Question

Explain giving two reasons why haloarenes are less reactive towards nucleophilic substitution reactions than haloalkanes.

Hint:

Resonance stabilization of the C–X bond and sp\(^2\) hybridisation make haloarenes much less reactive than haloalkanes in nucleophilic substitution reactions.

Answer 1

Reason 1: Partial double bond character of C–X bond In haloarenes, the lone pair of electrons on the halogen is involved in resonance with the aromatic ring. This gives the C–X bond partial double bond character, making it shorter and stronger than the C–X bond in haloalkanes. As a result, the bond is difficult to break during nucleophilic substitution. Reason 2: Instability of carbocation / SN\(_2\) hindrance \begin{itemize} \item Haloarenes do not undergo SN\(_1\) reactions because the phenyl carbocation formed is highly unstable. \item SN\(_2\) reactions are also difficult because the carbon atom bonded to halogen is sp\(^2\)-hybridised and the planar aromatic ring hinders backside attack by the nucleophile. \end{itemize}