The relative reactivity order of following halides towards \(S_N1\) reaction is:
(I) 2-Chloro-2-methylpropane
(II) Chloroethane
(III) Chlorormethane
(IV) Chlorodiphenylmethane
Answer & explanation
Correct answer: option 1
The correct answer is option 1. IV > I > II > III.
Let us explain why the correct reactivity order of the halides towards an \(S_N1\) reaction is IV > I > II > III in detail.
Understanding \(S_N1\) Reactions:
The \(S_N1\) (substitution nucleophilic unimolecular) reaction involves two key steps:
Formation of a carbocation: The halide ion leaves, forming a carbocation intermediate.
Nucleophilic attack: The nucleophile then attacks the carbocation to form the product.
The rate of an \(S_N1\) reaction is determined by the stability of the carbocation intermediate. The more stable the carbocation, the more reactive the halide is in the \(S_N1\) reaction.
Analysis of Each Halide:
IV. Chlorodiphenylmethane:
Carbocation Formed: Benzylic carbocation (a carbocation adjacent to a phenyl group).
Stability: The benzylic carbocation is highly stabilized by resonance with the phenyl rings. The resonance delocalizes the positive charge over the aromatic system, making this carbocation very stable.
Reactivity: Due to this high stability, chlorodiphenylmethane is extremely reactive in \(S_N1\) reactions. Hence, it ranks as the most reactive.
I. 2-Chloro-2-methylpropane:
Carbocation Formed: Tertiary carbocation.
Stability: The tertiary carbocation is stabilized by hyperconjugation and inductive effects from three alkyl groups, making it quite stable, but not as stable as the benzylic carbocation.
Reactivity: The stability of the tertiary carbocation makes 2-chloro-2-methylpropane very reactive in \(S_N1\) reactions, but it is less reactive than chlorodiphenylmethane.
II. Chloroethane:
Carbocation Formed: Primary carbocation (ethyl carbocation).
Stability: The primary carbocation is less stable compared to tertiary and benzylic carbocations due to the absence of significant alkyl group stabilization.
Reactivity: Since primary carbocations are less stable, chloroethane is less reactive in \(S_N1\) reactions compared to the tertiary and benzylic systems.
III. Chloromethane:
Carbocation Formed: Methyl carbocation.
Stability: The methyl carbocation is the least stable because it has no alkyl groups to stabilize the positive charge and is very reactive.
Reactivity: Due to its extreme instability, chloromethane is the least reactive in \(S_N1\) reactions.
Conclusion:
The reactivity order based on the stability of the carbocation intermediates is:
Chlorodiphenylmethane (IV): Most stable carbocation due to resonance stabilization.
2-Chloro-2-methylpropane (I): Stable tertiary carbocation, but less so compared to benzylic.
Chloroethane (II): Less stable primary carbocation.
Chloromethane (III): Least stable methyl carbocation.
Thus, the reactivity order for \(S_N1\) reactions is indeed IV > I > II > III, and option 1 is the correct choice.