Question

Arrange the following compounds as asked:
(a) In decreasing order of pKb, values:
C$_6$H$_5$NH$_2$, (C$_2$H$_5$)$_2$NH, C$_6$H$_5$NHCH$_2$CH$_3$, C$_6$H$_5$NH$_3$
(b) Increasing order of boiling point:
C$_6$H$_5$OH, C$_2$H$_5$NH$_2$, (C$_2$H$_5$)$_2$NH, C$_6$H$_5$NH$_2$
(c) Increasing order of solubility in water:
C$_6$H$_5$NH$_2$, (C$_2$H$_5$)$_2$NH, C$_6$H$_5$NHCH$_2$CH$_3$

Hint:

When comparing basicity, boiling points, or solubility, consider the electron-donating or electron-withdrawing effects and the size of the molecules.

Answer 1

(a) The order of basicity, which is inversely related to pKb, can be determined by considering the electron-donating and electron-withdrawing effects of the groups attached to the nitrogen atom. The more electron-donating the group, the more basic the compound. The correct order of pKb is: \[ \text{C}_6\text{H}_5\text{NH}_2>\text{C}_6\text{H}_5\text{NHCH}_2\text{CH}_3>(\text{C}_2\text{H}_5)_2\text{NH}>\text{C}_6\text{H}_5\text{NH}_3 \] (b) The boiling point increases with the size and intermolecular forces of the molecule. The correct order of boiling points is: \[ \text{C}_6\text{H}_5\text{OH}<\text{C}_6\text{H}_5\text{NH}_2<(\text{C}_2\text{H}_5)_2\text{NH}<\text{C}_6\text{H}_5\text{NH}_3 \] (c) The solubility of the compounds in water depends on their ability to form hydrogen bonds and interact with water molecules. The correct order of solubility in water is: \[ \text{C}_6\text{H}_5\text{NH}_2<\text{C}_6\text{H}_5\text{NHCH}_2\text{CH}_3<(\text{C}_2\text{H}_5)_2\text{NH} \]