CHM207 (ORGANIC CHEMISTRY)
NOMENCLATURE OF ALKANES USING IUPAC OR SYSTEMATIC NAMES
The IUPAC system uses the longest chain of carbon atoms as the main chain,
which is numbered to give the locations of side chains.
Four rules in naming the alkanes:
RULE 1: THE MAIN CHAIN (PARENT COMPOUND)

The first rule of nomenclature gives the base name of the compound:
Find the longest continuous chain of carbon atoms, and use the name of this
chain as the base name of the compound.

For example, the longest chain of carbon atoms in the following compound
contains six carbons, so the compound is named as a hexane derivative.
CH2CH3
H3C CH CH2-CH2-CH3

The groups attached to the main chain are called substituents because they are
substituted (in place of a hydrogen atom) on the main chain.

When there are two longest chains of equal length, use the chain with the greater
number of substituents as the main chain. For example:
CH3
CH3
CH
CH2
CH
CH
CH3 CH
CH3
CH3
CH2CH3
CH3
CH
CH2
CH
CH
CH3 CH
CH3
CH2CH3
CH3
CH3
WRONG
CORRECT
Seven-carbon chain, but only three
substituents
Seven-carbon chain, four substituents
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CHM207 (ORGANIC CHEMISTRY)
RULE 2: NUMBERING THE MAIN CHAIN

To give the locations of the substituents, assign a number to each carbon atom on
the main chain.
Number the longest chain, beginning with the end of the chain nearest a
substituent.

Start the numbering from the end nearest a branch so the numbers of the substituted
carbons will be as low as possible.
3
CH3
CH
4
CH
1CH
3
2
CH2
5
CH
CH2CH3
CH3 6CH
7CH

5
CH3
CH
4 CH
CH3
7 CH
3
6
CH2
3
CH
CH2CH3
CH3 2CH
1CH
3
CH3
3
INCORRECT
CORRECT
The first branch at C3
(carbon atom 3)
The first branch at C2
3-ethyl-2,4,5-trimethylheptane
When the numbering could begin with either of two alkyl groups or other
substituents, begin with the one that is alphabetically first. For example:
CH3 CH CH2 CH CH2 CH CH3
Cl
CH3
CH3
This compound is consists of Cl (chloro) and two methyl (CH3) groups as
substituents. According to alphabetically order, Cl (start with C) is come first
compared to methyl (start with m). Thus, the numbering could begin from the end
nearest the Cl atom.
2
CHM207 (ORGANIC CHEMISTRY)
7
6
CH3 CH
5
Cl
1
2
4
3
2
1
CH2 CH CH2 CH CH3
CH3
3
4
CH3
5
6
7
CH3 CH CH2 CH CH2 CH CH3
Cl
CH3
(INCORRECT)
(CORRECT)
2-chloro-4,6-dimethylheptane
CH3
RULE 3: NAMING ALKYL GROUPS
Name the substituent groups attached to the longest chain as alkyl groups. Give
the location of each alkyl group by the number of the main chain carbon atom to
which it is attached.

Alkyl groups are named by replacing the –ane suffix of the alkane name with –yl.
Methyl becomes methyl; ethane becomes ethyl.
Names and formulas of selected alkyl groups


Halogen substituents are named as F (fluoro-), Cl (chloro-), Br (bromo-) and I
(iodo-).
Other substituents i.e. –NO2 (nitro-), -NH2 (amino-), -CH=CH2 (vinyl-) and etc.
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CHM207 (ORGANIC CHEMISTRY)
RULE 4: ORGANIZING MULTIPLE GROUPS
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-, etc to avoid having to name the alkyl group twice.
di- means 2
penta- means 5
tri- means 3
hexa- means 6
tetra- means 4

When substituents are alphabetized, iso- is used as part of the alkyl group name, but
the hyphenated prefixes are not. Isobutyl is alphabetized with i, but n-butyl, t-butyl
and sec-butyl are alphabetized with b.

The number prefixes di-, tri-, tetra-, etc are ignored in alphabetizing.
NOMENCLATURE OF CYCLOALKANES




Named like acyclic alkanes.
Substituted cycloalkanes use the cycloalkane for the base name (parent compound),
with the alkyl groups named as substituents.
If there is just one substituent, no numbering is needed.
For examples:
CH3
CH3
C
CH3
Cl
CH3
methylcyclohexane
t-butylcycloheptane
chlorocyclopentane

If there are two or more substituents on the ring, the ring carbons are numbered to
give the lowest possible numbers for the substituted carbons.

The numbering begins with one of the substituted ring carbons and continues in the
direction that gives the lowest possible numbers to the other substituents.

In the name, the substituents are listed in alphabetical order.
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CHM207 (ORGANIC CHEMISTRY)

When the numbering could begin with either of two alkyl groups (as in
disubstituted cycloalkanes), begin with the one that is alphabetically first.

For examples:
CH3
1)
4 32
5 1
6
CORRECT
1-ethyl-3-methylcyclohexane
CH2CH3
(lowest possible numbers of substituents)
CH3
45 6
32 1
INCORRECT
1-ethyl-5-methylcyclohexane
CH2CH3
CH3
6 12
5
4 3
INCORRECT
3-ethyl-1-methylcyclohexane
(begin with the one that is alphabetically first)
CH2CH3
Other example:
CH3
CH3
5 12
4 3
1,1,3-trimethylcyclopentane
CH3

When the acyclic portion of the molecule contains more carbon atoms than the
cyclic portion (or when it contains an important functional group), the cyclic
portion is sometimes named as a cycloalkyl substituent.
8
9
8
7
6
5
4
3
2
7
6
5
4
3
2
1
1
4-cyclopropyl-3,5-dimethylnonane
5-cyclobutyl-3-methyl-2-octene
5