a. In benzene, there is no activating or orienting subsituent in present: therefore, halogenation occurs accoriding to the reactivity of halogens `F_(2) gt Cl_(2) gt Br_(2) gt l_(2)` because the `E_("act")` of halogens is: `F_(2) lt Cl_(2) lt Br_(2) lt I_(2)`
b. In the halogen-subsituted benzene, teh deactivitaig or orienting substituents `(F, Cl, Bror I)` are present, which direct the incoming `E^(o+)` in `SE` reaction.
b. In the halogen-substituded benzene is controlled by `-I` effect only, but the orientation (i.e.. `o.m.p-` position) is controlled by `+R` effect. Due to `+R` effect. they are `o,p-` directing.
More the `EN` of halogen, more the `-I` effect and lesser is the reactivity towards `SE` reaction. (`EN` order: `F gt Cl gt Cl gt Cl gt I`.) Hence the `SE` order is `Phl gt PhBr gt PhCl gt PhF`.
c. MOre the `EWG` stronger is the acid. The acidic character is determined by the combined effect (`-1` and `+R`) `+R` effect of `F` is more than other halogens due to hte overlpaof `2p` of `F` with `2p` of `C` of benzene ring. `-1` effect of `F` is also greatest due to high`EN`.
Teh net `bar(e)`- withdrawing effect is slightly less than `Cl.SO the acidic character is in the given order .
`-I`effects:`FgtClgtBrgtI,+R`effect:`FgtClgtBrgtI.`
Net`EW`:ClgtFgtBrgtI`.
