Chapter 3: Alkanes
and Cycloalkanes
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3-1
Organic Chemistry
Hydrocarb on s
Satu rated
Class
Alkan es
(Ch apter 3)
Carb on - Only carboncarbon
carb on single
bond ing
bonds
HH
Example
H-C-C-H
HH
N ame
Ethan e
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Un saturated
Alk enes
(Ch apters 4-5)
Alkynes
(Chap ter 4)
Arenes
(Ch apter 9)
One or more
carb on -carbon
doub le bonds
H
H
C C
H
H
Ethylene
On e or more
carb on -carbon
triple b on ds
On e or more
ben zenelike
rin gs
H-C C-H
Acetylene
Benzen e
3-2
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
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3-3
Structure
• Shape
• tetrahedral about carbon
• all bond angles are approximately 109.5°
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3-4
Representing Alkanes
Ball-an d-stick
model
Line-angle
formula
Stru ctural
formula
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CH3 CH2 CH2 CH3
CH3 CH2 CH2 CH2 CH3
Butane
Pen tane
3-5
Nomenclature
• Alkanes have the general formula CnH2n+2
• names of unbranched chain alkanes
N ame
Molecular
Formula
methane CH 4
ethan e
C2 H 6
propane C H
3
bu tane
8
C4 H 1 0
pen tane C5 H 1 2
hexan e
C6 H 1 4
hep tane C7 H 1 6
octane
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C8 H1 8
N ame
Molecular
Formula
nonane
C9 H2 0
decan e
C1 0 H 2 2
dodecan e
C1 2 H 2 6
tetrad ecane
C1 4 H 3 0
hexadecane C1 6 H 3 4
octadecan e C1 8 H 3 8
eicosane
C2 0 H 4 2
3-6
Constitutional Isomers
• Constitutional isomers: compounds with the
same molecular formula but a different
connectivity of their atoms
• there are two constitutional isomers with molecular
formula C4H10
CH3
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CH3 CH2 CH2 CH3
CH3 CHCH3
Butane
(bp -0.5°C)
2-Meth ylp ropan e
(bp -11.6°C)
3-7
Constitutional Isomerism
• the potential for constitutional isomerism is enormous
Molecular
Formul a
CH 4
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
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World population
is about
6,000,000,000
3-8
IUPAC Nomenclature
• suffix -ane specifies an alkane
• prefix tells the number of carbon atoms
Prefix Carbons
meth1
eth 2
prop3
but4
pent5
hex6
hept7
oct8
non 9
dec10
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Carb on s
Prefix
undec11
dod ec12
tridec13
tetradec14
pentadec- 15
hexadec16
heptadec- 17
octad ec18
non adec19
eicos 20
3-9
IUPAC Nomenclature
• 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 R• CH4 becomes CH3- (methyl)
• CH3CH3 becomes CH3CH2- (ethyl)
subs tituent
parent chain
1
2
3
4
5
6
8
7
4-Meth yloctan e
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3-10
IUPAC Nomenclature
• Common alkyl groups
N ame
methyl
Con dens ed
Structu ral Formula
-CH3
ethyl
-CH2 CH3
propyl
-CH2 CH2 CH3
isopropyl -CHCH3
CH3
bu tyl
-CH2 CH2 CH2 CH3
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N ame
isobu tyl
sec-butyl
Con dens ed
Structu ral Formula
-CH2 CHCH3
CH3
-CHCH2 CH3
CH3
CH3
t ert-bu tyl -CCH3
CH3
3-11
IUPAC Nomenclature
1.The name of an alkane with an unbranched chain
consists of a prefix and the suffix ane
2. For branched alkanes, the parent chain is the longest
chain of carbon atoms
3. Each substituent is given a name and a number
CH3
CH3 CHCH3
2-Methylp ropan e
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-Methylpen tane
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5
4 3
2
1
3-12
IUPAC 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
CH2 CH3
2
1
4
3
6
5
7
3-Ethyl-5-meth ylh eptane
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3-13
IUPAC Nomenclature
7. The prefixes di-, tri-, tetra-, etc. are not included
in alphabetization
CH3 CH2 CH3
CH3 CCH2 CHCH2 CH3
CH3
1
2
3
4
6
5
4-Ethyl-2,2-dimeth ylh exane
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3-14
Classification of Carbons
•
•
•
•
Primary (1°): a C bonded to one other carbon
Secondary (2°): a C bonded to two other carbons
Tertiary (3°): a C bonded to three other carbons
Quaternary (4°): a C bonded to four other carbons
a 2° carb on
a 4° carb on
a 3° carb on
CH3
CH3 -C-CH2 -CH-CH3
a 1° carb on
CH3
CH3
a 1° carb on
2,2,4-Trimeth ylp entane
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3-15
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
