### Alkanes

```Hydrocarbons
Hydrocarbon: A compound composed only of carbon and
hydrogen.
Alkanes
Alkanes: Hydrocarbons that contain only carbon-carbon
single bonds.
• The first two alkanes are methane and ethane.
Alkanes
Line-angle formula
• A line represents a carbon-carbon bond.
• A vertex and a line terminus represent a carbon atom.
• Hydrogen atoms are not shown in line-angle formulas.
Alkanes
Table 11.1 The first 10 alkanes with unbranched chains
Constitutional Isomerism
Constitutional isomers: Compounds that have the same
molecular formula but different structural formulas (a
different connectivity of their atoms).
• For the molecular formulas CH4, C2H6, and C3H8, only
one structural formula is possible. There are no
constitutional isomers for these molecular formulas.
• For the molecular formula C4H10, two constitutional
isomers are possible.
Constitutional Isomers
Problem: Do the structural formulas in each set represent
the same compound or constitutional isomers?
Constitutional Isomerism
Solution:
(a) They represent the same compound.
Solution:
(b) They represent constitutional isomers.
Constitutional Isomerism
Problem: Draw structural formulas for the five
constitutional isomers of molecular formula C6H14.
Constitutional Isomerism
Problem: Draw structural formulas for the five
constitutional isomers of molecular formula C6H14.
Solution:
2
1
3
4
6
5
Six carbons in an
unbranched chain
4
1
2
3
2
5
1
4
3
5
Five carbons in a chain;
one carbon as a branch
1
2
3
4
2
1
4
3
Four carbons in a chain;
two carbons as branches
IUPAC Names
The name of an alkane with a branched chain of carbon
atom consists of:
• a parent name: the longest chain of carbon atoms.
• substituent names: the groups bonded to the parent
chain.
IUPAC Names
The IUPAC name of an alkane with an unbranched chain
of carbon atoms consists of two parts:
(1) A prefix shows the number of carbon atoms in the
chain.
(2) The suffix -ane: shows that the compound is a
saturated hydrocarbon.
Alkyl Groups
Alkyl group: A substituent group derived from an alkane
by removal of a hydrogen atom (Table 11.3).
• Commonly represented by the symbol R-.
• Named by dropping the -ane from the name of the
parent alkane and adding the suffix -yl.
IUPAC Names
1. The name for an alkane with an unbranched chain of
carbon atoms consists of a prefix showing the number of
carbon atoms and the ending -ane.
2. For branched-chain alkanes, the longest chain of carbon
atoms is the parent chain and its name is the root name.
3. Give each substituent on the parent chain a name and a
number. Use a hyphen to connect the number to the
name.
IUPAC Names
4. If there is one substituent, number the parent chain from
the end that gives the substituent the lower number.
IUPAC Names
5. If the same substituent occurs more than once:
• Number the parent chain from the end that gives the
lower number to the substituent encountered first.
• Indicate the number of times the substituent occurs by
a prefix di-, tri-, tetra-, penta-, hexa-, and so on.
• Use a comma to separate position numbers.
IUPAC Names
6. If there are two or more different substituents:
• List them in alphabetical order.
• Number the chain from the end that gives the lower
number to the substituent encountered first.
• If there are different substituents at equivalent
positions on opposite ends of the parent chain, give
the substituent of lower alphabetical order the lower
number.
IUPAC Names
7. Do not include the prefixes di-, tri-, tetra-, and so on or the
hyphenated prefixes sec- and tert- in alphabetizing;
• Alphabetize the names of substituents first, and then
insert these prefixes. In the following example, the
alphabetizing parts are ethyl and methyl, not ethyl and
dimethyl.
Common Names
Common names; an older system
• The number of carbon atoms determines the name.
• The first three alkanes are methane, ethane, and
propane.
• All alkanes with the molecular formula C4H10 are named
butanes, all those with the molecular formula C5H12 are
named pentanes, etc.
