-OH group electronwithdrawing?

it depends.

O is highly electronegative element, which withdraws electrons strongly by inductive effect. inductive effect can be viewed as "propagate via sigma-bonds" -- nearer leads to stronger effect.
for example, lactic acid [CH3CHOHCOOH] has -OH group behind the -COOH. with its inductive effect, -OH results in stronger acidity when compared to propanoic acid [CH3CH2COOH] .


however, O also has lone pairs, which can be donated to the conjugated pi-bond system. this is particularly important in electrophilic substitution reactions on substituted benzene rings, as -OH activates the ring, as well as directing the positions of substitution.
phenol is more prone to nitration compared to benzene.


withdrawing: nitro -NO2, nitroso -NO, fluoro -F, chloro -Cl, oxo >C=O, carboxyl (-COOH, -COOCO-, -COCl, -CONR2) ....

donating: alkyl -R, phenyl -C6H5, alkoxy -OR, hydroxy -OH, amine -NR2, N-side of amide -NHCOR ....


2011-04-15 13:08:39 補充：
nitration is addition reaction that adds (substitute) nitro-group (-NO2) to the molecule.
typical example is nitration of aromatic compounds like benzene, toluene and phenol, producing nitrobenzene, trinitrotoluene (TNT) and trinitrophenol.

2011-04-15 13:09:03 補充：
nitro-compounds are ofter unstable, while compounds above are used as explosives.