SN2 reactiion

1.
The question is too broad.
However, a number of factors could be considered:
- the bond strength between the carbon and the leaving group (especially important for Sn1 reactions);
- the relative bond strength between carbon and the leaving group and the nucleophile (esp. important for Sn2);
- stability of the carbocation formed (for Sn1);
- polarity of the solvent (for Sn1);
- stability of the leaving group after leaving; and
- polarity of bond of carbon and leaving group.


2.
Encouraging Sn1:
- sterically-hindered center (prevents nucleophile to approach);
- sterically-bulky nucleophile (hindered approach to the reactive center);
- electron-donating groups attached to the carbocation center formed, e.g. alkyl groups, phenyl rings (stabilizes carbocation);
- highly polar / ionic solvent (solvates and stabilizes carbocation).

Encouraging Sn2:
- sterically unhindered reactive center and nucleophile (easier approach leads to lower activation energy).


3.
As mentioned above, highly polar / ionic solvents can solvates the carbocation formed in Sn1 reactions, thus stabilize the intermediates.

SN1 同 SN2 係 DSE 係 out-of-syllabus。
只係想提醒中學生知識友。