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3-16
Cycloalkanes
• Commonly written as line-angle formulas
• examples:
Isopropylcyclopentan e
1-tert -Bu tyl-4-methylcycloh exane
1-Isobutyl-2-meth ylcyclohexan e
1-Ethyl-1-methylcyclopropane
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3-17
IUPAC- A General System
• 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
Infi x
Nature of Carbon-Carbon
Bonds in the Parent Chain
Suffix
Cl ass
-e
hydrocarbon
-an-en-
all singl e bonds
-ol
alcohol
one or more double bonds
-al
aldehyde
-yn-
one or more triple bonds
-amine
-one
amine
-oic acid
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ketone
carboxyl ic acid
3-18
IUPAC - a general system
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
cyclohex-an-ol = cyclohexanol
CH3 CH2 NH 2
eth-yn-e = ethyne
O
eth-an-amine = ethanamine
H
O
CH3 CH2 OH
O
OH
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3-19
Conformation
• Conformation: any three-dimensional
arrangement of atoms in a molecule that results
from rotation about a single bond
• staggered conformation: a conformation about a
carbon-carbon single bond where the atoms on one
carbon are as far apart as possible from the atoms on
an adjacent carbon
H
H
H
H
H
H
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3-20
Conformation
• Eclipsed conformation: a conformation about a carboncarbon 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
• the lowest energy conformation of an alkane is a fully
staggered conformation
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3-21
Conformations
• Torsional strain: strain that arises when atoms
separated by 3 bonds are forced from a
staggered conformation to an eclipsed
conformation
• also called eclipsed interaction strain
• the torsional strain between staggered and eclipsed
ethane is approximately 3.0 kcal (12.6 kJ)/ mol
+3.0 kcal/mol
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3-22
Cycloalkanes
• Cyclopentane
• in planar cyclopentane, all C-C-C bond angles are 108°,
which differ only slightly from the tetrahedral angle of
109.5°
• consequently there is little angle strain
• angle strain: strain that arises when a bond angle is
either compressed or expanded compared with its
optimal value
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3-23
Cycloalkanes
• Cyclopentane (cont’d)
• in planar cyclopentane, there are 10 fully eclipsed C-H
bonds, which create torsional strain of approximately
10 kcal/mol
• puckering to an “envelope” conformation relieves part
of this strain
• in an envelope conformation, eclipsed interactions are
reduced but angle strain is increased slightly (105°)
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3-24
Cycloalkanes
• Cyclohexane
• the most stable conformation is a puckered chair
conformation
• in a chair conformation, all bond angles are approx.
109.5°, and all bonds on adjacent carbons are
staggered
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3-25
Cycloalkanes
• Chair cyclohexane
• six H are equatorial and six H are axial
• an axial bond extends from the ring parallel to the
imaginary axis of the ring
• equatorial bonds are roughly perpendicular to the
imaginary axis of the ring
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3-26
Cyclohexane
• chair cyclohexane (cont’d)
• there are two equivalent chair conformations
• all C-H bonds equatorial in one chair are axial in the
alternative chair, and vice versa
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3-27
Cyclohexane
• Boat conformation: a puckered conformation in
which carbons 1 and 4 are bent toward each other
• a boat conformation is less stable than a chair
conformation by 6.5 kcal (27 kJ)/mol
• torsional strain is created by four sets of eclipsed
hydrogen interactions
• steric strain (nonbonded interaction strain) is created
by one set of flagpole interactions
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3-28
Cyclohexane
• the alternative chair conformations interconvert via a
boat conformation
flip th is
end dow n
flip th is
end up
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3-29
Cyclohexane
• a group equatorial in one chair is axial in the alternative
chair
• the two chairs are no longer of equal stability
