Answer to a question

some anon tagged a question and i tagged the answers. but i figure i better post this, since its like a FAQ.

here goes:
anon: erm.. just wondering why OH is activating and Cl is deactivating when O is more electro-ve than Cl.

me: oh.. i mentioned it in the notes, the O atom is able to donate its lone pair to the benzene ring readily; the O atom does this(donation of LP) to a much greater extent compared to the Cl atom

but here's more

The donation( or delocalisation) of the lone pair to the benzne ring is the resonance effect
This is different from the shifting of electron density due to electronegativity differences, which is called the inductive effect.
In the case of the OH grp and the Cl grp attached to the benzene ring, you realise that the resonance effect and inductive effect are working antagonistically.
;(resonance effect means that the O and Cl atom donates electrons to the benzene ring, inductive effect means that they withdraw electron density from the ring)

its helpful if u realise that the O atom has a similar size to the C atom, so orbital overlap and electron delocalisation is an easy affair. IE, resonance effect for the O atom is very strong.

for the Cl atom, its much bigger, and so delocalisation of its LP to the C atoms in the benzene ring takes place to a much lesser extent

so in conclusion, the resonance effect for the O atom outweighs the inductive effect. The OH group is an activating group on the benzene ring.
whereas for the Cl atom, the inductive effect outweighs the resonance effect, so the Cl atom is deactivating on the benzene ring.