How do sn1 reactions differ from sn2 reactions?

Question

Nolan Allen · Accepted Answer

Consider the following points of difference.

SN1 Reaction

It is Substitution Nucleophilic Unimolecular reaction.
Proceeds in two steps.
First step is the slow one involving dissociation of the substrate to form carbocation intermediate. Second step is the fast one in which the carbocation combines with the attacking Nucleophile.
More stable carbocation #rarr# faster is the reaction
It is favored by polar solvents.
Mostly given by tertiary alkyl halides.
It occurs with Racemization.
It follows #1^(st)# order kinetics.
Rate=k[R-X]
(Image source only)

Elias Aguila · Answer

undefined SN1 (Substitution Nucleophilic Unimolecular) reactions involve a two-step mechanism with a rate-determining step where the leaving group dissociates to form a carbocation before the nucleophile attacks. SN2 (Substitution Nucleophilic Bimolecular) reactions occur in a single step where the nucleophile attacks the substrate as the leaving group leaves, leading to inversion of stereochemistry at the reaction center. SN1 reactions occur preferentially with tertiary substrates and polar protic solvents, while SN2 reactions occur preferentially with primary or methyl substrates and polar aprotic solvents.

Maya Allen · Answer

undefined SN1 (Substitution Nucleophilic Unimolecular) and SN2 (Substitution Nucleophilic Bimolecular) reactions are two mechanisms by which substitution reactions occur in organic chemistry.

Differences between SN1 and SN2 reactions:

1. Mechanism:
   - SN1: Proceeds in two steps, starting with the formation of a carbocation intermediate followed by nucleophilic attack.
   - SN2: Proceeds in a single step with simultaneous nucleophilic attack and expulsion of the leaving group.

2. Rate-determining step:
   - SN1: Formation of the carbocation intermediate is the slowest step, making the reaction rate dependent only on the concentration of the substrate.
   - SN2: The single-step process means that the rate is determined by the concentrations of both the substrate and the nucleophile.

3. Stereochemistry:
   - SN1: Often results in racemization or the formation of a mixture of stereoisomers due to the planar carbocation intermediate.
   - SN2: Typically proceeds with inversion of stereochemistry at the chiral center due to the backside attack of the nucleophile.

4. Reaction kinetics:
   - SN1: First-order kinetics.
   - SN2: Second-order kinetics.

5. Leaving group:
   - SN1: Can proceed with various leaving groups, including weak and stable leaving groups.
   - SN2: Requires a good leaving group, typically halides such as chloride, bromide, or iodide.

6. Solvent:
   - SN1: Often occurs in polar protic solvents, which stabilize the carbocation intermediate.
   - SN2: Typically occurs in polar aprotic solvents, which facilitate nucleophilic attack by minimizing ion pairing with the nucleophile.

7. Substrate:
   - SN1: Works well with tertiary and secondary substrates due to the stability of the carbocation intermediate.
   - SN2: Works best with primary substrates and unhindered secondary substrates, as steric hindrance can impede nucleophilic attack.

8. Rearrangement:
   - SN1: Can undergo rearrangement of carbocation intermediates, leading to the formation of different products.
   - SN2: Does not involve carbocation intermediates, hence no rearrangement occurs.

In summary, SN1 reactions proceed via a two-step mechanism involving carbocation intermediates, while SN2 reactions proceed via a single-step mechanism without the formation of intermediates. The choice between SN1 and SN2 mechanisms depends on factors such as the substrate, nucleophile, leaving group, and solvent conditions.