Sn is an abbreviation for ‘nucleophilic substitution reaction.’ 1 denotes unimolecular, and the Reaction rate is determined only by the concentration of one reactant; 2 denotes bimolecular, and the concentrations of both reactants determine the reaction rate.

The problem is that there are two viable products for Sn1 but only one feasible product for Sn2. Students frequently feel perplexed in this section, but if we examine the processes of those two substitution reactions, things will become apparent (with the aid of the graph). Several elements impact whether a reaction passes through Sn1 or Sn2.

However, at A-level, we need to examine the substrate’s steric barrier or whether it is bulky or not. If the substrate is bulky, such as (CH3)CBr, it will travel through the Sn1 pathway because it is difficult for nucleophiles to contact the centre carbon, and the only method is for the Br to leave first. Sn2 will occur if it is not bulky. 

In this article, we’ll deeply examine the difference between sn1 and sn2.

What is Nucleophilic Substitution Reaction?

A nucleophilic substitution reaction is an organic process in which another replaces one nucleophile. It is quite similar to the displacement reactions seen in chemistry.

In a displacement reaction, a less reactive element is replaced by a more reactive element from its salt solution. The leaving group is the group that accepts electron pairs and displaces them from the carbon, and the substitution occurs on a molecule known as the substrate. As a neutral molecule, the leaving group departs.

Nucleophilicity is the word used to describe the reactivity and strength of nucleophiles in nucleophilic substitution processes. A strong nucleophile replaces a lighter nucleophile in its compound in a nucleophilic substitution process. It is generally stated as follows:

R – LG + Nu٥ → R – Nu + LG٥

Where,

LG – Leaving Group ( Less Nucleophilic)

R – Alkyl Group

Nu٥ – Strongly Nucleophilic 

SN1 Prime Reaction

The rearranged product generated under SN1 conditions is known as the SN1 prime product, and the Reaction is known as the SN1 Prime reaction. It is a sort of nucleophilic Reaction in which the rate-controlling step has one molecularity. The rate of the reaction is related to the substrate concentration. It is a reaction to the first order.

SN2 Prime Reaction

When the alkyl halide depicted combines with an OH- or any nucleophile, the SN2′ prime Reaction occurs. As the attacking nucleophile faces Steric repulsions from the – e- cloud, the nucleophile OH- targets gamma carbon rather than alpha carbon in this reaction pathway. Many reactions include a succession of stages.

Difference Between SN1 and SN2 Reactions

Rate Law

SN1 Reaction: The SN1 Reaction is a first-order unimolecular Reaction. As a result, the substrate influences the reaction rate.

SN2 Reaction: The SN2 Reaction is a bimolecular reaction of the second order. As a result, the reaction rate is influenced by both the substrate and the nucleophile.

Rate Expression

SN1 Reaction: This is defined as rate = K [R-LG]

SN2 Reaction: This is defined as rate = K’ [R-LG] [Nu–]

Steps in the Reaction:

SN1 Reaction: SN1 Reaction consists of 1 step.

SN2 Reaction: SN2 Reaction consists of 2 steps.

Carbocation Formation

SN1 Reaction: During the process, a stable carbocation formed.

SN2 Reaction: A carbocation does not develop during the Reaction just because the leaving group is separated and new bonds are formed simultaneously.

Intermediate States

SN1 Reaction: This comes with two intermediate states.

SN2 Reaction: This comes with one intermediate state.

Key Factor of the Reaction/ Big Barrier

SN1 Reaction: The Reaction relies heavily on carbocation stability.

SN2 Reaction: The leading cause of the Reaction is a steric hindrance.

Reactivity Order based on –R group

SN1 Reaction: IIIry> IIry>> Iry

SN2 Reaction: Iry> IIry>> IIIry

Needs of Nucleophile to proceed the Reaction

SN1 Reaction: It is necessary to use a weak or neutral nucleophile.

SN2 Reaction: A powerful nucleophile is required.

Reaction Favorable Solvents

SN1 Reaction: Alcohol, a polar protic, is a good solvent.

SN2 Reaction: Polar aprotic solvents like DMSO and acetone are ideal. 

Characteristics of SN¹ reactions: 

1. The Reaction takes two steps.
2. K [R — L] = reaction rate
3. It is a reaction to the first order.
4. The intermediate carbocation is created. It is possible to reorganise carbocation.
5. The stability of the carbocation determines the rate. [3°>2°>1°]
6. The rate of Reaction is also affected by the stability of the cation and anion.
7. Polar Protic Solvent promotes the SN1 Reaction. (Because PPS dissolves both cation and anion, the leaving group must be strong, For example, a weak base (as in SN2).
8. A weak nucleophile carries out the SN1 Reaction. (A strong nucleophile will attack the substrate directly, resulting in an E1 reaction.)
9. The rate of the reaction is unaffected by nucleophile concentration or intensity.
10. In the case of mirror images, the products generated will have both R- and S-forms.
11. Low temperatures promote SN1 reactions.

Characteristics of SN² reactions: 

1. Reaction in a single step
2. K [R — L] = reaction rate [Nu(:)]
3. It’s a reaction of the second kind.
4. The rate is determined by the concentration and strength of the Nucleophile.
5. A transition state with an sp2 hybridised planar structure is generated. (Not sp3d because carbon lacks a d orbital.)
6. There is no intermediate carbocation formation.
7. The leaving group should be strong, i.e. not have a weak foundation.
8. The rate of Reaction is related to the bulkiness of the groups linked to the C atom. [CH3Cl > CH3CH2Cl > CH3CH(CH3)Cl > CH3C(CH3)²Cl]
9. 1° > 2° > 3° substrates reaction rate
10. A strong nucleophile carries out SN2 reactions. The nucleophile hits the substrate from the rear.
11. Polar aprotic solvents like DMA, DMSO, DMF, etc., promote SN2 reactions. (Because PAS does not dissolve/solvent cations and only anions in solution, by using PAS, cations are eliminated and only Nu(:) is present to attack the substrate.]
12. Walden inversion takes place.
13. Low temperature promotes Reaction. The E2 Reaction is caused by high temperatures.

Conclusion

Although SN1 and SN2 are both nucleophilic substitution processes, they vary in a few ways:

1. The rate-determining step in SN1 reactions is unimolecular, whereas it is bimolecular in SN2 reactions.
2. SN1 is a two-step method, whereas SN2 is a one-step procedure.
3. During SN1 reactions, the carbocation forms as an intermediate, but it does not develop during SN2 reactions.
4. In SN2 reactions, one can draw an intermediate structure in which the carbon has a partial bond with both the incoming nucleophile and the leaving group, but in SN1 reactions, this is not conceivable.