Question

There are three isomers of dichlorobenzene, one of which has now replaced naphthalene as the main constituent of mothballs.

(a) Identify the ortho, meta, and para isomers of dichlorobenzene.

(b) Predict the number of isomers for trichlorobenzene.

(c) The presence of one chlorine atom on a benzene ring will cause the next substituent to add ortho orpara to the first chlorine atom on the benzene ring. What does this tell you about the synthesis of m-dichlorobenzene?

(d) Which of the isomers of trichlorobenzene will be the hardest to prepare?

Short answer

a) The structures of dichlorobenzeneortho, meta, and para (3) isomers:

b) The number of isomers for trichlorobenzene is three

c) The synthesis of meta-dichlorobenzene cannot be accomplished in much quantity by a direct benzene-chlorine reaction because the electrophilic substitution favors the ortho/para substitution.

d) The 1,3,5-trichlorobenzene would be the most difficult to prepare.

Step-by-step solution

Subpart (a)The ortho, meta, and para isomers of dichlorobenzene.

Electrophilic aromatic substitution is a category of reactions that comprises a wide range of electrophilic reagent replacements.

The prefixes ortho(o), meta (m), and para(p)denote the relative location of substituents in double-substituted benzenes.

In ortho-substituted benzenes, the substituent occupies a 1,2-position.

In meta-substituted benzenes, it is 1,3, while inpara-substituted benzenes, it is1,4.

The structures of dichlorobenzene ortho, meta, and para isomers are depicted in the diagram below.

Subpart (b) The number of isomers for trichlorobenzene.

There are three isomers of trichlorobenzene as shown below:

Subpart (c) The synthesis of m-dichlorobenzene.

The yield of meta-dichlorobenzene in the product combination is rather minorcompared to the yields of ortho and para isomers.

The electrophilic substitution favors the ortho/para substitution.

The substitution of electrophile at ortho and meta position forms a tertiary carbocation once during the resonance. Hence a stable carbocation gives a stable product. The product meta-substituted is unstable.

As a result, the synthesis of meta-dichlorobenzene cannot be accomplished in good quantity by the direct interaction of benzene with chlorine; instead, a different reaction route must be utilized.

Subpart (d) The hardest to prepare among isomers of trichlorobenzene.

All threechlorine atoms are meta-substituted with respect to each other. Hence, 1,3,5-trichlorobenzene would be the most difficult to manufacture.

The electrophilic substitution favors the ortho/para substitution.