Organic Chemistry: NOMENCLATURE OF ALKANES AND CYCLOALKANES

1. Chemists have devised rules that allow compounds to be
named based on their structure.
This way, only the rules have to be learned.
Since the name is based on the structure, these rules also
allow the structure of a compound to be determined from
its name.
The method of nomenclature used is called Systematic
nomenclature, also known as IUPAC nomenclature.
This is because it was devised by a committee of
International Union of Pure and Applied Chemistry (IUPAC,
pronounced I-You-Pack) at a meeting in Geneva,
Switzerland. The IUPAC rules have been continually revised
since that time.
NOMENCLATURE OF ALKANES AND CYCLOALKANES

2. Unbranched alkanes named (after the first four) according
to the number of carbon atoms present, that is, the Greek
Latin roots, such as pent-, hex-, hept-, oct-, etc. are used to
designate the length of the chain.
The prefix “n” before the name stands for normal to
emphasize that the carbon atoms are in a continuous chain.
Some examples are:
Names such as isobutane and neopentane, are non-
systematic and are called common names.
Unbranched Alkanes
The names of alkanes have the ending “ane”.

3. Some Examples of Unbranched Alkanes

4. substituent
substituent
substituent
Branched Alkanes
For branched-chain alkanes, the longest continuous chain
of carbon atoms determines the name of the alkane. Some
examples are:
Hexane
Pentane
The groups attached to the main (parent) chain are called
substituents.
The main (parent) chain is numbered so as to give the lowest
number to the carbon atom with the first substituent
encountered along the chain. Some examples are:
CH3CH2CH2CH2CH CH3
CH3
CH3CHCHCH2CH3
CH3
CH3

5. CH3CH2CH2CH2CHCH3
CH3
1
2
3
4
5
6
CH3CHCHCH2CH3
CH3
CH3
1
2
3 4 5
If a single hydrogen is removed from an alkane, an alkyl
group is obtained. The name of the alkyl group is obtained
by replacing the “ane” ending of the alkane with “yl”. Some
examples are:
Alkane name Formula of alkane Formula of alkyl
group
Alkyl group name
Methane CH4 CH3- Methyl
Ethane CH3CH3 CH3CH2- Ethyl
Propane CH3CH2CH3 CH3CH2CH2- n-Propyl
The letter “R” is used to indicate an alkyl group.

6. The number allocated to the carbon atom having the
substituent is used to designate the location of the
substituent group.
The numbers designating the locations of the substituent
groups are separated from each other by a comma, while
they themselves are separated from words by a hyphen.
Some examples are:
The name obtained after combining all the different aspects
has three parts.
Number designating the location of the substituent group
along the main chain (locant).
The substituent group (prefix).
The parent (main) name.

7. When two or more substituents are present, each
substituent is given a number corresponding to its location
on the longest continuous chain of carbon atoms.
The substituent groups are listed alphabetically in the
name. For example:
CH3CH2CH2CH2CHCH3
CH3
1
2
3
4
5
6
2-Methylhexane
CH3CH2CHCH2CH2CH3
CH2
CH3
5
4
3
2
1 6
CH3CH2CH2CH2CHCH3
CH2
CH3
1
3
4
5
6
7
2
3-Methylheptane 3-Ethylhexane

8. When two or more substituents are identical, the prefixes
di-, tri-, tetra-, etc. are used to indicate this. For example:
4-ethyl-2-methylhexane
CH3CH CH2CH CH2CH3
CH2 CH3
CH3
5
4
3
2
1 6
CH3CHCHCH2CH3
CH3
CH3
1
2
3 4 5
2,3-Dimethylpentane
CH3CHCHCH3
CH3
CH3
1
2
3 4
2,3-Dimethylbutane
When two substituents are present on the same carbon
atom, the number indicating the location of the
substituents is used twice. For example:

9. When chains of equal length compete for selection as the
parent chain, the chain with the greater number of
substituents is chosen. For example:
3-Ethyl-3-methylhexane
CH3CH2C CH2CH2
CH2
CH3
CH3
CH3
2,3,5-Trimethyl-4-propylheptane (4 substituents)
Not 4-sec-Butyl-2,3-dimethylheptane (3 substituents)
CH3 CH2 CH CH CH CH CH3
CH3
CH3
CH3 CH2CH2CH3
5 4 3 2 1
6
7
1
2
3
4
5 6 7

