Question

Decreasing order of acidic strength of different (-OH) groups is

• A. $a>b>d>c$
• B. $a>c>b>d$
• C. $c>a>b>d$
• D. $c>b>a>d$

Answer 1

The correct option is A $a>b>d>c$

$\text{Stability of conjugate base}\propto \text{Strenght of acid}$
In the given compound, all the negative charge is on the electronegative atom so we have to check for other effect that determines the stability of conjugate base.
In (a), oxide ion formed is also called phenoxide ion, this conjugate base is more stable due to the delocalisation of negative charge in benzene ring.

Oxide ion formed in (b), (c) and (d) are not stabilised by delocalisation hence their conjugate base are less stable than (a).
Stability of conjugate base is also depends on Inductive effect
-I effect will stabilise the conjugate base and +I effect will destabilize the conjugate base

Oxide ion of (c) is less stabilised by less effective -I effect of phenyl group due to longer distance and destabilised by two +I effect of alkyl group.
Oxide ion of (d) is stabilised by effective -I effect phenyl group due to shorter distance whereas it destabilised by +I effect of one alkyl group.
Oxide ion of (b) is stabilised by effective -I effect of phenyl group and least +I effect is there.

Comparing the above discussion we conclude,
Stability of conjugated base follows the order
(a) > (b) > (d) > (c)
$\text{Stability of conjugate base}\propto \text{Strenght of acid}$
Decreasing order of acidic strength of OH group is
(a) > (b) > (d) > (c)