S_N1 reaction is favoured by heavy groups on the carbon atom attached to halogen i.e

Benzyl > allyl > tertiary > primary > secondary > primary > alkyl halides

3^o C is more stable due to the stabilization of the charge by three methyl groups (or inductive effect). It can also be explained on the basis of hyperconjugation.

Pair of optical isomerism and geometrical isomerism are able to exhibit the phenomenon of stereoisomerism because both type of isomers differ only in their orientation in space.