10. When branching first occurs at an equal distance from
either end of the longest chain, the name that gives the
lower number at first point of difference is chosen. For
example:
2,3,5-Trimethylhexane
Not 2,4,5-trimethylhexane
Using the upper numbering, the name is 2,3,5-
trimethylhexane. When the lower numbering is used, the
name is 2,4,5-trimethylhexane. The correct name is 2,3,5-
trimethylhexane.
CH3 CH CH2 CH CH CH3
CH3
CH3
CH3
5 4 3 2 1
6
1 2 3 4 5 6

11. If the same substituent numbers are obtained in both
directions, the first cited group (alphabetically) receives the
lower number. An example is:
3-Ethyl-5-methylheptane
Not 5-Ethyl-3-methylheptane
CH3 CH2 CH CH2 CH CH2 CH3
CH2
CH3
CH3
5 4 3 2 1
6
7
1 2 3 4 5 6 7

12. Alkyl Substituents
We observed that alkyl groups or substituents are named by
replacing “ane” ending of the alkane with “yl”.
Two alkyl groups – a propyl and an isopropyl – contain three
carbon atoms.
A propyl group is obtained when a hydrogen atom is
removed from a primary carbon of propane.
A primary carbon is one that is bonded to only one other
carbon.
It has three hydrogen atoms bonded to it.
These three hydrogen atoms are called primary hydrogen
atoms. Primary is denoted by 1˚.

13. Alkyl Substituents …
An isopropyl group is obtained when a hydrogen atom is
removed from a secondary carbon of propane.
A secondary carbon is one that is bonded to two other
carbon atoms.
It has two hydrogen atoms bonded to it.
These two hydrogen atoms are called secondary hydrogen
atoms.
Secondary is denoted by 2˚.

14. a propyl group
There are four alkyl groups that contain four carbon atoms.
Butyl and isobutyl groups result from removal of a primary
hydrogen atom from a primary carbon atom.
A sec-butyl is obtained by removal of a hydrogen atom from
a secondary carbon atom.
Sec- often abbreviated s- stands for secondary.
tert-butyl group has had a hydrogen removed from a
tertiary carbon (tert- offen abbreviated t-, stands for
tertiary).
an isopropyl group
CH3CH2CH2
A primary carbon
CH3CHCH3
A secondary carbon

15. A tertiary carbon atom is a carbon that is bonded to three
other carbon atoms. It has only one hydrogen atom bonded
to it. This hydrogen atom is called a tertiary hydrogen atom.
Tertiary is denoted by 3°.
an n-butyl group an isobutyl group
a sec-butyl group a tert-butyl group
CH3CH2CH2CH2
CH3CHCH2
CH3
CH3CH2CH
CH3
CH3C
CH3
CH3

16. A carbon atom bonded to four other carbon atoms is called
a quaternary carbon atom. It has no hydrogen atoms
bonded to it.
Alkyl group names are used so frequently that it is
important to learn them.
The most commonly used groups are listed below:

17. Nomenclature of small alkyl groups
Group structure Name
CH3CH─
│
CH3
Isopropyl or
(1-methylethyl)
CH3CHCH2─
│
CH3
Isobutyl or
(2-methylpropyl)
CH3CH2CH─
│
CH3
sec-butyl or s-butyl
(1-methypropyl)
CH3
│
CH3─C─
│
CH3
tert-butyl or t-butyl
(1,1-dimethylethyl)
CH3CHCH2CH2─
│
CH3
Isopentyl
(3-methylbutyl)
CH3
│
CH3─C─CH2─
│
CH3
Neopentyl
(2,2-dimethylpropyl)

18. Names such as isopropyl, sec-butyl, and tert-butyl are
acceptable substituent names in IUPAC system of
nomenclature, but systematic substituent names are
preferable. Systematic names are obtained by numbering
the substituent beginning at the carbon that is bonded to
the parent hydrocarbon. Some examples are:
CH3CH2CH2─
n-propyl group
CH3CH2CH3
1 2
CH3CHCH3 (isopropyl group)
│
1-methylethyl group
CH3CH2CH2CH2─
n-butyl group
CH3CH2CH2CH3
1 2 3
CH3CHCH2CH3 (sec-butyl group)
│
1-methylpropyl group

