The origin of acidity and basicity in organic compound is great interest and provides an extensive comparison. Among hydrocarbons % s character is taken in account while to decide the acidity, in simple aliphatic acids, more the number of alkyl groups, ( $+ I$ effect) less is the acidity and more the ( $- I$ effect) alkyl groups larger the acidity and vice-versa in the case of simple aliphatic bases. Benzoic acid is more stronger than aliphatic amine as the electron pair is less available in case of aromatic amines. The presence of solvent also plays a very important role and at times governs the order too.

Which of the following orders regarding acid strength is correct?

H C O O H < {C H}_{3} C O O H < P h C O O H

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H C O O H > P h C O O H > {C H}_{3} C O O H

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H C O O H > {C H}_{3} C O O H > P h C O O H

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{C H}_{3} C O O H > H C O O H > P h C O O H

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Solution

The correct option is B $H C O O H > P h C O O H > {C H}_{3} C O O H$
To see the strength of acid here need to compare stability of conjugate bases of these acids. $+ I$ effect of Methyl group $C H_{3}$ makes $C H_{3} C O O^{-}$ least stable.
Carbon shows positive inductive $(+ I)$ effect while hydrogen shows neither $+ I$ nor $- I$ . So hydrogen is more electronegative as compared to carbon.

$s p^{2}$ hybridised carbon shows weaker +I effect as compared $s p^{3}$ carbon. Hence phenyl group is more electronegative as compared to $- C H_{3}$ (methyl group) but still less electronegative than hydrogen.

So, summarizing, the electronegativity order of the three substituents is $H > P h > C H_{3}$ . Therefore the acidity order is $H - C O O H > P h - C O O H > C H_{3} - C O O H$ .

Option B is correct.

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