how do u predict whether halogenation will occur in the side chain or aromatic nucleus in arenes depending on reaction conditions

What?

how do i predict that halogenation will occur to the side chains of arenes

This is totally dependent on exactly what compound you're talking about. In general, halogenation of something like benzene occurs in the presence of a Lewis acid catalyst. Halogenation of a simple alkyl side chain would occur if the conditions promoted a radical reaction, such as the addition of UV light. Prediction of whether halogenation occurs on the ring or one some kind of side chain would require knowledge of what the side chain is.

 

Is this a homework question as well?