19. Examples of nomenclature of alkanes containing such
groups are:
CH3
│
CH3CHCH2─ (isobutyl group)
3 2 1
2-methylpropyl group
CH3CHCH3
│ CH3
CH3 2 1│
(isobutane) CH3CCH3 (tert-butyl group)
│
2-methylpropane 1,1-dimethylethyl group
4-(1-Methylethyl)heptane 4-isopropylheptane
heptane (parent chain)
1-methylethyl (substituent)
OR
CH3CH2CH2CH CH2CH2CH3
CH
CH3
CH3
5
4
3
2
1 6 7
1
2

20. In this name, the substituent is in brackets; the number
inside the brackets indicates a position on the substituent,
whereas the number outside the brackets indicates a
position on the parent hydrocarbon.
4-(1,1-Dimethylethyl)octane 4-tert-Butyloctane
octane (parent chain)
1,1-dimethylethyl (substituent)
CH3CH2CH2CH CH2CH2CH2
C
CH3
CH3
CH3
CH3
5
4
3
2
1 6 7 8
1
2
OR

21. The prefix “cyclo” is used before the name of the cyclic
parent alkane. Some examples are:
Cyclopropane Cyclobutane
Cyclopentane Cyclohexane
Monocyclic Alkanes
C
H2
C
H2
CH2
OR OR
OR
C
H2
CH2
CH2
C
H2
C
H2
C
H2
C
H2
CH2
CH2

22. When the cyclic compound has a substituent, then the
name of the substituent group occurs as a prefix. Some
examples are:
Methylcyclopropane Ethylcyclohexane n-Propylcyclobutane
When two substituents are present, the ring is numbered
beginning with the substituent first in the alphabet, and the
direction of the numbering is continued so as to give the
next substituent the lower number possible. Some
examples are:
CH3
CH2CH3
CH2CH2 CH3

23. When three or more substituents are present, numbering is
done beginning at the substituent that leads to the lowest
set of locants. Some examples are:
CH3
CH2CH3
CH3
CH3
CH3
C
H3
1-Ethyl-3-methylcyclohexane
Not 1-ethyl-5-methylcyclohexane
1,2-Dimethylcyclopentane
Not 1,5-dimethycyclopentane
1,1-Dimethylcyclopentane

24. The ring is rigid and rotation about the carbon-carbon
bonds is not possible, its structure is fixed in space. Two
ways of arranging two substituents on the ring are possible.
For example:
CH3
CH2CH3
CH3
Cl
CH2CH3
CH3
2-Ethyl-1,4-dimethylcyclohexane
4-Chloro-2-ethyl-1-methylcyclohexane

25. When the two groups are on the same side, the prefix “cis”
(Latin word for “on the same side”) is used.
If the two groups are on opposite sides, the prefix “trans”
(Latin word for “across”) is used.
The two isomers are called geometric isomers.
They have different physical properties. For example:
CH3 CH3
H H
CH3 H
H CH3

26. cis-1,2-Dimethylcyclopentane
trans-1,2-Dimethylcyclopentane
bp 99˚C
mp -62˚C
bp 92˚C
mp -120˚C
CH3
H
CH3
H
CH3
H
CH3
H
CH3
H
H
CH3
CH3
H
H
CH3

27. Bridged Bicycloalkanes
Bridged bicyclic alkanes have two rings that share two
carbon atoms. For example:
CH2
CH2
C
H
C
H
C
H2
C
H2
CH2
2-carbon
bridge
1 carbon bridge
bridgehead
carbon atom
OR OR

28. Numbering begins at a bridgehead along the longest chain
(bridge), then back to the first bridgehead by the second
longest chain.
The shortest chain (bridge) is then numbered progressively
starting from the first bridgehead.
The number of carbon atoms forming each chain (bridge)
are indicated in square brackets starting from the longest
chain to the shortest chain. Some examples are:
1
2
3
4
5
6
7
Bicyclo[2.2.1]heptane
OR
Norbornane
CH3
1
2
3
4
5
6
7
8
8-Methylbicyclo[3.2.1]octane
1
2
3
4
Bicyclo[1.1.0]butane

29. 8-Methylbicyclo[4.3.0]nonae
2-Methylbicyclo[4.2.2]decane
C
H3
6
9
8
1
2
3
4
5
7
CH3
1
2
3
4
5
6
7
8
9
10