| 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,
