Difference Between SN1 and SN2 Reactions

Key Difference – SN1 vs SN2 Reactions
 

The SN1 and SN2 reactions are nucleophilic substitution reactions and most commonly found in Organic Chemistry. The two symbols SN1 and SN2 refer to two reaction mechanisms. The symbol SN stands for “nucleophilic substitution”. Even though both SN1 and SN2 are in the same category, they have many differences including the reaction mechanism, nucleophiles and solvents participated in the reaction, and the factors affecting the rate determining step. The key difference between SN1 and SN2 reactions is that SN₁ reactions have several steps whereas SN₂ reactions have only one step.         

What are SN1 Reactions?

In SN1 reactions, 1 indicates that the rate determining step is unimolecular. Thus, the reaction has a first-order dependence on electrophile and zero-order dependence on nucleophile. A carbocation is formed as an intermediate in this reaction and this type of reactions commonly occur in secondary and tertiary alcohols. SN1 reactions have three steps.

1. Formation of the carbocation by removing the leaving group.

2. The reaction between the carbocation and the nucleophile (Nucleophilic attack).

3. This happens only when the nucleophile is a neutral compound (a solvent).

What are SN2 Reactions?

In SN2 reactions, one bond is broken, and one bond is formed simultaneously. In other words, this involves the displacement of the leaving group by a nucleophile. This reaction happens very well in methyl and primary alkyl halides whereas very slow in tertiary alkyl halides since the backside attack is blocked by bulky groups.

The general mechanism for SN2 reactions can be described as follows.

What is the difference between SN1 and SN2 Reactions?

Characteristics of SN1 and SN2 Reactions:

Mechanism:

SN1 Reactions: SN₁ reactions have several steps; it starts with the removal of the leaving group, resulting a carbocation and then the attack by the nucleophile.  

SN2 Reactions: SN₂ reactions are single step reactions where both nucleophile and substrate are involved in the rate determining step. Therefore, the concentration of the substrate and that of the nucleophile will affect to the rate determining step. 

Barriers of the reaction:

SN1 Reactions: The first step of SN1 reactions is removing the leaving group to give a carbocation. The rate of the reaction is proportional to the stability of the carbocation. Therefore, the formation of the carbocation is the greatest barrier in SN1 reactions. The stability of the carbocation increases with the number of substituents and the resonance. Tertiary carbocations are the most stable and primary carbocations are the least stable (tertiary > secondary > primary).

SN2 Reactions: Steric hindrance is the barrier in SN₂ reactions since it proceeds through a backside attack. This happens only if the empty orbitals are accessible. When more groups are attached to the leaving group, it slows the reaction. So the fastest reaction occurs in the formation of primary carbocations whereas slowest is in tertiary carbocations (primary-fastest > secondary > tertiary -slowest).

Nucleophile:

SN1 Reactions: SN₁reactions require weak nucleophiles; they are neutral solvents such as CH₃OH, H₂O, and CH₃CH₂OH.

SN2 Reactions: SN₂ reactions require strong nucleophiles. In other words, they are negatively charged nucleophiles such asCH₃O^–, CN^–, RS^–, N₃^– and HO^–.

Solvent:

SN1 Reactions: SN1 reactions are favoured by polar protic solvents. Examples are water, alcohols, and carboxylic acids. They can also act as the nucleophiles for the reaction.

SN2 Reactions: SN2 reactions proceed well in polar aprotic solvents such as acetone, DMSO, and acetonitrile.

Definitions:

Nucleophile: a chemical species that donates an electron pair to an electrophile to form a chemical bond in relation to a reaction.

Electrophile: a reagent attracted to electrons, they are positively charged or neutral species having vacant orbitals that are attracted to an electron rich centre.