Methyl bromide (1) undergoes reaction by SN2 pathway. As the substitution is increased at the α-carbon it becomes increasingly difficult for the nucleophile to approach the carbon center to attack. Thus, there is a drop in the SN2 reaction rate progressively along the series. In (4), however, there is a very significant crowding the T.S. for SN2 pathway having 5 bulky groups around a carbon center as compared to 4 in the starting material. Such steric crowding increases the energy of T.S. and, thus, it is less likely to be formed and hence slows the SN2 reaction extremely.
Now in a SN1 attack, there is considerable charge separation on going from substrate to the T.S. to the carbocation intermediate. Thus, factors which stabilize the carbocation will evidently increase the rate of the reaction. In the series depicted above, the stabilization of the carbocation on account of electron donating inductive effect and hyperconjugation progressively increases from left to right across the series. In steric terms, the formation of a planar carbocation with 3 substituents on the central carbon as compared to alkyl halide having 4 substituents on the same carbon is less demanding sterically (as compared to the T.S. for SN2 ). The steric factor gets more emphasis from the fact that on going from left to right across the series, the relative relief in steric strain on going from alkyl halide to planar carbocation is more (CH3 is more bulky than H). These factors is turn stabilized the carbocation intermediate and consequently the SN1 pathway is preferred on going from left to right in the series (Scheme 5).
Scheme 5
The effect of substitution at α-carbon of an alkyl halide is much more pronounced than substitution at the β-carbon. However, steric factors do play role in determining the reactivity of alkyl halides substituted at the β-carbon. Thus, in the following series of alkyl halides, for the solvolysis by ethanol, the given order of relative rates of reaction was observed. It must be noted that these are all SN2 rates.
This occurs because of increase in relative crowding in T.S. of the SN2 reaction, which makes it increasingly difficult for the nucleophile to approach the α-carbon carrying the bromide ion for attack “from the back”. The steric effect is not very pronounced till there is a possibility of adoption of a conformation (by rotation along the Cα-Cβ bond). The large difference on going from (7) to (8) is that there is no scope for the Cα-Cβ bond to adopt a favourable conformation in (8) as all conformations are equally crowded while in (6) and (7) there is at least one conformation where the nucleophile C2H5O− is hindered by only one hydrogen.
The structure of a compound also plays an important role in the rate of the reaction. Thus, alkyl halides bearing the halogen at bridge head carbons undergo hydrolysis extremely slowly. The more is the rigidness of the structure, the less is the rate of substitution. Thus, among the alkyl halides, 1-bromobicyclo[2.2.1]heptane (9) and 1-bromobicyclo[2.2.2]octane (10), for the solvolysis by ethanoate anion, the latter exhibits improved rate of reaction compared to the former, however, both the alkyl halides are extremely slow in undergoing solvolysis. These two compounds evidently undergo reaction by SN1 pathway as both the halides are 3° halides which rules out any possibility of attack from the back by nucleophile (SN2 pathway). But the possibility of forcing these rigid structures to adopt a planar conformation required for the carbocation is also very demanding and has a very high energy requirement. This causes the carbocation to be formed only reluctantly at a very slow rate which explains the slow rate of the reaction. The improvement of rate for (10) as compared to (9) is due to less rigidity in the ring (2 carbons in bridge in (10) compared to one in (9))
