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  4. Among formic acid, acetic acid, propanoic acid and phenol, the strongest acid in water is

Q. Among formic acid, acetic acid, propanoic acid and phenol, the strongest acid in water is

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Solution:

• The substituents attached to benzoic acid having effects tends to decrease its acidity.
• Carboxylic acids are more acidic than phenol as they are more resonance stabilised. Among the three carboxylic acids given in the options, the effect of group intensifies the negative charge on the carboxylate ion thereby making acetate ion less stable than formate ion. As a result, the release of ion from acetic acid will become more difficult as compared to formic acid. Hence, formic acid is a stronger acid than acetic acid.
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Further since +1 effect of alkyl groups increases in the order $CH_{3} —< CH_{3} CH_{2]—$ the relative acid strength will decrease in the same order, i.e.,
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Now between carboxylic acids and phenol, carboxylic acids are stronger acids than phenol because carboxylate ion is more resonance stabilised. This is because the negative charge on the carboxylate ion is delocalised over two electronegative oxygen atoms.
image image
Thus, the order of acidic strength of given compounds is
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Hence, formic acid is a strongest acid.