The correct answer is (1) (IV) > (III) > (II) >(I)
The order of reactivity in \(S_N1\) reactions is determined by the stability of the carbocation intermediate formed during the reaction. A more stable carbocation is more likely to form, leading to a faster reaction.
(I) Chlorobenzene:
Chlorobenzene does not undergo \(S_N1\) reactions readily because it does not contain a leaving group (like a halide) that can easily dissociate. In \(S_N1\) reactions, the first step involves the formation of a carbocation through the departure of a leaving group. In chlorobenzene, the chlorine atom is not a good leaving group because it does not readily leave the molecule to form a carbocation. Therefore, chlorobenzene is the least reactive among the given compounds.
(II) Benzyl chloride:
Benzyl chloride contains a benzyl group ($\text{C}_6\text{H}_5\text{CH}_2-$) attached to a chlorine atom. The presence of the benzyl group stabilizes the carbocation that would be formed during the \(S_N1\) reaction. The benzyl carbocation is resonance-stabilized by the delocalization of the positive charge onto the aromatic ring. This resonance stabilization makes benzyl chloride more reactive than chlorobenzene.
(III) Benzyl bromide:
Benzyl bromide is similar to benzyl chloride but contains a bromine atom instead of a chlorine atom. Bromine is a better-leaving group than chlorine because it is larger and has a more polarizable electron cloud. This means that the bromine atom can more easily dissociate from the molecule to form a carbocation. Therefore, benzyl bromide is more reactive than benzyl chloride.
(IV) Benzyl iodide:
Benzyl iodide contains an iodine atom, which is an excellent leaving group. Iodine is much larger and more polarizable than chlorine or bromine, making it very prone to leaving the molecule. As a result, benzyl iodide is the most reactive among the given compounds. The carbocation formed from benzyl iodide is also highly stabilized by resonance, similar to benzyl chloride and benzyl bromide.
So, the correct order of reactivity is $(IV) > (III) > (II) > (I)$, with benzyl iodide being the most reactive and chlorobenzene being the least reactive in \(S_N1\) reactions. |
