Chemistery

The reactivity of a SN1 reaction is determined by 3 factors: 1. the substituent 2. leaving group 3. solvent effect Note that the nucleophilicity is not considered here but a weak nucleophile is usually preferred. Substituent Substituent is the things that attached to the carbocation. As the carbocation formation is rate-determining step, a stable carbocation is necessary. A carbocation is positively charged and it is electron deficient. So it needs some groups/ substituents that are electron-donating. Methyl group (not hydrogen anymore) stabilizes the carbonation by the inductive effect (donating electrons through the sigma bond) hyperconjucation (donating electrons through the overlap of the sp3 orbital and the empty p orbital) Therefore, a more substituted alkyl halide is more favored than a primary alkyl halide, which is the opposite of the SN2 reaction. Resonance (typically double bond or oxygen lone pair e-) also stabilized the carbocation. Leaving Group ...

As we have already learned SN2, what would happen if the substrate is hindered and the nucleophile is a weak one? SN1 will happen. Unimolecular : SN1 is called unimolecular substitution because only one molecule (the substrate) is involved in determining the rate of reaction. This would not happen if the reaction is one step (nucleophile attacks and leaving group leaves the substrate). This reaction must involves multi steps: one is rate-liming (slow) and the other is fast (doesn't affect the rate). As it is not a concerted reaction, an intermediate is formed. Substitution : the nucelophile (from the solvent) substitutes the leaving group. SN1 is also called solvolysis because the solvent acts as the nucleophile (SN1 happens when we put a bulky substrate in a solvent). Let's look at the general mechanism: Unimolecular Substitution Step 1 is a rate-determining step which is a slow ionization that gives a carbocation intermediate. This step is slow and has a high...