Question
Explain the factors affecting $S_N1$ and $S_N2$ mechanism.
✓
Answer
Factors that affect the $S_N1$ and $S_N2$ mechanisms:
- Nature of substrate
- Nucleophilicity of the reagent
- Solvent polarity
- $S_N2$ mechanism: The transition state (T.S.) of the $S_N2$ mechanism is pentacoordinate and thus crowded. As a result, $S_N2$ mechanism is favoured in primary halides and least favoured in tertiary halides. Reactivity order of alkyl halides towards $S_N2$ mechanism: $3^\circ < 2^\circ < 1^\circ < CH_3X.$
- $S_N1$ mechanism: A planar carbocation intermediate is formed in the $S_N1$ reaction. It has no steric crowding as bulky alkyl groups can be easily accommodated in planar carbocation.
Formation of planar carbocation intermediate results in a relief from steric crowding present in the tertiary halide substrate.
Secondly, the carbocation intermediate is stabilized by +I effect of alkyl substituents and by the hyperconjugation effect of alkyl substituents containing $\alpha -$hydrogens.
As a result, $S_N1$ mechanism is most favoured in tertiary halides and least favoured in primary halides.
Reactivity order of alkyl halides towards $S_N1$ mechanism: $3^\circ > 2^\circ > 1^\circ > CH_3X.$
- $S_N2$ mechanism: A more powerful nucleophile attacks the substrate faster and favours the $S_N2$ mechanism.
- $S_N1$ mechanism: The rate of the $S_N1$ mechanism is independent of the nature of nucleophile as nucleophile does not react in the slow step of $S_N1.$ It waits till the carbocation intermediate is formed, and reacts fast with it.
- $S_N2$ mechanism: Polar protic solvents usually decrease the rate of an $S_N2$ reaction. In the rate-determining step of the $S_N2 $ mechanism, the substrate, as well as nucleophile, is involved.
A polar solvent stabilizes nucleophile (one of the reactants) by solvation. Thus, solvent deactivates the nucleophile by stabilizing it. Hence, aprotic solvents or solvents of low polarity will favour the $S_N2$ mechanism. - $S_N1$ mechanism: $S_N1$ mechanism proceeds via the formation of a carbocation intermediate. A good ionizing solvent, polar solvent, stabilizes the ions by solvation.
Solvation of carbocation is relatively poor and solvation of anion is particularly important. Anions are solvated by hydrogen-bonding solvents, that is, protic solvents.
Thus, the $S_N1$ reaction proceeds more rapidly in polar protic solvents than in aprotic solvents.
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