Which is the correct order of decreasing acidity of the C-H bonds in the following compounds? $CH X_{3} NO X_{2}$ , $CH X_{3} COCH X_{3}$ , $CH X_{3} COOC X_{2} H X_{5}$ , $CH X_{3} CH X_{3}$

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

Answer: A

The acidity follows the stability of the conjugate carbanion. The nitronate ion is the best stabilised (

p K_{a} \approx 10

), then the ketone enolate (

p K_{a} \approx 19

20

). The ester enolate is less stable (

p K_{a} \approx 25

) because the ester oxygen lone pair donates into

C = O

. Ethane gives an unstabilised carbanion (

p K_{a} \approx 50