1). - Nucleophilic substitution of primary leaving groups, goes by SN2 mechanism.
- Nucleophilic substitution of tertiary leaving groups, goes by SN1 mechanism.
- But nucleophilic substitution of secondary leaving groups, may go either by SN2 or SN1 mechanism, depending on the reaction governing factors.
In this case:
- Acetolysis of the cis- isomer goes by SN2 mechanism, as usual.
- But acetolysis of the trans- isomer by SN2 mechanism, is not favored due to steric hindrance and thus, it finally goes by SN1 mechanism.
2). There is no neighbouring group participation in the case of cis- isomer.
But in the case of trans- isomer, neighbouring group participation stabilizes the so formed carbocation via the positive charged-cyclic intermediate, making acetolysis 670 folds faster.
3). No! A positive charged intermediate cannot be formed in the case of cis- isomer because carbocations are not formed during SN2 nucleophilic substitution.