Question

c-x bond length in halobenzene is small than c-x bond length in ch3-x why

Answer 1

In halobenzene, halogen atom has lone pair of electrons, which contribute towards the resonance. Out of the five possible resonanting structures, 3 structures have partial double bond which are difficult to break, secondly the carbon attached to the halogen is a sp2 hybridsed carbon, whereas in haloalkanes, carbon is sp3

Answer 2

In halobenzene there is partial double bond character in C-X bond due to resonance and also C is sp2 hybridised.
  While in CH3-X there is only single bond character in C-X bond and C is sp3 hybridised. 

This is the reason.