Though the above two reactions looks identical in nature, but they differ widely in the mechanistic pathway. If the first reaction is carried out by increasing the amount of either nucleophile or substrate, then there is an increase in the rate of the reaction. However, in the second case, there is hardly any effect in the rate of the reaction on increasing the concentration of nucleophile but the rate increases on increasing the concentration of substrate. So, for reaction (1) , the rate law may be written as:

Rate = k[RBr][OH ⁻ ]

This indicates that both the substrate and the nucleophile are involved in rate determining step. In other words, it is a second order reaction. Traditionally, this type of reactions are called S_N² (Substitution Nuclephilic Bimolecular) reactions. This type of reaction is therefore concerted in nature and, therefore, must involve only one transition state as shown in Scheme 3.

Scheme 3

In reaction (2) the rate law has to be written as:

Rate = k[RBr]

Since the rate of the reaction is independent of the concentration of nucleophile, therefore, the reaction must have more than one step involving at least one intermediate. This type of reaction is depicted as S_N¹ (Substitution Nucleophilic Unimolecular) reaction. It has been observed that these reactions involve the actual formation of a carbocation in the slow rate limiting step. The carbocation so formed reacts with nucleophile in a fast step to give the product. The mechanism may thus be depicted as shown in Scheme 4.

Scheme 4

Since S_N¹ reactions involve the formation of a carbocation, so factors stabilizing the carbocation would make the reaction more facile. One such factor is the effect of solvent. Polar solvents having high dielectric constants will stabilize the carbocation and, hence, increase the rate of the reaction. Thus, the rate of solvolysis of tertiary halide, (CH₃)₃ Br is found to be 30000 times faster in 50% aqueous ethanol than in pure ethanol. This is due to the fact that the charge is developed and concentrated in the transition state (T.S.) as compared to the starting material. The energy requirement for this process can be decreased by increasing the dielectric constant of the solvent. The process is also facilitated by solvation by the solvent molecules.

The effect of solvent in S_N² reaction is much less pronounced than in case of S_N¹ reactions as the reaction proceed through a transition state having no new charge separation but rather the existing charge is being dispersed. Thus, the solvation of the T.S. may only slightly be unfavoured as compared to the nucleophile. This leads to only a marginal decrease in the rate of S_N² reactions on going from non-polar to polar solvent.

However, there is a pronounced effect on the rate of S_N² reactions on going from polar protic to polar aprotic solvent. This is because in a polar protic solvent like methanol, the nucleophile is solvated efficiently as compared to a polar aprotic solvent like N , N -dimehylformamide. This solvation reduces the nucleophililicity of the nucleophile.

Another factor that decides whether a particular nucleophilic substitution will follow S_N¹ or S_N² pathway is the structure of the substrate. If the structure of the substrate that the carbocation generated from it has some degree of stabilization owing to some factors, then it is quite possible for the substrate to follow S_N¹ pathway. In the hydrolysis of series of the given alkyl halides, the following trends are observed:

The alkyl halides (1) and (4) readily undergo hydrolysis.

The S_N¹ rates increase from left to right in the series and the S_N² rates decrease in the same direction.

The alkyl halides (2) and (3) follow a mixed S_N² and S_N¹ rate laws. The rate of S_N² reaction is increased by increasing the concentration of the nucleophile.