Question

How to know that what reaction among sn1 and sn2 a secondary substrate undergo.

Answer 1

There are many factors which need to be considered to say whether a reaction is going to proceed through SN1 or SN2 path.
1. Nature of substrate - Sterically hindered substrates go through either SN1 or E1 mechanism. Methyl and primary substrates go through SN2 mechanism. A secondary substrate may go through either of the two depending on the temperature and the solvent taken. Always check whether there is a chance of formation of a highly stable carbocation such as cyclopropyl methyl carbocation, allylic carbocation, benzylic carbocation etc. Also check whether there are chances of ring expansion in order to relieve strain. If such factors are present then the reaction will proceed through SN1 path.
2. Solvent - Aprotic solvents like DMSO, DMF generally favour SN2 path whereas polar protic solvents like water favour SN1 path.