Q5 P - Organic Chemistry (Mcmurry)

Question

Which of the following alkyl halides would you expect to undergo Friedel–

Crafts reaction with rearrangement and which without? Explain.

(a)  ${\mathbf{CH}}_3{\mathbf{CH}}_2\mathbf{Cl}$

(b) ${\mathbf{CH}}_3{\mathbf{CH}}_2\mathbf{CH}\left(\mathbf{Cl}\right){\mathbf{CH}}_3$

(c)  ${\mathbf{CH}}_3{\mathbf{CH}}_2{\mathbf{CH}}_2\mathbf{Cl}$

(d)  ${\left({\mathbf{CH}}_3\right)}_3{\mathbf{CCH}}_2\mathbf{Cl}$

(e) Chlorocyclohexane

Step-by-Step Solution

Answer

(a)This alkyl halide does not undergo rearrangement during Friedelcraft’s reaction.

(b) This alkyl halide does not undergo rearrangement during Friedelcraft’s reaction.

(c) This alkyl halide undergo rearrangement during Friedelcraft’s reaction.

(d) This alkyl halide undergo rearrangement during Friedelcraft’s reaction.

(e) This alkyl halide does not undergo rearrangement during Friedelcraft’s reaction

Step 1: FriedelCraft’s reaction

FriedelCrafts reaction possesses several limitations. One limitation is that only alkyl halides can be used for this particular reaction. Aryl and vinyl halides do not react as their carbocations possess high energy to react under these conditions.

Step 2: Alkyl halides that undergo Friedelcraft’s reaction with and without rearrangment

(a)In this reaction benzene reacts with $C{H}_{3}C{H}_{2}Cl$(ethyl chloride ) and $AlC{l}_3$ to form ethylbenzene. The reaction can be given as:

                                           Reaction (a)

In this FriedelCraft’s reaction, the reaction occurs without rearrangement as halogen substituted carbons give more stable carbocations.

(b)In this reaction, benzene reacts with 2-chlorobutane and $AlC{l}_3$to produce sec-butylbenzene. The reaction can be given as:

                                                   Reaction (b)

This reaction happens without rearrangement.

(c)In this reaction, benzene reacts with n-chloropropane in the presence of $AlC{l}_3$to form sec-Butylbenzene. The reaction of n-chloropropane with $AlC{l}_3$generates a primary carbocation. This undergoes rearrangement to form secondary carbocation. The reaction can be given as:

                                        Reaction (c)

(d)The reaction of 1-chloro-2,2-dimethylpropane with $AlC{l}_3$generates a primary carbocation that rearranges to a secondary carbocation. Benzene reacts with this secondary carbocation in the presence of $AlC{l}_3$ to generate tert-pentyl benzene. The reaction can be given as:

                                           Reaction (d)

(e)In this reaction benzene reacts with chlorocyclohexane to produce cyclohexylbenzene as the product. The reaction can be given as:

                                         Reaction (e)

This reaction happens without rearrangement.