Question

Explain the factors affecting S_N1 and S_N2 mechanism.

Answer 1

Factors that affect the S_N1 and S_N2 mechanisms:

1. Nature of substrate
2. Nucleophilicity of the reagent
3. Solvent polarity

i. Nature of substrate:

1. 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° < 2° < 1°< CH₃X.
2. 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 α-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° > 2° > 1°> CH₃X.

ii. Nucleophilicity of the reagent:

1. S_N2 mechanism: A more powerful nucleophile attacks the substrate faster and favours the S_N2 mechanism.
2. 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.

iii. Solvent polarity:

1. 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.
2. 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.