91Ó°ÊÓ

To obtain a particular organic end-product with multiple substitutions on benzene, the working backward method is used by deconstructing the target molecule. This gives an idea about the conditions or reagents required.

For example, the carbonyl group can be obtained at ortho or para position by acylation on benzene. However, for obtaining a meta product, oxidation of alkene is preferable.

a. Working backward:

1. Add methyl group by Friedel-Craft alkylation.
2. Substitute the ketone by Friedel-Craft acylation.
3. Substitute the ${\text{SO}}_{\text{3}}$ group on the ortho isomer.

Synthesis process of a

b. Working backward:

• Bromine and alkyl groups are ortho, para directing, but in the product, they are at meta positions. Therefore, acylation is not possible.

The alkyl group can be formed by the reduction of carbonyl.

c. Working backward:

• The aldehyde group can be formed by reducing alcohol.
• Hydroboration-oxidation leads to the formation of alcohol from an alkene.
• Alkenes can be obtained by eliminating HBr from the benzylic halide.
• Benzylic halide can be synthesized by the radical bromination of alkylbenzene.
• Alkylation of benzene results in the formation of alkylbenzene.
• Chlorine can be directly substituted for benzene using the chlorinating agent.