The Foundations of Chemistry

OTHER AROMATIC HYDROCARBONS

Benzene molecules bearing alkyl substituents are called alkylbenzenes.The simplest of

these is methylbenzene (common name, toluene), shown in Figure 27-11c. The dimethyl-

benzenes are called xylenes. Three different compounds (Table 27-6) have the formula

C 6 H 4 (CH 3 ) 2 (see margin). These three xylenes are constitutional isomers.In naming these

(as well as other disubstituted benzenes), we use prefixes ortho- (abbreviated o-), meta-

(m-), or para- (p-) to refer to relative positions of substituents on the benzene ring. The

ortho- prefix refers to two substituents located on adjacentcarbon atoms; for example,

1,2-dimethylbenzene is o-xylene. The meta- prefix identifies substituents on C atoms 1

and 3, so 1,3-dimethylbenzene is m-xylene. The para- prefix refers to substituents on

C atoms 1 and 4, so 1,4-dimethylbenzene is p-xylene.

Summary of Rules for Naming Derivatives of Benzene

1.If there is only one group on the ring, no number is needed to designate its posi-

tion.

2.If there are two groups on the ring, we use the traditional designations.

ortho- or o- for 1,2-disubstitution

meta- or m- for 1,3-disubstitution

para- or p- for 1,4-disubstitution

3.If there are three or more groups on the ring, location numbers are assigned to

give the minimum sumof numbers.

Examples are

When an H atom is removed from a benzene molecule, C 6 H 6 , the resulting group,

C 6 H 5 Xor , is called “phenyl.” Sometimes we name mixed alkyl-aromatic

hydrocarbons on that basis.

C C

CH 3 CH 3

H

2

1

3

4

phenylcyclohexane 2-phenyl-cis-2-butene

H

H

CH 2 CH 3

H

H

H

ethylbenzene

H

H

CH 2 CH 3

CH 2 CH 3

H

H

m-diethylbenzene, or

1,3-diethylbenzene

H

H

CH 2 CH 3

CH 2 CH 3

CH 2 CH 3

H

1,2,4-triethylbenzene

27-6

1062 CHAPTER 27: Organic Chemistry I: Formulas, Names, and Properties

CH 3

CH 3

4

5

6

3

2

1

para-xylene

bp  138 C

mp  13 C

CH 3

CH 3

4

5

6

3

2

1

CH 3

CH 3

4

5

6

3

2

1

ortho-xylene

bp  144 C

mp   27 C

meta-xylene

bp  139 C

mp   54 C