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Question

Which of the following phenols has the largest pKa value (i.e., least acidic)?

A
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B
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C
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D
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Solution

The correct option is C
Acidic strengthKa1pKa

The electron density on oxygen atom is stabilised by Electron withdrawing group and destabilised by Electron donating group at para position.

NO2,CN and Cl are electron withdrawing group, hence it will stabilise the conjugate base and it will be more acidic. Thus its pKa value will be low.
NO2 and CN shows -R effect which will decrease the electron density on O and stabilise the conjugate base and are more acidic. Hence they will have less pKa value.
CH3 is an electron donating group, hence it will destabilise the conjugate base and it will be less acidic. Thus its pKa value will be high.

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