We discussed earlier what are ortho and para directing groups.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.
Phenols give 2,4,6-trinitrophenol(Picric acid) when it reacts with a mixture of conc. HNO3 and H2SO4. This is because of the production of negative charge on the three locations of the benzene ring due to the strong activating effect(+R effect) of the –OH group (due to the presence of lone pairs on Oxygen atom) on the benzene ring towards electrophilic reaction where the electrophile(-NO2 ) can easily attack.
Such is the effect of –OH group on benzene that instead of producing mono nitro derivative on the ring, the reaction readily proceeds to form tri nitro derivative that is 2,4,6-trinitrophenol.
Same thing we observed in case of the reaction of phenol with bromine water.
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