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Question

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

A
PhCO2H>PhSO3H>PhCH2OH>PhOH
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B
PhSO3H>PhOH>PhCO2H>PhCH2OH
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C
PhCO2H>PhOH>PhCH2OH>PhSO3H
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D
PhSO3H>PhCO2H>PhOH>PhCH2OH
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Solution

The correct option is D PhSO3H>PhCO2H>PhOH>PhCH2OH
PhCH2OH is least acidic than others because its conjugate base is not stabilised by any resonating structures like others.
Conjugate bases of PhSO3H 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 PhSO3H and PhCOOH, the negative charge is delocalised on highly electronegative oxygen atoms which makes PhSO3H and PhCOOH stronger acid than PhOH , due to increased stability of their conjugate base.
Now,
Conjugate PhSO3H 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 PhSO3H has most stable conjugate base.
Stability of conjugate base Strength of acid
Therefore, the decreasing order of acidity is
PhSO3H>PhCO2H>PhOH>PhCH2OH





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