Nomenclature of Alkanes (
)
Chemists have developed a method of nomenclature that assigns a systematic, unique name to
each possible compound. The system presently used is called the IUPAC system, an
abbreviation for the International Union of Pure and Applied Chemistry.
The nomenclature of all other classes of compounds is based on the names of the alkane series.
The names of the first ten straight-chain (unbranched) alkanes is shown in the table below.
No. carbons
Name
Formula
Structure
No. isomers
1
methane
CH4
CH4
1
2
ethane
C2H6
CH3CH3
1
3
propane
C3H8
CH3CH2CH3
1
4
butane
C4H10
CH3CH2CH2CH3
2
5
pentane
C5H12
CH3CH2CH2CH2CH3
3
6
hexane
C6H14
CH3(CH2)4CH3
5
7
heptane
C7H16
CH3(CH2)5CH3
9
8
octane
C8H18
CH3(CH2)6CH3
18
9
nonane
C9H20
CH3(CH2)7CH3
35
10
decane
C10H22
CH3(CH2)8CH3
75
Removal of a terminal hydrogen from an alkane generates an alkyl group.
Below are the names of some common alkyl groups.
H3C
methyl
H3C CH2 CH2
propyl (or n-propyl)
H3C CH2
ethyl
H3C CH CH3
isopropyl
CH3
H3C CH
CH3
CH3
H3C CH2 CH2 CH2
H3C CH CH2
H3C CH2 CH CH3
H3C C CH3
butyl (or n-butyl)
isobutyl
sec-butyl
tert-butyl
n = normal – indicates a straight chain alkane (all 1º and 2 º carbons)
The IUPAC rules for naming alkanes are given below and will be illustrated by naming the
alkane below:
CH3
CH2
H3C CH2 CH2 CH CH CH3
H2C
CH3
Rule 1: Selecting and Naming the Longest Chain.
Name the compound as a derivative of the alkane represented by the longest chain or continuous
sequence of carbon atoms. In the above compound, the longest chain contains 7 carbons (do you
see it?). The compound is therefore named as a heptane, and is considered to be a derivative of
this parent compound. The name must end in the suffix –ane to indicate the compound is a
member of the alkane series. Except for the first four members of the alkane series, the prefix of
the name donates the number of carbon atoms in the chain. Thus, for example, hept indicates 7
carbons.
Rule 2: Numbering the Longest Carbon Chain.
Number the carbon atoms of the longest chain so that the branching group(s) are on the lowest
numbered carbon atoms.
Rule 3: Locating the Branching Groups.
The location of the branching hydrocarbon groups is indicated by the number of the carbon atom
of the parent chain to which they are attached. Thus, in our illustrative compound, we find a
CH3 group on C-3 and a CH3CH2 group on C-4.
Rule 4: Naming the Branched Groups.
Each branched group is named according to the number of carbons on its longest chain. To
name the group, replace the suffix –ane by –yl. Thus, CH3 is called methyl since it stems from
methane, and CH3CH2 is named ethyl from ethane.
Rule 5: Arranging the Names of the Branched Groups.
The names of these groups are prefixed alphabetically with the number of the carbon atom to
which each group is attached. Thus, the name of the compound illustrated above is:
_____________________________
Rule 6: Punctuation.
The rules of punctuation are:
1. words are run together
2. a dash is used to separate a number from a word
3. a comma is used to separate numbers.
The following examples further illustrate the method:
CH3
CH3 CH2 CH2 C
CH3
CH3
2,2-dimethylpentane
CH3 CH2 CH3 CH3
H3C CH C
CH CH3
CH3
2,3,3,4-tetramethylhexane
Note the repetition of numbers when a group appears twice on the same carbon, and the use of
di- and tri- to indicate the number of times the groups appears in as a branch.
A problem that is sometimes encountered is to draw all of the isomers with a certain molecular
formula. This is done on the next page for the formula C7H16. This is easy to do if it is done
systematically. The best way is to start with the longest possible carbon chain (seven in this
case), draw it, then proceed to all possible structures with six carbons in the chain, etc. One way
to insure that you do not duplicate a structure is to give it an IUPAC name. If two structures
have the same correct IUPAC name, then the are simply two representations of the same
compound.
Constitutional isomers of heptane: C7H16
Constitutional isomers have the same molecular formula, but differ in way in their atoms are
connected.
H3C CH2 CH2 CH2 CH2 CH2 CH3
heptane
CH3
CH3 CH2 CH2 CH2 CH CH3
CH3
2-methylhexane
3-methylhexane
CH3 CH2 CH2 CH CH2 CH3
CH3
CH2
CH3 CH2 CH CH2 CH3
3-ethylpentane
CH3
CH3 CH2 CH2 C CH3
2,2-dimethylpentane
CH3
CH3 CH3
CH3 CH2 CH CH CH3
CH3
2,3-dimethylpentane
CH3
CH3 CH CH2 CH CH3
2,4-dimethylpentane
CH3
CH3 CH2 C CH2 CH3
3,3-dimethylpentane
CH3
CH3 CH3
CH3 CH C CH3
CH3
2,2,3-trimethylbutane