Question

The reactivity order of the following molecules towards SN1 reaction is:
Allyl chloride (I), Chlorobenzene (II), Ethyl chloride (III)

• A. I>II>III
• B. I>III>II
• C. II>I>III
• D. III>I>II

Answer 1

The Correct Option is D

The reactivity order towards an \(S_N1\) reaction is determined by the stability of the carbocation intermediate.

Allyl chloride forms an allyl carbocation, which is resonance-stabilized, making it more stable.

Ethyl chloride forms a primary carbocation, which is less stable than the allyl carbocation.

Chlorobenzene, however, does not undergo \(S_N1\) reactions readily because the carbocation would be destabilized due to the aromatic ring.

Thus, the reactivity order is III>I>II.

Answer 2

Correct Answer:

Option 2: I > III > II

Explanation:

SN1 Reaction Mechanism:

The SN1 reaction involves the formation of a carbocation intermediate. The stability of the carbocation determines the reaction rate.

Analysis of Molecules:

• Allyl chloride (I):
  Forms an allylic carbocation (CH₂=CH-CH₂⁺), which is stabilized by resonance. This is the most stable carbocation among the given options.
• Chlorobenzene (II):
  Forms a phenyl carbocation, which is highly unstable. It is very difficult to form, resulting in a very slow SN1 reaction.
• Ethyl chloride (III):
  Forms a primary carbocation (CH₃CH₂⁺), which is relatively unstable compared to the allylic carbocation but more stable than the phenyl carbocation.

Reactivity Order:

The reactivity order is determined by the stability of the carbocation formed. Thus, the order is I > III > II.