Answer :

When a +R (Positive resonance effect) showing functional group joins phenol or benzene ring, the acidic nature decreases. These groups have their lone pairs with them and they participate in resonance or in delocalization.They show electron releasing tendency. The electrons are released towards benzene ring through delocalization. As a result of which the acidic strength of the compound is reduced due to the destabilization of the phenoxide ion due to concentration of negative charge. So, the acidic strength is much lesser than that of phenol. Eg. –NH2, -OR, R, OH, where R is an alkyl group. These are called ortho, para directing groups.


Similarly for –R (Negative resonance effect) showing functional groups, when they join phenol, acidic nature of the resulted compound is more. This is because the phenoxide ion is stabilized by the dispersal of negative charge as these groups show electron withdrawing tendency. Eg. –NO2, -CHO, -CN, -COOH, etc. These are called meta directing groups.



From the equivalent resonance structures above, in the second, third and fourth figure, we observe negative charge is developed at certain position. The position of the charge at the second and fourth position is same and it is termed as ‘ortho’(o) position and the position at the third figure is termed as ‘para’(p) position.


Here, at these positions, the electron density is more and the electrophile is most likely to attack at these positions. So if we substitute any –R effect showing groups(like nitro), at these positions on a phenol, the compound thus formed would be more acidic than phenol as discussed earlier.


So, p-nitrophenol is more acidic than phenol.


Note:


1. There is another position called ‘meta’(m) which lies in between ortho and para.


2. para substituted isomer with electron withdrawing group is more acidic than ortho substituted one. This is due to the more stable intermolecular Hydrogen bonding(bonding with other molecules of the same or different species) for p isomer and intramolecular hydrogen bonding(bonding formed in the same molecule) for o- isomer.


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