CH3
+1.74 k cal/mol
CH3
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3-30
Cis-trans Isomerism
• Cis-trans isomers have
• the same molecular formula
• the same connectivity of their atoms
• an arrangement of atoms in space that cannot be
interconverted by rotation about sigma bonds
H
H
H
H
H H
H
H
H3 C
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H
H H H3 C
H
CH 3
H
CH3
CH3
cis-1,2-D imethylcyclopentan e
H
H
CH3
H3 C
H
CH 3
t rans-1,2-D imeth ylcyclopentane
3-31
Cis-trans isomerism
• the ring is commonly viewed through an edge or from
above
H
H
H
H
H H
H
H
H3 C
CH3
CH3
cis-1,2-D imethylcyclopentan e
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H
H H H3 C
H
CH 3
H
H
H
CH3
H3 C
H
CH 3
t rans-1,2-D imeth ylcyclopentane
3-32
Cis-trans isomerism
• cyclohexanes may be viewed as planar hexagons
H
H3 C
CH3
H
CH3
H3 C
t rans-1,4-D imeth ylcyclohexan e
H
H3 C
H
CH3
CH3 H 3 C
cis-1,4-D imethylcyclohexane
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3-33
Cis-trans isomerism
• or we can represent them as chair conformations
• in viewing chair conformations, remember that groups
equatorial in one chair are axial in the alternative chair
• for trans-1,4-dimethylcyclohexane, the diequatorial
chair is more stable than the diaxial chair
CH3
axial
H
equatorial
H
equ atorial
CH3
H
CH3
axial
CH3
H
(less s table)
(more stable)
t rans -1,4-D imethylcyclohexan e
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3-34
Cis-trans isomerism
• for cis-1,4-dimethylcyclohexane, the alternative chairs
are of equal stability
equ atorial
H
CH3
H
axial
CH3
H
CH3
H
axial
CH3
cis-1,4-D imethylcycloh exane
(th ese conformation s are of equ al stability)
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3-35
Cis-trans isomerism
• Problem: draw the alternative chair conformations of
this trisubstituted cyclohexane and state which is the
more stable
H3 C
CH3
CH3
H
H
H
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3-36
Physical Properties
• Alkanes are nonpolar compounds and have only
weak interactions between their molecules
• Dispersion forces: weak intermolecular forces of
attraction resulting from interaction of temporary
induced dipoles
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3-37
Physical Properties
• Boiling point
• low-molecular-weight alkanes (1 to 4 carbons) are
gases at room temperature; e.g., methane, propane,
butane
• higher-molecular-weight alkanes (5 to 17 carbons) are
liquids at room temperature (e.g., hexane, decane,
gasoline, kerosene)
• high-molecular-weight alkanes (18 or more carbons)
are white, waxy semisolids or solids at room
temperature; e.g., paraffin wax
• Density
• average density is about 0.7 g/mL
• liquid and solid alkanes float on water
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3-38
Physical Properties
• Constitutional isomers are different compounds
and have different physical properties
N ame
hexan e
3-methylp entane
2-methylp entane
Meltin g Boiling
Point Point D en sity
(°C)
(°C) (g/mL)
-95
69
0.659
-6
-23
2,3-dimethylbutan e -129
64
62
58
0.664
0.653
0.662
2,2-dimethylbutan e -100
50
0.649
Hexane
2,2-D imethylbutan e
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3-39
Reactions of Alkanes
• Oxidation is the basis for the use of alkanes as
energy sources for heat and power
• heat of combustion: heat released when one mole of a
substance is oxidized to carbon dioxide and water
CH4 + 2 O2
Methan e
CH3 CH2 CH3 + 5 O2
Propan e
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CO2 + 2 H 2 O
H° = -212 kcal/mol
3 CO2 + 4 H2 O H° = -530 kcal/mol
3-40
Sources of Alkanes
• Natural gas
• 90-95% methane, 5-10% ethane
• 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
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3-41
Synthesis Gas
• a mixture of carbon monoxide and hydrogen in varying
proportions, depending on how it is produced
C + H2 O
Coal
+ 1 O2
CH4
heat
catalyst
2
CO + H2
CO + 2 H2
Methan e
• methanol and acetic acid are produced from synthesis
gas
catalyst
CO + 2 H2
CH 3 OH + CO
Methan ol
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CH 3 OH
Methan ol
catalyst
O
CH 3 COH
Acetic acid
3-42
Alkanes and
Cycloalkanes
End Chapter 3
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3-43