced abnormally close to one another
• butane - gauche conformation; nonbonded interaction
strain is approx. 3.8 kJ (0.9 kcal)/mol
CH3
H
CH3
H
H
H
2-29
2 Conformations
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2-30
2 Cyclopropane
• angle strain: the C-C-C bond angles are
compressed from 109.5° to 60°
• torsional strain: there are 6 sets of eclipsed
hydrogen interactions
• strain energy is about 116 kJ (27.7 kcal)/mol
H
H
H
H
H
H
2-31
2 Cyclobutane
• puckering from planar cyclobutane reduces torsional
strain but increases angle strain
• the conformation of minimum energy is a puckered
“butterfly” conformation
• strain energy is about 110 kJ (26.3 kcal)/mol
2-32
2 Cyclopentane
• puckering from planar cyclopentane reduces torsional
strain, but increases angle stain
• the conformation of minimum energy is a puckered
“envelope” conformation
• strain energy is about 42 kJ (6.5 kcal)/mol
2-33
2 Cyclohexane
 Chair
conformation: the most stable puckered
conformation of a cyclohexane ring
• all bond angles are approx. 109.5°
• all bonds on adjacent carbons are staggered
2-34
2 Cyclohexane
 In
a chair conformation, six H are equatorial and
six are axial
2-35
2 Cyclohexane
 For
cyclohexane, there are two equivalent chair
conformations
• all C-H bonds equatorial in one chair are axial in the
alternative chair, and vice versa
2-36
2 Cyclohexane
 Boat
conformation: a puckered conformation of a
cyclohexane ring in which carbons 1 and 4 are
bent toward each other
• less stable than a chair conformation by 27 kJ (6.5
kcal)/mol
2-37
2 Cyclohexane
 Twist-boat
conformation
• approx. 41.8 kJ (5.5 kcal)/mol less stable than a chair
conformation
• approx. 6.3 kJ (1.5 kcal)/mol more stable than a boat
conformation
2-38
2 Cyclohexane
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2-39
2 Methylcyclohexane
 Equatorial
and axial methyl conformations
CH3
+7.28 kJ/mol
CH3
2-40
2 G° axial ---> equatorial
axial --> equatorial
 G°
Group (kJ/mol)
Group
 G°
(kJ/mol)
5.9
5.9
7.1
7.28
C N
F
0.8
1.0
C CH
I
1.7
1.9
Cl
2.2
CH2 CH3
Br
2.4
3.9
CH( CH 3 ) 2 9.0
C( CH3 ) 3
21.0
OH
N H2
COOH
CH= CH2
CH3
7.3
2-41
2 Cis,trans Isomerism
 Cis,trans
isomers have:
• the same molecular formula
• the same connectivity
• a different arrangement of their atoms in space due to
the presence of either a ring or a carbon-carbon
double bond (see Chapter 5)
2-42
2 Cis,trans Isomerism
 1,2-dimethylcyclopentane
H
H
H
H
H H
H
H
H
H H
H
CH3
H
CH3
cis-1,2-Dimethylcyclopentane
CH3
H
H
CH3
H
trans-1,2-Dimethylcyclopentane
2-43
2 Cis,trans Isomerism
 1,4-dimethylcyclohexane
• planar hexagon representations
H
H3 C
CH3
H
trans-1,4-Dimethylcyclohexane
H
H3 C
H
CH3
cis-1,4-Dimethylcyclohexane
2-44
2 Cis,trans Isomerism
 trans-1,4-dimethylcyclohexane
• the diequatorial-methyl chair conformation is more
stable by approximately 2 x (7.28) = 14.56 kJ/mol
CH3
H
H
H3 C
H
CH3
CH3
(less stable)
H
(more stable)
2-45
2 Cis,trans Isomerism
 cis-1,4-dimethylcyclohexane
H
H
CH3
H3 C
H
H
CH3
conformations are of equal stability
CH3
2-46
2 Cis,trans Isomerism
H
H
H
H
trans-decalin
cis-decalin
2-47
2 Physical Properties
 Intermolecular
•
•
•
•
forces of attraction
ion-ion (Na+ and Cl- in NaCl)
ion-dipole (Na+ and Cl- solvated in aqueous solution)
dipole-dipole and hydrogen bonding
dispersion forces (very weak electrostatic attraction
between temporary dipoles)
2-48
2 Physical Properties
 Low-molecular-weight
alkanes
(methane....butane) are gases at room
temperature
 Higher molecular-weight alkanes (pentane,
decane, gasoline, kerosene) are liquids at room
temperature
 High-molecular-weight alkanes (paraffin wax) are
semisolids or solids at room temperature
2-49
2 Physical Properties
 Constitutional
