Arrange in decreasing order of basic strength in aqueous solution using approximate pKaH values (methylamine 10.6, pyridine 5.2, aniline 4.6, acetamide $- 0.5$ ): $CH X_{3} NH X_{2}$ , pyridine, $C X_{6} H X_{5} NH X_{2}$ , $CH X_{3} CONH X_{2}$ .

A $CH X_{3} CONH X_{2}$ > $C X_{6} H X_{5} NH X_{2}$ > pyridine > $CH X_{3} NH X_{2}$
Bpyridine > $CH X_{3} NH X_{2}$ > $C X_{6} H X_{5} NH X_{2}$ > $CH X_{3} CONH X_{2}$
C $CH X_{3} NH X_{2}$ > pyridine > $C X_{6} H X_{5} NH X_{2}$ > $CH X_{3} CONH X_{2}$
D $CH X_{3} NH X_{2}$ > pyridine > $CH X_{3} CONH X_{2}$ > $C X_{6} H X_{5} NH X_{2}$

Answer: C

Basic strength follows the pKaH of the conjugate acid: 10.6 > 5.2 > 4.6 >

- 0.5

. Methylamine has a localised lone pair; pyridine has an in-plane sp

^{2}

lone pair; aniline's is conjugated with the ring; acetamide's is conjugated with C=O.