Question

Among the following, the one which reacts most readily with ethanol is:
A. p-Nitrobenzyl bromide
B. p-Chloro benzyl bromide
C. p-Methoxy benzyl bromide
D. p-Methyl benzyl bromide

Answer 1

Hint: Here, all the given compounds are benzene substituted compounds. We have to understand the effects of each of the compounds such as inductive effect, electromeric effect, etc.

Complete Step by Step Answer:
Let’s understand the inductive effect (+I effect)and mesomeric effect (+M)in detail. It describes an effect in which polarity is results in a compound because of the interaction of a \[\pi \] bond and a lone pair or due to the interaction of two \[\pi \] bonds in neighboring atoms. The resonance stabilization of \[{\rm{C}}{{\rm{H}}_{\rm{2}}} = {\rm{CH}} - {\rm{Cl}}\]is shown as follows:

\[{\rm{C}}{{\rm{H}}_{\rm{2}}} = {\rm{CH}} - {\rm{Cl}} \to {}^ - {\rm{C}}{{\rm{H}}_{\rm{2}}} - {\rm{CH}} = {\rm{C}}{{\rm{l}}^ + } \to {}^ + {\rm{C}}{{\rm{H}}_{\rm{2}}} - {\rm{CH}} = {\rm{C}}{{\rm{l}}^ - }\]

The mesomeric effect defines the development of polarity between atoms in a conjugated system because of the transfer of electrons of the pi bond.
Now, come to the question. Option A is p-nitrobenzyl bromide. The structure is,

Here, the Nitro group is an electron-withdrawing group. Therefore, -I and –M effects were observed in the compound.
Option B is p-chloro-benzyl bromide. The structure is,


In the above compound, -I and +M effect was observed.
The third option is p-methoxyl benzyl bromide.

Here also, due to the electron-withdrawing nature of the methoxy group, the effect observe are –I and +M.
The fourth option is p-methyl benzyl bromide.

Due to the methyl group which is electron donating in nature, the effect observed is +I effect.
Option 1 and 4 is wrong because of +M and +I effect respectively.
Now, the remaining options are 2 and 4. Both have the same effect but the size of the oxygen atom is smaller than the chlorine atom and when the halogen atom is removed, the lone pair on the oxygen atom undergoes hyperconjugation to stabilise the compound. Therefore, p-methoxy benzyl chloride undergoes a reaction with ethanol easily.
Hence, option C is right.

Note: It is to be noted electromeric effect defines a complete shifting of pi electrons in the presence of a reagent. The nature of this effect is temporary. When the reagent is removed, the pi bond again gets reformed.