isomers have different physical
properties
mp
(°C)
bp
(°C)
Density
(g/mL)
-95
68.7
0.659
-154
60.3
0.653
2,3-dimethylbutane
-118
-129
63.3
58.0
0.664
0.661
2,2-dimethylbutane
-98
49.7
0.649
Name
hexane
2-methylpentane
3-methylpentane
2-50
2 Oxidation of Alkanes
 Oxidation
is the basis for their use as energy
sources for heat and power
• heat of combustion: heat released when one mole of a
substance in its standard state is oxidized to carbon
dioxide and water
H°
kJ (kcal)/mol
CH4 + 2 O 2
CO 2 + 2 H2 O
-890 (-212)
3 CO 2 + 4 H2 O
-2220 (-530)
Methane
CH3 CH2 CH3 + 5 O 2
Propane
2-51
2 Heat of Combustion
 Heat
of combustion for constitutional isomers
Hydrocarbon
Structural formula
H0
[kJ (kcal)/mol]
Octane
-5470.6 (-1307.5)
2-Methylheptane
-5465.6 (-1306.3)
2,2-Dimethylhexane
-5458.4 (1304.6)
2,2,3,3-Tetramethylbutane
-5451.8 (1303.0)
2-52
2 Heats of Combustion
 For
constitutional isomers [kJ (kcal)/mol]
-5470.6 (-1307.5)
-5465.6 (-1306.3)
-5458.4 (1304.6) -5451.8 (1303.0)
8 CO 2 + 9 H2 O
2-53
2 Heat of Combustion
• ring strain as determined by heats of combustion
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2-54
2 Sources of Alkanes
 Natural
gas
• 90-95% methane
 Petroleum
•
•
•
•
•
•
gases (bp below 20°C)
naphthas, including gasoline (bp 20 - 200°C)
kerosene (bp 175 - 275°C)
fuel oil (bp 250 - 400°C)
lubricating oils (bp above 350°C)
asphalt (residue after distillation)
 Coal
2-55
2 Gasoline
 Octane
rating: the percent 2,2,4-trimethylpentane
(isooctane) in a mixture of isooctane and heptane that
has equivalent antiknock properties
CH 3 CH 3
CH 3 ( CH 2 ) 5 CH 3
CH 3 CCH2 CH CH 3
CH 3
Heptane
(octane rating 0)
2,2,4-Trimethylpentane
(octane rating 100)
2-56
2 Synthesis Gas
A
mixture of carbon monoxide and hydrogen in
varying proportions which depend on the means
by which it is produced
C +
Coal
CH4 +
Methane
H2 O
1
2
O2
heat
catalyst
CO + H2
CO + 2 H2
2-57
2 Synthesis Gas
 Synthesis
gas is a feedstock for the industrial
production of methanol and acetic acid
CO + 2 H 2
catalyst
CH 3 OH
Methanol
CH 3 OH +
Methanol
CO
catalyst
O
CH 3 COH
Acetic acid
• it is likely that industrial routes to other organic
chemicals from coal via methanol will also be
developed
2-58
2 Prob 2.19
Which sets represent pairs of constitutional isomers?
(a) CH3 CH2 OH and CH3 OCH3
O
O
(b) CH3 CCH3 and CH3 CH2 CH
O
O
(c) CH3 COCH 3 and CH3 CH2 COH
OH
O
(d) CH3 CHCH2 CH3 and CH3 CCH2 CH 3
(e)
(f)
and CH3 CH2 CH2 CH2 CH3
and CH2 = CHCH2 CH2 CH3
2-59
2 Prob 2.22
Write the IUPAC name of each compound.
(a)
(d)
(b)
(c)
(e)
2-60
2 Prob 2.24
Explain why each is an incorrect IUPAC name, and write
the correct IUPAC name.
(a) 1,3-dimethylbutane
(b) 4-methylpentane
(c) 2,2-dimethylbutane
(d) 2-ethyl-3-methylpentane
(e) 2-propylpentane
(f) 2,2-diethylheptane
(g) 2,2-dimethylcyclopropane
(h) 1-ethyl-5-methylcyclohexane
2-61
2 Prob 2.26
Write the IUPAC name of each compound.
O
(a) CH3 CH2 CCH 3
O
(b) CH3 CH2 CH
O
(c) CH3 CH 2 CH2 CH2 CH 2 COH
OH
(d) CH3 CHCH3
(e)
(f)
O
(g) CH3 CH= CH 2
OH
(h)
2-62
2 Prob 2.40
Complete the table for cis,trans and equatorial,axial
substituted cyclohexanes.
Position of
Substitution
1,41,31,2-
cis
a,e or e,a
____ or ____
trans
e,e or a,a
____ or ____
____ or ____
____ or ____
2-63
2 Prob 2.43
Draw alternative chair conformations for each.
HO
OH
(a)
H
OH
(b)
OH
H
H
H
CH2 OH
OH
H
(c)
HO
CH3
H
H
OH
(d)
H
HO
OH
H
H
2-64
2 Prob 2.44
Draw the more stable chair conformation of glucose.
OH
HO
O
HO
OH
OH
2-65
2 Prob 2.51
Given these heats of combustion, which constitutional
isomer is the more stable?
O
O
CH3 -CH
H2 C CH2
Acetaldehyde
-1164 kJ (-278.8 kcal)/mol
Ethylene oxide
-1264 kJ (-302.1 kcal)/mol
2-66
2
Alkanes
and
Cycloalkanes
End Chapter 2
2-67