Question

The reactivity order of compounds with KI in acetone is?

A.iii > iv > ii > i
B.ii > iii> iv > i
C.iv > ii > iii > i
D.iv > iii > ii > i

Answer 1

Electron withdrawing substituent decreases the electron density and increases the electrophilicity of the carbon.

Complete Step by step answer:
All the compounds have halogen attached in the aliphatic part of the molecule. The difference among the four compounds which is shown below.
The compound (i), the halide attached with electron donating methyl group in its adjacent.
The compound (ii), the halide attached with electron donating methoxy group in its adjacent
The compound (iii), is benzyl chloride the presence of pi electron cloud aryl ring.
The compound (iv), presence of electron withdrawing carbonyl group
The reaction is the attack of Iodine to the electrophilic carbon and replaces the chlorine. It is an example of nucleophilic substitution reaction. The SN2 type reaction will undergo the polar aprotic solvent and in concerted mechanism. If a nucleophile is going to attack, the carbon center should be electropositive in nature. Hence the electron withdrawing group attached molecule (iv) will have higher reactivity.
All other molecules have electron donating substituent in the carbon center where the Iodine attack happens. Among the other three molecules, two alkyl substituted $2^\circ$ alkyl halide compounds (i) are least reactive.
In compounds (iii), pi electron clouds are more and make electron rich electrophilic C-center. The lone pair electron present in oxygen moves towards the carbon attached with chlorine makes the C electron rich, but lesser than the phenyl ring.
Hence the answer is option (C).
The reactivity order of compounds with KI in acetone will follow as (iv) > (ii) > (iii) > (i).

Note: An electrophilic substitution occurs by the attack of an electrophile. The characteristic reactions of aromatic compounds involve electrophilic substitution in which resonance – stabilised ring system is preserved.