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Solution


Q.

Given below are two statements :

Statement-I: The acidic strength of monosubstituted nitrophenol is higher than phenol because of electron withdrawing nitro group.

Statement-II: o-Nitrophenol, m-nitrophenol and p-nitrophenol will have same acidic strength as they have one nitro group attached to the phenolic ring.

In the light of the above statements, choose the most appropriate answer from the options given below:                         [2022]
1 Both statement-I and statement-II are correct.  
2 Both statement-I and statement-II are incorrect.  
3 Statement-I is correct but statement-II is incorrect.  
4 Statement-I is incorrect but statement-II is correct.  

Ans.

(3)

Electron withdrawing groups (e.g. –NO2) stabilise the phenoxide ion more by dispersing the negative charge relative to phenol (i.e. release of proton becomes easy) and thus, increase the acidic strength of phenols. The particular effect is more when the substituent is present on o- and p- positions than in m- position to the phenolic group. Thus, acidic strength of nitrophenols decreases in the order:

p-nitrophenol > o-nitrophenol > m-nitrophenol > phenol.

→pKa IncreasesAcidic strength decreases