Hi guys,
I'd really appreciate any help with this question! I've tried to make a stab at it but I'm not really sure I'm getting anywhere. This is what I thought, in decreasing order of pKaH:
meta-NO2, para-chlorine, unsubstituted, para-OMe, para-NO2.
I figured that the electron-withdrawing NO2 in the meta position is going to destabilise the +ve charge. Then and similar argument for the chlorine. O-Me is electron-donating, so is likely to stabilise said +ve charge, and the para NO2 can delocalise the +ve charge over itself, lending stability. That leaves the unsubstituted analine in the middle.
However, from having a look at some pKaH values, there seems to be something a bit wrong with my answer! So yeah, I'd really appreciate any help.
Cheers, Josh