Question Bank [2022] — Chemistry STD 12 Science — Question
Maharashtra BoardEnglish MediumSTD 12 ScienceChemistryQuestion Bank [2022]2 Marks
Question
Explain. Aryl halides are less reactive than alkyl halides towards nucleophilic substitution reactions.
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Answer
The low reactivity of aryl halides is due to the resonance effect and $sp ^2$ hybrid state of carbon to which halogen atom is attached.
In aryl halides, one of the lone pairs of electrons on the halogen atom is in conjugation with $\pi$-electrons of the ring. Due to resonance, the $C - X$ bond acquires partial double bond character. Thus, the $C - X$ bond in aryl halides is stronger and shorter than alkyl halides. Hence, it is difficult to break $C - X$ bond in aryl halides.
Further, the phenyl cation produced due to the self-ionization of aryl halide will not be stabilised by resonance. This rules out the possibility of $S_N 1$ mechanism. Also, the backside attack of nucleophiles is blocked by the aromatic ring. This rules out the possibility of $S_N 2$ mechanism. As a result, nucleophilic substitution reaction involving cleavage of $C - X$ bond in haloarenes proceeds with difficulty.
Therefore, aryl halides are less reactive than alkyl halides towards nucl71eophilic substitution reactions.
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