Organic
Chemistry
William H. Brown
Christopher S. Foote
Brent L. Iverson
2-1
Alkanes
and
Cycloalkanes
Chapter 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 an open chain
 Aliphatic hydrocarbon: another name for an
alkane
2-3
Hydrocarbons
Hydrocarbons
Saturated
Class
Alkanes
(Chapter 2)
Carbon- Only carboncarbon carbon single
bonding
bonds
HH
Example
H-C-C-H
Name
HH
Ethane
Unsaturated
Alkenes
(Chapters 5-6)
Alkynes
(Chapter 7)
Arenes
(Chapter 21-22)
One or more
carbon-carbon
double bonds
One or more
carbon-carbon
triple bonds
One or more
benzenelike
rings
H
H
H-C C-H
C C
H
H
Ethene
Acetylene
Benzene
2-4
Structure
 Shape
• tetrahedral about carbon
• all bond angles are approximately 109.5°
2-5
Drawing Alkanes
 Line-angle
formulas
• an abbreviated way to draw structural formulas
• each vertex and line ending represents a carbon
Ball-andstick
model
Line-angle
formula
Structural
formula
CH3 CH2 CH3
CH3 CH2 CH2 CH3
CH3 CH2 CH2 CH2 CH3
Propane
Butane
Pentane
2-6
Constitutional Isomerism
 Constitutional
isomers: compounds with the
same molecular formula but a different
connectivity of their atoms
• example: C4H10
CH3 CH2 CH2 CH3
Butane
(bp -0.5°C)
CH3
CH3 CHCH3
2-Methylpropane
(bp -11.6°C)
2-7
Constitutional Isomerism
• do these formulas represent constitutional isomers?
CH3
CH3
CH3
CH3 CHCH2 CH and CH3 CH2 CHCHCH3
CH3
CH3
(each is C7 H16)
• find the longest carbon chain
• number each chain from the end nearest the first
branch
• compare chain lengths as well the identity and location
of branches
CH3 5 CH3
4
CH3 CHCH2 CH
1
2 3
CH3
1
2
3
4
and
5
CH3
CH3 CH2 CHCHCH3
2 1
CH3
5
4
3
5
4
3
2
2-8
1
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
2-9
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
2-10
Nomenclature - IUPAC
 Parent
name: the longest carbon chain
 Substituent: a group bonded 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
CH 4
Methane
CH 3 -
Methyl group
CH 3 CH3
Ethane
CH 3 CH 2 -
Ethyl group
2-11
Nomenclature - IUPAC
1.The name of a saturated hydrocarbon with an
unbranched chain consists of a prefix and suffix
2. The parent chain is the longest chain of carbon atoms
3. Each substituent is given a name and a number
CH3
CH3 CHCH3
1
2
3
2-Methylpropane
4. If there is one substituent, number the chain from the
end that gives it the lower number
CH3
CH3 CH2 CH2 CHCH3
5
4
32
1
2-Methylpentane
1
2
34
5
(not 4-methylpentane)
2-12
Nomenclature - IUPAC
5. If there are two or more identical substituents, number
the chain from the end that gives the lower number to
the substituent encountered first
• indicate the number of times the substituent appears
by a prefix di-, tri-, tetra-, etc.
• use commas to separate position numbers
6
5
4
3
2
1
1
2
3
4
5
6
2,4-Dimethylhexane (not 3,5-dimethylhexane)
2-13
Nomenclature - I UPAC
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
1
2
3
4
5
6
7
3-Ethyl-5-methylheptane
7
6
5
4
3
2
1
(not 3-methyl-5-ethylheptane)
2-14
Nomenclature - IUPAC
7. The prefixes di-, tri-, tetra-, etc. are not included in
alphabetization
• alphabetize the names of substituents first and then
insert these prefixes
1
2 3
4
5
6
4-Ethyl-2,2-dimethylhexane
(not 2,2-dimethyl-4-ethylhexane)
2-15
Nomenclature - IUPAC
 Alkyl
groups
Name
Condensed
Structural Formula
butyl
- CH 2 CH2 CH2 CH 3
Name
methyl
Condensed
Structural Formula
- CH 3
2-methylpropyl
(isobutyl)
- CH 2 CHCH3
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
2-16
Nomenclature - Common
 The
number of carbons in the alkane determines
the name
• all alkanes with four carbons are butanes, those with
five carbons are pentanes, etc.