• For alkanes beyond propane, iso shows that one end of
an otherwise unbranched chain terminates in (CH3)2CH-
For more complex alkanes, use the IUPAC system.
Source of Hydrocarbons: Petroleum
Figure 11.3
Fractional
distillation of
petroleum.
Cycloalkanes
Cyclic hydrocarbon: A hydrocarbon that contains carbon
atoms joined to form a ring.
Cycloalkane: A cyclic hydrocarbon in which all carbons of
the ring are saturated (have only carbon-carbon single
bonds).
Cycloalkanes with ring sizes of from 3 to over 30 carbon
atoms are found in nature.
• Five-membered (cyclopentane) and six-membered
(cyclohexane) rings are especially abundant in nature.
Cycloalkanes
Nomenclature
• To name a cycloalkane, prefix the name of the
corresponding open-chain alkane with cyclo-, and
name each substituent on the ring.
• If there is only one substituent on the ring, there is no
need to give it a location number.
• If there are two substituents, number the ring
beginning with the substituent of lower alphabetical
order.
Conformations of Alkanes
Conformation: Any three-dimensional arrangement of
atoms in a molecule that results by rotation about a single
bond.
• Figure 11.4 Three conformations for a butane molecule.
Cyclopentane
Figure 11.5 The most stable conformation of a
cyclopentane ring is an envelope conformation.
Cyclohexane
The most stable conformation of a cyclohexane ring is the
chair conformation.
• All bond angles are approximately 109.5°.
Cyclohexane
In a chair conformation,
• six C-H bonds are equatorial.
• six C-H bonds are axial.
• Figure 11.6 Chair conformation of cyclohexane
Cyclohexane
The more stable conformation of a substituted
cyclohexane ring has substituent group(s) equatorial
rather than axial.
Figure 11.7 Methylcyclohexane
Cis/Trans Isomers
Cis: on the same side of the ring.
Trans: on the opposite side of the ring,
• In drawing cis-trans isomers of disubstituted
cyclopentanes, we can view a cyclopentane ring edgeon.
Cis-Trans Isomers
• Alternatively, we can view the cyclopentane ring from
above. Substituents are shown by solid wedges
(above) or dashed wedges (below).
Cis-Trans Isomerism
• To determine cis-trans isomers in disubstituted
cyclohexanes, we can view a cyclohexane ring either
as a planar hexagon or viewed from above.
• Because cis-trans isomers differ in the orientation of
their atoms in space, they are stereoisomers.
• Cis-trans isomers are one type of stereoisomer.
• In Ch 15, we study another type called enantiomers.
Physical Properties
Physical Properties
• Alkanes that are constitutional isomers are different
compounds and have different physical and chemical
properties.
Reactions
Oxidation (combustion)
• Oxidation of hydrocarbons, including alkanes and
cycloalkanes, is the basis for their use as energy
sources for heat [natural gas, liquefied petroleum gas
(LPG), and fuel oil] and power (gasoline, diesel fuel,
and aviation fuel).
Reactions of Alkanes
Reaction with halogens (halogenation)
• Halogenation of an alkane is a substitution reaction.
The Chlorofluorocarbons
Chlorofluorocarbons (CFCs)
• Manufactured under the trade name Freon.
• CFCs are nontoxic, nonflammable, odorless, and
noncorrosive.
• Among the CFCs most widely used were CCl3F (Freon11) and CCl2F2 (Freon-12).
CFCs were used as;
• Heat-transfer agents in refrigeration systems.
• Industrial cleaning solvents to prepare surfaces for
coatings and to remove cutting oils from millings.
• Propellants for aerosol sprays.
CFC Replacements
Chlorofluorocarbons (CFCs) cause destruction of the
Earth’s stratospheric ozone layer.
The most prominent replacements are the
hydrofluorocarbons (HFCs) and the
hydrochlorofluorocarbons (HCFCs).
• These compounds are chemically more reactive than
CFCs and are destroyed before they reach the
stratosphere.
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