Aryl Halides and Phenols - Result Question 9
####10. The increasing order of the $p K _a$ values of the following compounds is
(2019 Main, 10 Jan I)
(a) $D<A<C<B$
(b) $B<C<A<D$
(c) $C<B<A<D$
(d) $B<C<D<A$
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Answer:
Correct Answer: 10. (a)
Solution:
- Acidic strength is inversely proportional to $p K _a$ value. The acidity of phenols is due to greater resonance stabilisation of phenoxide ion relative to phenol. Therefore, any substituent which stabilises the phenoxide ion more by dispersal of negative charge will tend to increase the acidity of phenol. Electron withdrawing groups $\left(-NO _2\right)$ increases the acidic strength of phenol whereas electron donating group $\left(-OCH _3\right)$ decreases the acidic strength of phenol. In case of $-NO _2$ group attached to phenol, the dispersal of negative charge is more pronounced at $o$ - and $p$-position than at $m$-position.
Thus, order of acidic strength of nitrophenol is:
$p$-nitrophenol $>o$-nitrophenol and the correct order of the $p K _a$ values of give option is
