Organic Chemistry

624 CHAPTER 16 Reactions of Substituted Benzenes

16.1 Nomenclature of Disubstituted

and Polysubstituted Benzenes

Disubstituted Benzenes

The relative positions of two substituents on a benzene ring can be indicated either by

numbers or by the prefixes ortho, meta, and para. Adjacent substituents are called

ortho, substituents separated by one carbon are called meta, and substituents located

opposite one another are designated para. Often, only their abbreviations (o,m,p) are

used in naming compounds.

If the two substituents are different, they are listed in alphabetical order. The first

stated substituent is given the 1-position, and the ring is numbered in the direction that

gives the second substituent the lowest possible number.

If one of the substituents can be incorporated into a name (Section 15.7), that name is

used and the incorporated substituent is given the 1-position.

A few disubstituted benzenes have names that incorporate both substituents.

CH 3

NH 2

CH 3 CH 3

OH

para-cresol

used as a wood preservative

until prohibited for

environmental reasons

ortho-toluidine meta-xylene

CH 3

Cl

CH 3

NO 2

NH 2 OH

2-chlorotoluene

ortho-chlorotoluene

not

ortho-chloromethylbenzene

CH 2 CH 3

4-nitroaniline

para-nitroaniline

not

para-aminonitrobenzene

2-ethylphenol

ortho-ethylphenol

not

ortho-ethylhydroxybenzene

Cl

I

1-chloro-3-iodobenzene

meta-chloroiodobenzene

not

1-iodo-3-chlorobenzene or

meta-iodochlorobenzene

Br

NO 2 CH 2 CH 3

Cl

1-bromo-3-nitrobenzene

meta-bromonitrobenzene

1-chloro-4-ethylbenzene

para-chloroethylbenzene

Br

Br

Br Br

Br

Br

1,4-dibromobenzene

para-dibromobenzene

p-dibromobenzene

1,3-dibromobenzene

meta-dibromobenzene

m-dibromobenzene

1,2-dibromobenzene

ortho-dibromobenzene

o-dibromobenzene

3-D Molecules:

ortho-Toluidine;

meta-Xylene;

para-Cresol