Organic Chemistry

Section 2.3 Nomenclature of Cycloalkanes 71

2.3 Nomenclature of Cycloalkanes

Cycloalkanesare alkanes with their carbon atoms arranged in a ring. Because of the

ring, a cycloalkane has two fewer hydrogens than an acyclic (noncyclic) alkane with

the same number of carbons. This means that the general molecular formula for a

cycloalkane is Cycloalkanes are named by adding the prefix “cyclo”to the

alkane name that signifies the number of carbon atoms in the ring.

Cycloalkanes are almost always written as skeletal structures. Skeletal structures

show the carbon–carbon bonds as lines, but do not show the carbons or the hydrogens

bonded to carbons. Atoms other than carbon and hydrogens bonded to atoms other

than carbon are shown. Each vertex in a skeletal structure represents a carbon. It is un-

derstood that each carbon is bonded to the appropriate number of hydrogens to give

the carbon four bonds.

Acyclic molecules can also be represented by skeletal structures. In a skeletal struc-

ture of an acyclic molecule, the carbon chains are represented by zigzag lines. Again,

each vertex represents a carbon, and carbons are assumed to be present where a line

begins or ends.

The rules for naming cycloalkanes resemble the rules for naming acyclic alkanes:

1. In the case of a cycloalkane with an attached alkyl substituent, the ring is the par-
   ent hydrocarbon unless the substituent has more carbon atoms than the ring. In
   that case, the substituent is the parent hydrocarbon and the ring is named as a sub-
   stituent. There is no need to number the position of a single substituent on a ring.


2. If the ring has two different substituents, they are cited in alphabetical orderand
   the number 1 position is given to the substituent cited first.


3. If there are more than two substituents on the ring, they are cited in alphabetical
   order. The substituent given the number 1 position is the one that results in a second

methylcyclopentane ethylcyclohexane

CH 2 CH 3

1-cyclobutylpentane

CH 2 CH 2 CH 2 CH 2 CH 3

CH 3

cyclopropane cyclobutane cyclopentane cyclohexane

cyclopropane

H 2 CCH 2

CH 2

H 2 C

CH 2

CH 2

CH 2

H 2 C CH 2

cyclobutane

H 2 CCH 2

H 2 CCH 2

cyclopentane cyclohexane

H 2 CCH 2

H 2 CCH 2

CH 2

CnH 2 n.

If there is only one substituent on a

ring, do not give that substituent a

number.

butane 2-methylhexane 3-methyl-4-propylheptane 6-ethyl-2,3-dimethylnonane

64

2

31 5 31

42

1-methyl-2-propylcyclopentane

H 3 C

CH 2 CH 2 CH 3

1-ethyl-3-methylcyclopentane

CH 3 CH 2

CH 3

1,3-dimethylcyclohexane

CH 3

CH 3

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