Organic Chemistry

Section 2.1 Nomenclature of Alkyl Substituents 63

There are nine alkanes with molecular formula We can name only two of

them (heptane and isoheptane) without defining new structural units. Notice that neo-

heptane cannot be used as a name because three different heptanes have a carbon that

is bonded to four other carbons and a name must specify only one compound.

The number of constitutional isomers increases rapidly as the number of carbons in

an alkane increases. For example, there are 75 alkanes with molecular formula

and 4347 alkanes with molecular formula To avoid having to memorize the

names of thousands of structural units, chemists have devised rules that name com-

pounds on the basis of their structures. That way, only the rules have to be learned. Be-

cause 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 nomenclaturebecause it was designed by a commission of the International

Union of Pure and Applied Chemistry (abbreviated IUPAC and pronounced “eye-you-

pack”) at a meeting in Geneva, Switzerland, in 1892. The IUPAC rules have been con-

tinually revised by the commission since then. Names such as isobutane and

neopentane—nonsystematic names—are called common namesand are shown in red

in this text. The systematic or IUPAC names are shown in blue. Before we can under-

stand how a systematic name for an alkane is constructed, we must learn how to name

alkyl substituents.

2.1 Nomenclature of Alkyl Substituents

Removing a hydrogen from an alkane results in an alkyl substituent(or an alkyl

group). Alkyl substituents are named by replacing the “ane”ending of the alkane with

“yl.”The letter “R”is used to indicate any alkyl group.

CH 3

a methyl group

CH 3 CH 2

an ethyl group

CH 3 CH 2 CH 2

a propyl group

CH 3 CH 2 CH 2 CH 2

a butyl group

R

any alkyl group

C 15 H 32.

C 10 H 22

CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3

heptane

heptane

common name:

systematic name:

CH 3 CHCH 2 CH 2 CH 2 CH 3

isoheptane

2-methylhexane

CH 3

CH 3 C CHCH 3

2,2,3-trimethylbutane

CH 3

CH 3 CH 3

CH 3 CH 2 CHCH 2 CH 2 CH 3

3-methylhexane 2,3-dimethylpentane

CH 3 CH CHCH 2 CH 3

CH 3 CH 3

2,4-dimethylpentane

CH 3 CHCH 2 CHCH 3

CH 3 CH 3 CH 3

CH 3 CCH 2 CH 2 CH 3

2,2-dimethylpentane

CH 3

CH 3

CH 3 CH 2 CCH 2 CH 3

3,3-dimethylpentane

CH 3

CH 3

CH 3 CH 2 CHCH 2 CH 3

3-ethylpentane

CH 2 CH 3

C 7 H 16.

A compound can have more than one

name, but a name must specify only one

compound.

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