• iso- indicates the chain terminates in -CH(CH3)2; neothat it terminates in -C(CH3)3
CH3 CH2 CH2 CH3
Butane
CH3
CH3 CHCH3
Isobutane
CH3
CH3
CH3 CH2 CH2 CH2 CH3 CH3 CH2 CHCH3 CH3 CCH3
CH3
Pentane
Isopentane
Neopentane
2-17
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
2-18
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
2-19
Cycloalkanes
 Line-angle
drawings
• each line represents a C-C bond
• each vertex and line ending represents a C
C
C
C
C
C C
C
C
H2 C
H2 C
CH2
CH3
CH CH
CH2
CH3
C8 H1 6
2-20
Cycloalkanes

Example: name these cycloalkanes
(a)
(b)
(c)
(d)
2-21
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)
2-22
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
1
6
2
7
5
3
4
Bicyclo[2.2.1]heptane
2-23
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
-an-en-yn-
Nature of Carbon-Carbon
Bonds in the Parent Chain
Suffix
Class
hydrocarbon
all single bonds
-e
-ol
alcohol
-al
aldehyde
-amine
amine
-one
ketone
-oic acid carboxylic acid
one or more double bonds
one or more triple bonds
2-24
IUPAC - General
prop-en-e = propene
CH3 CH=CH 2
eth-an-ol = ethanol
OH
but-an-one = butanone
HC CH
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
2-25
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 single bond
2-26
Conformations
 Staggered
conformation: a conformation about a
carbon-carbon single bond in which the atoms or
groups on one carbon are as far apart as
possible from the atoms or groups on an
adjacent carbon
H
H
H
H
H
H
2-27
Conformations
 Eclipsed
conformation: a conformation about a
carbon-carbon single bond in which the atoms or
groups of atoms on one carbon are as close as
possible to the atoms or groups of atoms on an
adjacent carbon
H
H
H
H
HH
2-28
Conformations
 Torsional
strain
• also called eclipsed interaction strain
• strain that arises when nonbonded atoms separated by
three bonds are forced from a staggered conformation
to an eclipsed conformation
• the torsional strain between eclipsed and staggered
ethane is approximately 12.6 kJ (3.0 kcal)/mol
+12.6 kJ/mol
2-29
Conformations
angle (Q): the angle created by two
intersecting planes
 Dihedral
2-30
Conformations
 Ethane
as a function of dihedral angle
2-31
Conformations
 The
origin of torsional strain in ethane
• originally thought to be caused by repulsion between
eclipsed hydrogen nuclei
• alternatively, caused by repulsion between electron
clouds of eclipsed C-H bonds
• theoretical molecular orbital calculations suggest that
the energy difference is not caused by destabilization
of the eclipsed conformation but rather by stabilization
of the staggered conformation
• this stabilization arises from the small donor-acceptor
interaction between a C-H bonding MO of one carbon
and the C-H antibonding MO on an adjacent carbon;
this stabilization is lost when a staggered
conformation is converted to an eclipsed conformation
2-32
Conformations
 anti
conformation
• a conformation about a single bond in which the
groups lie at a dihedral angle of 180°
CH3
H
H
H
H
CH3
2-33
Conformations
 Steric
strain (nonbonded interaction strain):
• the strain that arises when atoms separated by four or
more bonds are forced closer to each other than their
atomic (contact) radii will allow
 Angle
strain:
• strain that arises when a bond angle is either
compressed or expanded compared to its optimal
value
 The
total of all types of strain can be calculated
by molecular mechanics programs
• such calculations can determine the lowest energy
arrangement of atoms in a given conformation, a
process called energy minimization
2-34
Conformations
• conformations of butane as a function of dihedral
angle
2-35
Anti Butane
 Energy-minimized
anti conformation
• the C-C-C bond angle is 111.9° and all H-C-H bond
angles are between 107.4 and 107.9°
• the calculated strain is 9.2 kJ (2.2 kcal)/mol
2-36
Eclipsed Butane
• calculated energy difference between (a) the nonenergy-minimized and (b) the energy-minimized
eclipsed conformations is 5.6 kJ (0.86 kcal)/mol
2-37
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
2-38
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
2-39
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
2-40
Cyclohexane
 Chair
conformation: the most stable puckered
conformation of a cyclohexane ring
• all bond C-C-C bond angles are 110.9°
• all bonds on adjacent carbons are staggered
2-41
Cyclohexane
 In
a chair conformation, six H are equatorial and
six are axial
2-42
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
2-43
Cyclohexane
 Boat
conformation: a puckered conformation of a
cyclohexane ring in which carbons 1 and 4 are
bent toward each other
• there are four sets of eclipsed C-H interactions and
one flagpole interaction
• a boat conformation is less stable than a chair
conformation by 27 kJ (6.5 kcal)/mol
2-44
Cyclohexane
 Twist-boat
conformation
• approximately 41.8 kJ (5.5 kcal)/mol less stable than a
chair conformation
• approximately 6.3 kJ (1.5 kcal)/mol more stable than a
boat conformation
2-45
Cyclohexane
QuickTime™ and a
Photo - JPEG decompressor
are needed to see this picture.
