SN1 and SN2 Reactions and their Mechanisms for JEE

A nucleophile is defined as an atom or a molecule that contains a pair of electrons to donate to another atom or molecule. In simple terms, we can say that these are electron-rich species with unused electrons in their orbit. Nucleophiles can be anions or neutral anions that are negatively charged ions. For example, Hydroxide ion (OH^-).

Neutral nucleophiles are molecules that contain a lone pair of electrons with zero charges on them. For example, Ammonia (NH₃). Even though the molecule ammonia is neutral, the nitrogen atom contains a lone pair of electrons, which can be donated to electron-deficient species.

A substitution reaction is a type of chemical reaction, where the atom or the functional group of a molecule gets substituted by another atom or a functional group. Substitution reactions can be of two types: Electrophilic Substitution Reactions and Nucleophilic Substitution Reactions.

A nucleophilic substitution reaction is a reaction in which one nucleophile replaces the other nucleophile in a molecule. The molecule on which substitution occurs is known as the ‘substrate’, and the group that leaves is called ‘leaving group’. In this topic, we will learn about the types of nucleophilic substitution reactions along with their mechanisms. 

What are S_N1 and S_N2 Reactions?

S_N1 and S_N2 reaction mechanisms belong to nucleophilic substitution reactions. To understand these reaction mechanisms better, it is important to learn the basics of nucleophilic substitution reactions. S_N2 reaction (substitution, nucleophilic, bimolecular ) is a one-step mechanism, following second-order kinetics whereas S_N1 is a two-step mechanism, following first-order kinetics. In the S_N1 mechanism, the product is formed via the formation of carbocation, whereas in the S_N2 mechanism the reaction occurs through a transition state.

Mechanism of S_N2 Reaction

Let us consider the reaction between methyl chloride and hydroxide ions to produce methanol and chloride ions. In the S_N2 reaction, the rate of reaction depends on the concentration of both the reactant, i.e. methyl chloride and hydroxide ions. In other words, rate $=\mathrm{k}\left[\mathrm{CH}_{3} \mathrm{Cl}\right]\left[\mathrm{OH}^{-}\right]$.

Image: Mechanism of SN2 reaction

Hence, in this reaction both the alkyl halide and nucleophile simultaneously take part in the rate-determining step of the S_N2 reaction. Thus, S_N2 is a concerted reaction i.e. step reaction. S_N2 reaction occurs through the transition state in which the carbon atoms are simultaneously bound to the incoming nucleophile OH- ion and the outgoing leaving group chloride ion.

Due to the instability of the transition state, it decomposes into product methanol and leaves a group of chloride ions. In the S_N2 reaction, the attack of the nucleophile occurs from the back side where the leaving group leaves from the front side. It results in the inversion of the configuration which is also known as ‘Walden Inversion’.

Mechanism of S_N1 Reaction:

Let us consider the reaction between tert-butyl bromide and the hydroxide ion to yield tert-butyl alcohol and chloride ion.

S_N1 reaction follows first-order kinetics, that is, the rate of reaction depends on the concentration of only one reactant, which is tert-butyl bromide and is independent of the concentration of hydroxide ion. Rate $=k\left[\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CBr}\right]$. Hence, the reaction occurs in two steps.

Step 1: This step is slow, reversible and the rate-determining step. Here, tert-butyl bromide undergoes ionisation to produce tert-butyl carbocation and the bromide ions.

Step 1. Formation of carbocation

Step 2: Since the carbocation is very reactive, it is immediately attacked by the nucleophile, i.e, OH^- to give the substitution, that is tert-butyl alcohol.

The step is fast and therefore, does not affect the rate of reaction.

Step 2. Attack of nucleophile

Stereochemistry of S_N2 and S_N1 

Stereochemistry of S_N2 reaction: As we previously discussed, in an S_N2 reaction, the attack of a nucleophile occurs to form the back side, that is, in the direction opposite to the one where the halogen leaves. As a result, S_N2 is accompanied by inversion of configuration just the same way an umbrella turns inside out. This inversion of configuration is also called ‘Walden inversion’. Hence in S_N2, there is a complete inversion of configuration.

Stereochemistry of S_N2

Stereochemistry of S_N1: In the S_N1 reaction, if the alkyl halide is optically active, then the product is a racemic mixture. In the S_N1 reaction, nucleophile attacks occur from both the backside and front side. However, backside attack is predominant. We can say that the nucleophile attack in S_N1 carbocation can occur with equal ease from both the front and the backside. Given the racemic mixture of the product, the overall result is generally racemisation with some inversion i.e, partial racemisation occurs.

Stereochemistry of S_N1

Distinguish Between S_N1 and S_N2 Mechanism 

Conclusion

To sum it up, S_N1 and S_N2 are two types of nucleophilic substitution reactions explained by taking S_n1 and S_n2 reaction examples. A nucleophile is defined as an electron-rich atom or molecule. It can be in the form of an anion (negatively charged ion) or in the form of a compound that has one lone pair of electrons.

A nucleophilic substitution reaction involves the replacement of one functional group or atom with another anion or atom.