Q55E - Organic Chemistry (Mcmurry)

Question

How would you synthesize the following substances starting from benzene or phenol? Assume that ortho- and para-substitution products can

be separated.

(a) o-Bromobenzoic acid (b) p-Methoxytoluene

(c) 2,4,6-Trinitrobenzoic acid (d) m-Bromoaniline

Step-by-Step Solution

Answer

Answer

Step-by-step Solution Step 1: Isomers

Isomers are molecules or polyatomic ions having identical molecular formulas but the distinct arrangement of atoms in space.

In organic chemistry ortho, meta and para are isomers of a compound generally cyclic in nature. They are used to indicate the position of non-hydrogen substituents on a hydrocarbon.

Ortho, para and meta substituent occupies position corresponding to R in the diagram

Step 2: Synthesis of compound (a) and (b)

It is necessary to introduce substituents in the proper order when synthesizing substituted aromatic rings. A group that is introduced out of order will not have the proper directing effect. 

Mostly in these types of reactions, a mixture of ortho and para isomer may be formed.

(a) Treatment of benzene with methyl chloride in the presence of Lewis acid and catalyst, gives toluene, which reacts with bromine in the presence of AlBr₃, gives 1-Bromo-2-methylbenzene. Now, the methyl group undergoes oxidation with KMnO₄ and yields the required product as shown below:

                                  Synthesis of o-bromobenzoicacid

(b) Treatment of phenol with methyl chloride in the presence of Lewis acid and catalyst, gives p-methyltoluene, which reacts with NaOH in the presence of CH₃Br₃, gives 1-p-methoxytoluene. 

                                     Synthesis of p-methoxytoluene

 

The reaction (b) can be performed in either order.

Step 3: Synthesis of compound (c) and (d)

(c)Treatment of benzene with methyl chloride in the presence of Lewis acid and catalyst, gives toluene, which reacts with nitric acid in the presence of sulphuric acid, gives 2,4,6-trinitrobenzoic acid. Now, the compound undergoes oxidation with KMnO₄ and yields the required product as shown below

                              Synthesis of 2,4,6-trinitrobenzoic acid

(d) Treatment of benzene with nitric acid in the presence of sulphuric acid, gives p- nitrobenzoic acid, which further react with bromine molecule in the presence of FeBr₃ and after hydrolysis, we obtain the desired product as shown below:

                                        Synthesis of m-bromoaniline