Question

Reactivity of $HCHO$ with the given Grignard reagent in the decreasing order:

a.) (iv) ˃ (iii) ˃ (ii) ˃ (i)
b.) (i) ˃ (ii) ˃ (iii) ˃ (iv)
c.) (iii) ˃ (ii) ˃ (i) ˃ (iv)
d.) (ii) ˃ (iii) ˃ (i) ˃ (iv)

Answer 1

Alkyl or aryl magnesium halide is known as Grignard reagent (RMgX). We can synthesize Grignard reagents by the reaction of magnesium metal with alkyl or alkenyl halides. Grignard reagent acts as a good nucleophile, reacts with electron deficient carbons like carbonyl compounds and etc.

Complete step by step answer:
Here we are supposed to find the reactivity of formaldehyde towards the given Grignard reagents.
We know that in Grignard reagent, the hydrocarbon part acts as a nucleophile from its starting material.
i. $PhMgBr\xrightarrow[{}]{}P{{h}^{-}}$
The benzyl magnesium bromide gives the following nucleophile from its starting material.

ii. $PhC{{H}_{2}}MgBr\xrightarrow{{}}PhC{{H}_{2}}^{-}$

The allyl magnesium bromide gives the following nucleophile from its starting material.

The vinyl magnesium bromide gives the following nucleophile from its starting material.

Now among the four Grignard reagents the benzyl anion (ii) is more nucleophilic towards formaldehyde due to its stability and resonance.
After that allyl anion (iii) is more nucleophilic towards formaldehyde due to its stability and resonance.
After that phenyl anion (i) is more nucleophilic towards formaldehyde.
After that vinyl anion (iv) is more nucleophilic towards formaldehyde.
Therefore the decreasing reactivity order of Grignard reagent towards formaldehyde is as follows.
(ii) ˃ (iii) ˃ (i) ˃ (iv)
So, the correct answer is “Option D”.

Note: In the Grignard reagent the hydrocarbon part will have a negative charge and MgBr gets positive charge. Due to the negative charge on the hydrocarbon part it acts as a good nucleophile.