Comparing Substitution Nucleophilic 1 (SN1) vs. Substitution Nucleophilic 2) SN2 Reactions
Let’s consider all the factors:
The Mechanism of SN1 and SN2
SN1 – Unimolecular means that only one molecule (substrate) affects the rate of the reaction. This will be the step in which the substrate is involved. And this is the first step, the formation of a carbocation.
SN1 – As we already know that tertiary carbocation is the most stable form of carbocations, the tertiary alkyl halide will be the most suitable starting material. With this substrate, the reaction will be frequently. But, if we have a secondary alkyl halide with a good leaving group in a polar protic solvent, the reaction occurs relatively slow. Then, if we have secondary carbocation with secondary carbon next to the quaternary, the rearrangement occurs and formation of a more stable carbocation. Methyl and primary alkyl halides are too high in energy and these reactions have not been observed.
SN2 – This reaction works frequently and rapidly with primary alkyl halides. The steric hindrance is a crucial thing and how it increases so the rate of reaction decreases. With secondary substrates relatively slow (best with high concentrations of good nucleophiles in polar aprotic solvents), and with tertiary extremely slow, often says it is not observed.
SN1 – Nucleophiles are weak and neutral. These are usually solvents such as CH3OH, CH3CH2OH, H2O, and others.
SN2 – Nucleophiles are strong and negatively charged, and these can be CH3O‾, CN‾, RS‾, N3‾, HO‾, and others.
Sv1 – Solvents in these reactions are polar protic such as water, alcohols, and carboxylic acids.
SN2 – Solvents in these reactions are polar aprotic such as acetone, DMSO, acetonitrile, and DMF that are polar enough to dissolve the substrate and nucleophile but do not participate in hydrogen bonding with the nucleophile.