The reaction mechanism of Sn 1 is gradual. The reactants first dissociate into carbocation and negatively charged leaving group, which requires energy and is a slow step to control the reaction rate. When the molecule dissociates, the carbocation immediately combines with the nucleophile, which is a fast step.

Therefore, the stability of carbocation determines the activity of Sn 1 reaction, and the reaction activity is A >: C >; D> B.

In SN2 reaction, nucleophiles attack electron-deficient central atoms of electrophiles with a pair of lone pairs, forming transition states and leaving leaving groups at the same time.

No carbocation is generated in the reaction, and the rate control step is the above-mentioned synergistic step. The reaction rate is directly proportional to the concentration of the two substances, so it is called bimolecular nucleophilic substitution reaction. In inorganic chemistry, the reaction mechanism of bimolecular nucleophilic substitution reaction is often called "exchange mechanism"

Therefore, the size of the attacking group, that is, steric hindrance, determines the reactivity of Sn2.

Extended data:

Sn 1, a single molecule nucleophilic substitution, is a kind of nucleophilic substitution reaction, in which s stands for substitution, n stands for nucleophilic substitution, and 1 stands for the decisive step of the reaction involving one molecule.

Bimolecular nucleophilic substitution reaction (SN2) is a kind of nucleophilic substitution reaction, in which S stands for substitution, N stands for nucleophilic substitution, and 2 stands for the decisive step of the reaction involving two molecules.

Baidu encyclopedia -Sn 1

Baidu Encyclopedia -Sn2