Explain why allylic and benzylic halides are more reactive towards `S_(N)1` substitution while 1-halo and 2-halobutanes preferentially undergoes `S_(N) 2` substitution .

Explanation

1) Allylic and benzylic halides show high reactivity towards the `S_N 1` reaction .
Reason : The carbocation thus formed gets stabilised through resonance phenomenon as shown below .

2) 1- halo and 2-halo butanes preferentially under goes `S_(N)2` substitute .
Reason : `S_N 2` reactions involve transition state formation . Higher the steric hindrance lesser the stability of transition state . The given 1-halo and 2-halo butanes have less steric hindrance so these are preferentially undergo `S_(N) 2` reaction .