Arrange the following in the order of decreasing basicity:
C2H5NH2, iso-(C3H7)3N, CH3CONH2, CH3NH-Na+
CH3NH-Na > C2H5NH2 > (iso-C3H7)3N > CH3CONH2
CH3NH- is the conjugate base of the very weak acid CH3NH2 and therefore is the strongest base. The three bulky isopropyl groups on cause steric strain, but with an unshared electron pair on N, the strain is partially relieved by increasing the normal C—N—C bond angle (109°) to about 112°. If the unshared electron pair forms a bond to H, as in R3NH+, relief of strain by angle expansion is prevented. 3° Amines, therefore, resist forming the fourth bond and suffers a decrease in basicity. In CH3CONH2, the N atom is weakly basic because its electron pair can be delocalised to O of the CH3CO— group by extended 
bonding.
