ALKANES
Families of organic molecules are classified according to their reactive parts, called
functional groups. Alkanes are composed of only carbon atoms and hydrogen atoms
and contain only single bonds. Compounds that contain only carbon and hydrogen are
called hydrocarbons, so an alkane is a hydrocarbon that has only single bonds. Alkanes
in which the carbons form a continuous chain with no branches are called straight-
chain alkanes. The names of several straight-chain alkanes are given in the table below.
Condensed formula Name
CH4 Methane
CH3CH3 Ethane
CH3CH2CH3 Propane
CH3CH2CH2CH3 Butane
CH3CH2CH2CH2CH3 Pentane
CH3CH2CH2CH2CH2CH3 Hexane
CH3CH2CH2CH2CH2CH2CH3 Heptane
CH3CH2CH2CH2CH2CH2CH2CH3 Octane
CH3CH2CH2CH2CH2CH2CH2CH2CH3 Nonane
The family of alkanes shown in the table is an example of a homologous series. A
homologous series (homos is Greek for “the same as”) is a family of compounds in
which each member differs from the next by one methylene group (CH2). The members
of a homologous series are called homologs. Propane (CH3CH2CH3) and butane
(CH3CH2CH2CH3) are homologs.
The alkanes are the simplest and least reactive class of organic compounds because they
contain only hydrogen and sp3 hybridized carbon, and they have no reactive functional
groups. Although alkanes undergo reactions such as cracking and combustion at high
temperatures, they are much less reactive than other classes of compounds having
functional groups.
The general molecular formula for an alkane is CnH2n+2 where n is an integer.
A hydrocarbon with no double or triple bonds is said to be saturated because it has the
maximum number of bonded hydrogens. Alkanes are therefore saturated
hydrocarbons.
Nomenclature of Alkanes
Common Names
If all alkanes had unbranched (straight-chain) structures, their nomenclature would be
simple. Most alkanes have structural isomers, however, and we need a way of naming
all the different isomers. For example, there are two isomers of formula C4H 10. The
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, unbranched isomer is simply called butane (or n-butane, meaning "normal" butane),
and the branched isomer is called isobutane, meaning an "isomer of butane.”
CH3
H2 H2
H 3C C C CH3 H 3C C CH
H 3
butane (n-butane) isobutane
The three isomers of C5H12 are called pentane (or n -pentane), isopentane, and
neopentane. N
CH3
CH 3
H2 H2 H2 H 3C C CH3
H2
H3C C C C CH 3 H3 C C C CH 3
H CH3
pentane (n-pentane) isopentane
neopentane
Isobutane, isopentane, and neopentane are common names or trivial names, meaning
historical names arising from common usage. Common names cannot easily describe
the larger, more complicated molecules having many isomers, however. The number of
isomers for any molecular formula grows rapidly as the number of carbon atoms
increases. For example, there are 5 structural isomers of hexane, 18 isomers of octane,
and 75 isomers of decane! To avoid having to memorize the names of thousands of
structural units, chemists have devised rules that name compounds on the basis of their
structures. That way, only the rules have to be learned. Because the name is based on
the structure, these rules make it possible to deduce the structure of a compound from
its name.
This method of nomenclature is called systematic nomenclature. It is also called IUPAC
nomenclature because it was designed by a commission of the International Union of
Pure and Applied Chemistry (abbreviated IUPAC) at a meeting in Geneva, Switzerland,
in 1892. Alkanes
To name an alkane, we follow four rules:
1. Find the longest continuous chain of carbon atoms, and use this chain as the base
name.
2. Number the longest chain, beginning with the end nearest a branch.
3. Name the substituents on the longest chain (as alkyl groups). Give the location of
each substituent by the number of the main chain carbon atom to which it is attached.
4. When two or more substituents are present, list them in alphabetical order. When two
or more of the same alkyl substituent are present, use the prefixes di-, tri, tetra-, and so
on (ignored in alphabetizing) to avoid having to name the alkyl group twice.
