Question

Which compound undergoes SN1 reaction faster?

Answer 1

Why Benzyl Chloride Undergoes SN1 Reaction Faster Than Cyclohexyl Chloride

The compound with the benzene ring, benzyl chloride (\({C6H5CH2Cl}\)), undergoes the SN1 reaction faster compared to cyclohexyl chloride. This is due to the difference in the stability of the carbocation intermediates formed during the reaction.

Benzyl Chloride:

When the chloride ion (\({Cl-}\)) departs from benzyl chloride, a benzyl carbocation (\({C6H5CH2^+}\)) is formed. This carbocation is highly stabilized by resonance with the aromatic ring. The positive charge on the carbon is delocalized over the ring, making the intermediate stable and favorable for the SN1 mechanism, where the rate-determining step is the formation of the carbocation.

Resonance stabilization:

\[ {C6H5CH2^+} \xrightarrow{\text{Resonance}} {C6H5CH2^+} \]

Cyclohexyl Chloride:

In contrast, when cyclohexyl chloride undergoes the SN1 reaction, the cyclohexyl carbocation (\({C6H11^+}\)) formed does not benefit from resonance stabilization. The positive charge on the carbon in the cyclohexyl carbocation is localized, making it less stable. This instability makes the formation of the carbocation slower, and as a result, cyclohexyl chloride does not undergo the SN1 reaction as easily as benzyl chloride.

Conclusion: The benzyl carbocation is stabilized by resonance with the aromatic ring, making the SN1 reaction faster for benzyl chloride compared to cyclohexyl chloride, which does not have such stabilization.