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Nucleophilic substitution reactions are of two types – SN1 reactions and SN2 reactions.

i) Write any two differences between SN1 and SN2 reactions.

ii) Write any two reasons for the less reactivity of aryl halides towards nucleophilic substitution reactions.

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SN1 reaction Sn2 reaction
1. Molecularity is 1. 1. Molecularity is 2.
2. Rate of reaction is dependent only on the concentration of the alkyl halide. 2. Rate of reaction is dependent on the concentration of the alkyl halide as well as a nucleophile.
3. Mechanism involves two steps – formation of carbocation followed by the nucleophilic attack. 3. Mechanism involves one step via the formation of a transition state.
4. Starting with an optically active alkyl halide results in partial racemization. 4. Starting with an optically active alkyl halide results in a complete inversion of configuration.

ii) Aryl halides are much less reactive than haloalkane or alkyl halides towards nucleophilic substitution reaction due to

1) Resonance effect – in haloalkanes the electron pairs on halogen atom are in conjugation with -IT electrons of the ring and thus the C – X bond acquires a partial double bond character. Asa result the bond cleavage in haloarenes is difficult than in haloalkanes.

2) Difference in hybridization of carbon atom in C – X bond – in haloalkanes, the carbon atom attached to halogen is sp3 hybridized while in case of haloarenes the carbon atom attached to halogen is sp2 hybridized which is more electronegative. Hence, the C – X bond length in haloarenes (169 pm) is less than that in haloalkanes (177 pm). It is difficult to break a shorter bond than a longer bond. Therefore, Haloarenes are less reactive towards nucleophilic substitution reaction.

3) InstabilIty of phenyl cation – in case of haloarenes, the phenyl cation formed as a result of self-ionization will not be stabilized by resonance and therefore, SN1 mechanism Is ruled out.

4) Because of the possible repulsion, it is less likely for the electron-rich nucleophile to approach electron-rich arenes,

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