Question

Which of the following shows the correct order of decreasing acidity?
(Here $Ph-$ indicates phenyl group)

• A. $PhC{O}_{2}H>PhS{O}_{3}H>PhC{H}_{2}OH>PhOH$
• B. $PhS{O}_{3}H>PhOH>PhC{O}_{2}H>PhC{H}_{2}OH$
• C. $PhC{O}_{2}H>PhOH>PhC{H}_{2}OH>PhS{O}_{3}H$
• D. $PhS{O}_{3}H>PhC{O}_{2}H>PhOH>PhC{H}_{2}OH$

Answer 1

The correct option is D $PhS{O}_{3}H>PhC{O}_{2}H>PhOH>PhC{H}_{2}OH$

$PhC{H}_{2}OH$ is least acidic than others because its conjugate base is not stabilised by any resonating structures like others.
Conjugate bases of $PhS{O}_{3}H$ and $PhCOOH$ have equivalent structures but conjugate base of $PhOH$ has non-equivalent resonating structures.
Also, the negative charge is delocalised on less electronegative carbon atom while in conjugate bases of $PhS{O}_{3}H$ and $PhCOOH$, the negative charge is delocalised on highly electronegative oxygen atoms which makes $PhS{O}_{3}H$ and $PhCOOH$ stronger acid than $PhOH$ , due to increased stability of their conjugate base.
Now,
Conjugate $PhS{O}_{3}H$ has 3 equivalent resonating structures while $PhCOOH$ has one less i.e. 2 equivalent resonating structures as shown.



Higher the number of equivalent structure more will be the stability of conjugate base. Hence $PhS{O}_{3}H$ has most stable conjugate base.
Stability of conjugate base $\propto$ Strength of acid
Therefore, the decreasing order of acidity is
$PhS{O}_{3}H>PhC{O}_{2}H>PhOH>PhC{H}_{2}OH$