SN1:
Forms carbocation intermediate.
Thus rate depends only on the molecule that makes the carbocation. (because formation of the carbocation is the slow step)
Favored by substituted carbons (CH3's donate electron density to the carbocation, and thus stabilize it)
Favored in polar, protic (H's) solvents. The protic solvent stabilizes the carbocation.
Since it forms a carbocation, it is able to undergo rearrangement in order to make the carbon more substituted. (e.g. a CH3 group will shift over to make the carbocation more substituted)
SN2:
Single step nucleophilic attack.
Backside attack of the electrophile, resulting in inversion of stereochemistry.
Favored by less substituted carbons (i.e. H's) because larger groups (e.g. CH3) block the nucleophile from getting to the electrophile.
Since it is a direct nucleophilic attack, there is no carbocation, and thus no rearrangement.
Favored in polar, aprotic solvents (e.g. dimethyl ether)
Discussion Zen 5 Speculation (EPYC Turin and Strix Point/Granite Ridge - Ryzen 9000)
Discussion Intel current and future Lakes & Rapids thread
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