Question

Reaction of alkyl halides with aromatic compounds in presence of anhydrous \[{\rm{AlC}}{{\rm{l}}_{\rm{3}}}\] is known as:
A. Friedel-Craft reaction
B. Hoffmann degradation
C. Kolbe’s synthesis
D. Beckmann rearrangement

Answer 1

Hint: It is one of the most important methods for electrophilic substitution reactions of arenes (aromatic compounds). Aluminium chloride acts as an acid catalyst.

Complete Step by Step Solution:
Friedel-Crafts reaction can be divided into two parts which are Friedel-Crafts alkylation and Friedel-Craft’s acylation. Friedel-Crafts alkylation reaction involves the introduction of an alkyl group in the benzene ring for the synthesis of alkylbenzenes which are not generally available. The alkylating agent used in Friedel-Craft’s alkylation is an alkyl halide. The catalyst employed is a Lewis acid which may be \[{\rm{AlC}}{{\rm{l}}_{\rm{3}}}\], \[{\rm{B}}{{\rm{F}}_{\rm{3}}}\], \[{\rm{FeC}}{{\rm{l}}_{\rm{3}}}\]etc. Friedel-Crafts acylation reaction involves the introduction of an acyl group \[({\rm{RCO}} - )\] in the aromatic ring in the presence of anhydrous \[{\rm{AlC}}{{\rm{l}}_{\rm{3}}}\] or other Lewis acid catalysts like \[{\rm{B}}{{\rm{F}}_{\rm{3}}}\], \[{\rm{FeC}}{{\rm{l}}_{\rm{3}}}\], \[{\rm{ZnC}}{{\rm{l}}_{\rm{2}}}\]etc. The acylating agents employed are acid chlorides, acid anhydrides and esters. Hence, option (A) is correct.

Hoffman degradation is a simple laboratory method for conversion of an amide to a primary amine having one less carbon atom. The amide is treated with bromine \[{\rm{(B}}{{\rm{r}}_2})\] in the presence of alkali \[{\rm{(NaOH)}}\]. Hence, option (B) is incorrect.

In Kolbe’s reaction, sodium phenoxide on heating to \[{\rm{400}}\,\,{\rm{K}}\]with \[{\rm{C}}{{\rm{O}}_2}\]under pressure \[{\rm{(6 - 7}}\,\,{\rm{atm)}}\]yields sodium salicylate. This on acidification gives salicylic acid. Hence, option (C) is incorrect.
In Beckmann’s rearrangement reaction, a ketoxime is treated with an acidic catalyst such as sulphuric acid, polyphosphoric acid, thionyl chloride, benzene sulphonyl chloride etc., where it gets converted into a substituted amide. Hence, option (D) is incorrect.
Therefore, option A is correct.

Note: Friedel-Crafts acylation reaction can be used in preference to Friedel-Crafts alkylation as it is free from the two chief drawbacks of the later; skeletal rearrangement and polysubstituted.