2-46
Methylcyclohexane
 Equatorial
and axial methyl conformations
CH3
+7.28 kJ/mol
CH3
2-47
G0 axial ---> equatorial
• given the difference in strain energy between axial and
equatorial conformations, it is possible to calculate the
ratio of conformations using the following relationship
G0 = -RT ln Keq
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
2-48
Cis,Trans Isomerism
 Stereoisomers:
compounds that have
• the same molecular formula
• the same connectivity
• a different orientation of their atoms in space
 Cis,trans
isomers
• stereoisomers that are the result of the presence of
either a ring (this chapter) or a carbon-carbon double
bond (Chapter 5)
2-49
Isomers
 relationships
among isomers
2-50
Cis,Trans Isomers
 1,2-Dimethylcyclopentane
H
H
H
H
H H H
H
H
CH3
H3 C
CH3
CH3
cis-1,2-Dimethylcyclopentane
H
H
H HH3 C
H
H
CH3
H3 C
H
CH3
trans-1,2-Dimethylcyclopentane
2-51
Cis,Trans Isomerism
 1,4-Dimethylcyclohexane
H
H3 C
CH3
H
H
H3 C
CH3
H3 C
trans-1,4-Dimethylcyclohexane
H
CH3
CH3
H3 C
cis-1,4-Dimethylcyclohexane
2-52
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)
2-53
Cis,Trans Isomerism
 cis-1,4-Dimethylcyclohexane
H
H
CH3
H3 C
H
H
CH3
conformations are of equal stability
CH3
2-54
Cis,Trans Isomerism
 The
decalins
H
H
A
B
H
trans-Decalin
H
H
H
A
B
H
H
cis-Decalin
2-55
Steroids
 The
steroid nucleus
C
A
D
B
 Cholestanol
CH3
H
H
HO
H
H
HO
H
H
CH3
H
H
Cholestanol
2-56
Bicycloalkanes
 Norbornane
drawn from three different
perspectives
one carbon bridge between carbons
1 and 4 of the six-membered ring
1
6
2
5
3
4
(a)
1
1
4
1
2
3
5
4
(b)
O
6
(c)
4
Camphor
2-57
Bicycloalkanes
 Adamantane
join the three axial
bonds to a CH group
add three
axial bonds
Cyclohexane
Adamantane
NH2
Amantadine
(antiviral agent)
2-58
Physical Properties
 Intermolecular
•
•
•
•
forces of attraction (example)
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)
2-59
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
2-60
Physical Properties
 Constitutional
isomers have different physical
properties
mp
(°C)
bp
(°C)
Density
(g/mL)
-95
68.7
0.659
3-methylpentane
2,3-dimethylbutane
-154
-118
-129
60.3
63.3
58.0
0.653
0.664
0.661
2,2-dimethylbutane
-98
49.7
0.649
Name
hexane
2-methylpentane
2-61
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
0
dioxide and water
H
kJ(kcal)/mol
CH4 + 2 O2
Methane
CH3 CH2 CH3 + 5 O2
Propane
CO2 + 2 H2 O
-890.4 (-212.8)
3 CO2 + 4 H2 O
-2220 (-530.6)
2-62
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)
2-63
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 CO2 + 9 H2 O
2-64
Heat of Combustion
• strain in cycloalkane rings as determined by heats of
combustion
2-65
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
2-66
Gasoline
 Octane
rating: the percent 2,2,4-trimethylpentane
(isooctane) in a mixture of isooctane and heptane that
has equivalent antiknock properties
Heptane
(octane rating 0)
2,2,4-Trimethylpentane
(octane rating 100)
2-67
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
O2
2
heat
catalyst
CO + H2
CO + 2 H2
2-68
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
2-69
Alkanes and
Cycloalkanes
End Chapter 2
2-70