CBSE Class 12-science Answered

(a)P-nitrophenol> o-nitrophenol > m-nitrophenol.
Nitro group has both M-effect (mesmeric effect) and I-effect (inductive Effect) but M-effect predominates over I-effect. NO2 group at o and p position withdraws electron of o-h bond towards it by stronger M-effect while the NO2 group at m position withdraws electron of o-h bond by weaker I -effect . Thus o and p are nitrophenols are more acidic than m-nitrophenol.
However o-nitrophenol is little less acidic than p-nitrophenol due to intramolecular h-bonding which makes loss of proton little more difficult... So, p-nitrophenol is strongest.

 

 

 

(b)m-cresol m  > p cresol > o-cresol
m-cresol is acidic and can be explained on the basis of hyper conjugation .
The methyl group leads to increase in electron density on oxygen atom.The ion that is formed as a result of loosing proton is not that stable in case of ortho and para forms .Due to this meta is more acidic .