Which of the following are stronger acids than water (pKa 15.7)?

A $CH X_{3} NO X_{2}$
B $CH X_{3} COCH X_{3}$
C $CH X_{3} COCH X_{2} COOC X_{2} H X_{5}$
D $CH X_{3} CN$

Answer: A, C

The pKa values are about: nitromethane 10.2, ethyl acetoacetate 10.7, acetone 19.3 and acetonitrile 25. Only nitromethane and ethyl acetoacetate have pKa below 15.7, because their conjugate bases are stabilised by the nitro group or by two carbonyl groups. Hence options 1 and 3.

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Which of the following are stronger acids than water (pKa 15.7)?

Answer: A, C

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