William H. Brown
Thomas Poon
www.wiley.com/college/brown
Chapter Three
Alkanes and Cycloalkanes
Organic Chemistry
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.
Structure
• Shape
– Tetrahedral about carbon.
– All bond angles are approximately 109.5°.
Representing Alkanes
– Line-angle formula
• Each line represents a single bond.
• Each line ending represents a CH3 group.
• Each vertex (angle) represents a carbon atom.
Ball-an d-stick
model
Line-angle
formula
Stru ctural
formula
CH3 CH2 CH2 CH3
Butane
CH3 CH2 CH2 CH2 CH3
Pen tane
Alkanes
• Alkanes have the general formula CnH2n+2
– Names of unbranched alkanes with 1 to 20 carbon
atoms.
Molecular
Molecular
N ame
Formula
methane CH4
C2 H6
ethan e
propane C3 H8
bu tane
C4 H1 0
pen tane C5 H1 2
hexan e
C6 H1 4
hep tane C7 H1 6
octane
C8 H1 8
N ame
nonane
Formula
C9 H2 0
decan e
C1 0 H2 2
dodecan e
C1 2 H2 6
tetrad ecane
C1 4 H3 0
hexadecane C1 6 H3 4
octadecan e C1 8 H3 8
eicosane
C2 0 H4 2
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
CH3 CH2 CH2 CH3
CH3 CHCH3
Butane
(bp -0.5°C)
2-Meth ylp ropan e
(bp -11.6°C)
Constitutional Isomerism
– The potential for constitutional isomerism is
enormous.
Molecular
Formula
CH4
Constitutional
Is omers
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
IUPAC Nomenclature
– Suffix -ane specifies an alkane.
– Prefix tells the number of carbon atoms.
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
IUPAC Nomenclature
• Common alkyl groups
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
5
4 3
2
1
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
3-Ethyl-5-meth ylheptane
2
1
3
4
5
6
7
IUPAC Nomenclature
7. The prefixes di-, tri-, tetra-, etc. are not included
in alphabetization. In the following example, the
alphabetizing names are ethyl and methyl.
CH3 CH2 CH3
CH3 CCH2 CHCH2 CH3
CH3
4-Ethyl-2,2-dimeth ylh exane
1
2
3
4
6
5
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
2,2,4-Trimeth ylp entane
a 1° carb on
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 there is only one substituent, no need to give it a
number.
– If there are two substituents, number from the
substituent of lower alphabetical order.
– If there are three or more substituents, number to give
them the lowest set of numbers, and then list
substituents in alphabetical order.
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
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.
IUPAC - a general system
CH3 CH=CH 2
prop-en-e = propene
CH3 CH2 OH
eth-an-ol = ethanol
O
but-an-one = butanone
but-an-al = butanal
pent-an-oic acid = pentanoic acid
cyclohex-an-ol = cyclohexanol
eth-yn-e = ethyne
eth-an-amine = ethanamine
O
OH
HC CH
CH3 CH2 NH 2
H
O
OH
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. On the right is a Newman
projection formula.
Conformation
– 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
– The lowest energy conformation of an alkane is a fully
staggered conformation.
Conformations
• Torsional strain: Strain that arises when atoms
separated by three 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 12.6 kJ/ mol(3.0 kcal/mol).
+3.0 k cal/mol
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.
Cycloalkanes
• Cyclopentane (cont’d)
– In planar cyclopentane, there are 10 fully eclipsed C-H
bonds, which create torsional strain of approximately
42 kJ/mol (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°).
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.
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.
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.
Cyclohexane
• Boat conformation: A puckered conformation in which carbons 1 & 4 are bent
toward each other.
– A boat conformation is less stable than a chair conformation by 27 kJ)/mol
(6.5 kcal/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.
Cyclohexane
– The alternative chair conformations interconvert via a
boat conformation.
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
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.
Cis-trans isomerism
– The ring is commonly viewed through an edge or from
above.
H
H
H
H
H H
H
H
H
H H H3 C
H
CH3
H
CH3
H3 C
CH3
cis-1,2-D imethylcyclopentan e
H
H
CH3
H3 C
H
CH3
t rans-1,2-D imeth ylcyclopentane
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 H3 C
cis-1,4-D imethylcyclohexane
CH3
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
Cis-trans isomerism
– For cis-1,4-dimethylcyclohexane, the alternative chairs
are of equal stability.
equ atorial
H
CH 3
H
axial
CH3
H
CH3
H
axial
CH3
cis-1,4-D imethylcycloh exane
(th ese conformation s are of equ al stability)
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
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.
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.
Physical Properties
• Constitutional isomers are different compounds
and have different physical properties.
Reactions of Alkanes
• Oxidation is the basis for the use of alkanes as
energy sources for heat and power.
– Heat of combustion: the heat released when one
mole of a substance is oxidized to carbon dioxide and
water.
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
Sources of Alkanes
• Figure 3.13
Fractional
distillation
of
petroleum.
Synthesis Gas
– A mixture of carbon monoxide and hydrogen in
varying proportions, depending on how it is
produced. C + H2 O heat CO + H2
Coal
+ 1 O2
CH4
catalyst
2
CO + 2 H2
Methan e
CO + 2 H2
CH 3 OH + CO
Methan ol
cataly st
cataly st
CH 3 OH
Methan ol
O
CH 3 COH
Acetic acid
– Methanol and acetic acid are produced from synthesis
gas.
Methane Economy
• If the United States moves from a petroleum
economy to a natural gas economy as some
advocate, synthesis gas and synthesis gas-derived
methanol will become key building blocks.
Alkanes and
Cycloalkanes
End Chapter 3