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Families of organic molecules are classified according to their reactive parts, called
functional groups. Alkanes are composed of only carbon atoms and hydrogen atoms
and contain only single bonds. Compounds that contain only carbon and hydrogen are
called hydrocarbons, so an alkane is a hydrocarbon that has only single bonds. Alkanes
in which the carbons form a continuous chain with no branches are called straight-
chain alkanes. The names of several straight-chain alkanes are given in the table below.
Condensed formula Name
CH4 Methane
CH3CH3 Ethane
CH3CH2CH3 Propane
CH3CH2CH2CH3 Butane
CH3CH2CH2CH2CH3 Pentane
CH3CH2CH2CH2CH2CH3 Hexane
CH3CH2CH2CH2CH2CH2CH3 Heptane
CH3CH2CH2CH2CH2CH2CH2CH3 Octane
CH3CH2CH2CH2CH2CH2CH2CH2CH3 Nonane
The family of alkanes shown in the table is an example of a homologous series. A
homologous series (homos is Greek for “the same as”) is a family of compounds in
which each member differs from the next by one methylene group (CH2). The members
of a homologous series are called homologs. Propane (CH3CH2CH3) and butane
(CH3CH2CH2CH3) are homologs.
The alkanes are the simplest and least reactive class of organic compounds because they
contain only hydrogen and sp3 hybridized carbon, and they have no reactive functional
groups. Although alkanes undergo reactions such as cracking and combustion at high
temperatures, they are much less reactive than other classes of compounds having
functional groups.
The general molecular formula for an alkane is CnH2n+2 where n is an integer.
A hydrocarbon with no double or triple bonds is said to be saturated because it has the
maximum number of bonded hydrogens. Alkanes are therefore saturated
hydrocarbons.
Nomenclature of Alkanes
Common Names
If all alkanes had unbranched (straight-chain) structures, their nomenclature would be
simple. Most alkanes have structural isomers, however, and we need a way of naming
all the different isomers. For example, there are two isomers of formula C4H 10. The
Page 1 of 7
, unbranched isomer is simply called butane (or n-butane, meaning "normal" butane),
and the branched isomer is called isobutane, meaning an "isomer of butane.”
CH3
H2 H2
H 3C C C CH3 H 3C C CH
H 3
butane (n-butane) isobutane
The three isomers of C5H12 are called pentane (or n -pentane), isopentane, and
neopentane. N
CH3
CH 3
H2 H2 H2 H 3C C CH3
H2
H3C C C C CH 3 H3 C C C CH 3
H CH3
pentane (n-pentane) isopentane
neopentane
Isobutane, isopentane, and neopentane are common names or trivial names, meaning
historical names arising from common usage. Common names cannot easily describe
the larger, more complicated molecules having many isomers, however. The number of
isomers for any molecular formula grows rapidly as the number of carbon atoms
increases. For example, there are 5 structural isomers of hexane, 18 isomers of octane,
and 75 isomers of decane! To avoid having to memorize the names of thousands of
structural units, chemists have devised rules that name compounds on the basis of their
structures. That way, only the rules have to be learned. Because the name is based on
the structure, these rules make it possible to deduce the structure of a compound from
its name.
This method of nomenclature is called systematic nomenclature. It is also called IUPAC
nomenclature because it was designed by a commission of the International Union of
Pure and Applied Chemistry (abbreviated IUPAC) at a meeting in Geneva, Switzerland,
in 1892. Alkanes
To name an alkane, we follow four rules:
1. Find the longest continuous chain of carbon atoms, and use this chain as the base
name.
2. Number the longest chain, beginning with the end nearest a branch.
3. Name the substituents on the longest chain (as alkyl groups). Give the location of
each substituent by the number of the main chain carbon atom to which it is attached.
4. When two or more substituents are present, list them in alphabetical order. When two
or more of the same alkyl substituent are present, use the prefixes di-, tri, tetra-, and so
on (ignored in alphabetizing) to avoid having to name the alkyl group twice.
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