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E d+ d+ 16 d+

16 The Chemistry of Benzene and Its Derivatives

As we learned in Chapter 15, benzene and its derivatives are aromatic compounds. In this chapter, we’ll learn how aromaticity affects the spectroscopic properties and the reactivity of benzene and its derivatives. In particular, we’ll learn that benzene and its derivatives do not un- dergo most of the usual addition reactions of alkenes. Instead, they undergo a type of reaction called electrophilic aromatic substitution, in which a ring hydrogen is substituted by another group. Such substitution reactions can be used to prepare a variety of substituted benzenes from benzene itself. Most of this chapter is concerned with the substitution reactions of ben- zene and its derivatives. Chapters 17 and 18 consider other aspects of aromatic chemistry.

16.1 NOMENCLATURE OF BENZENE DERIVATIVES

The nomenclature of benzene derivatives follows the same rules used for other substituted hy- drocarbons:

Cl NO2 CH2CH3 L L L

chlorobenzene nitrobenzene ethylbenzene

The nitro group, abbreviated NO2, which is a part of the nitrobenzene structure shown here, may be less familiar than Lthe other substituent groups. The nitro group can be repre-

sented in more detail as a resonance hybrid of two equivalent dipolar structures: 1

O 1 O _ 3 3 3 R ||N* R N) R NO2 L L = L $O _ @O 3 1 3 3 3

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16.1 NOMENCLATURE OF BENZENE DERIVATIVES 741

Some monosubstituted benzene derivatives have well-established common names that should be learned.

CH3 CHA CH2 OH OCH3 L L L L

toluene styrene phenol anisole

The positions of substituent groups in disubstituted benzenes can be designated in two ways. Modern substitutive nomenclature utilizes numerical designations in the same manner as that for other compound classes. However, an older system, which is still used, employs special let- ter prefixes. The prefix o (for ortho) is used for substituents in a 1,2-relationship; m (for meta) for substituents in a 1,3-relationship; and p (for para) for substituents in a 1,4-relationship.

Cl Br

Cl " " M

" M F " I

i i i NO2 o-dichlorobenzene m-bromonitrobenzene p-fluoroiodobenzene 1,2-dichlorobenzene 1-bromo-3-nitrobenzene 1-fluoro-4-iodobenzene

As these examples illustrate, when none of the substituents qualifies as a principal group, the substituents are cited and numbered in alphabetical order. In contrast, if a substituent is eligi- ble for citation as a principal group, it is assumed to be at carbon-1 of the ring.

OH " M

i NO2 m-nitrophenol (3-nitrophenol) —OH group is the principal group

Some disubstituted benzene derivatives also have time-honored common names. The di- methylbenzenes are called xylenes, and the methylphenols are called cresols.

CH3 OH

CH3 " " M M r i CH3 o-xylene m-cresol

The hydroxyphenols also have important common names.

HO OH HO ) ) ) OH HO OH vvL LvL catechol resorcinol hydroquinone 16_BRCLoudon_pgs4-3.qxd 11/26/08 9:06 AM Page 742

742 CHAPTER 16 • THE CHEMISTRY OF BENZENE AND ITS DERIVATIVES

When a benzene derivative contains more than two substituents on the ring, the o, m, and p designations are not appropriate; only numbers may be used to designate the positions of sub- stituents. The usual nomenclature rules are followed (Secs. 2.4C, 4.2A, 8.1).

F OH 3 1

2 F " M ) M CH CH O NO 3 2 2 5 2 i1 LcL Br 34 alphabetical citation: bromodifluoro 2-ethoxy-5-nitrophenol numbering: 1,2,3 name: 1-bromo-2,3-difluorobenzene

Sometimes it is simpler to name a benzene ring as a substituent group. A benzene ring or substituted benzene ring cited as a substituent is referred to generally as an aryl group; this term is analogous to alkyl group in nonaromatic compounds (Sec. 2.9B). When an unsubsti- tuted benzene ring is a substituent, it is called a phenyl group. This group can be abbreviated

Ph . It is also sometimes abbreviated by its group formula, C6H5 . L L

O C6H5 O C6H5 PhO Ph ccLL LL LL

diphenyl ether (phenoxybenzene) three different ways to write the structure

The Ph—CH2— group is called the benzyl group.

PhCH2 Cl L benzyl chloride or (chloromethyl)benzene

Carefully note the difference between the phenyl group, Ph ,andthebenzyl group,

Ph CH2 .Somestudentserroneouslythinkbothofthesenamesrefertothephenylgroup.L L L

PROBLEMS 16.1 Name the following compounds.

(a) CH2CH3 (b) CH2CH3 (c)

H2C A CH NO2

CH2CH3 " M " LcL M i i Cl

(d) OH (e) F Cl (f)

M M CH Cl M M 2 " i ccLL M I Br i "NO2 "Cl 16.2 Draw the structure of each of the following compounds. (a) p-chlorophenol (b) m-nitrotoluene (c) 3,4-dichlorotoluene (d) 1-bromo-2-propylbenzene (e) methyl phenyl ether (f) benzyl methyl ether (g) p-xylene (h) o-cresol (i) 2,4,6-trichloroanisole (a compound present in tainted wine corks)