Question

Arrange the following in the increasing order of their boiling point:
${{C}_{2}}{{H}_{5}}OH$ , ${{(C{{H}_{3}})}_{2}}NH$ , ${{C}_{2}}{{H}_{5}}N{{H}_{2}}$

Answer 1

The boiling points of different functional groups are different, boiling points of different functional groups depend on the intermolecular hydrogen bonding of the respective functional group. The stronger the intermolecular hydrogen bonding, the higher the boiling point.

Complete answer:
This order means that ethanol has the highest boiling point amongst the three and dimethylamine has the lowest boiling point.
Boiling point of a compound depends on the intermolecular hydrogen bonding between the compound in liquid state. The stronger the intermolecular hydrogen bonding, the harder it is to break the bond, and thus higher the boiling point.
Thus, whichever of these three substances has the strongest hydrogen bonding has the highest boiling point. Now, polar substances form hydrogen bonds between each other as their electronegativity is one sided.
Amines form intermolecular hydrogen bonding as they are polar, thus their boiling point is higher than simple hydrocarbons. Also, the more hydrogen atoms attached to the primary nitrogen atom, the more hydrogen bonds are formed.
Therefore, the compound with more hydrogen atoms attached to the nitrogen atom, has higher boiling point as it has stronger hydrogen bonding. So, ${{C}_{2}}{{H}_{5}}N{{H}_{2}}$ (Ethylamine) has higher boiling point than ${{(C{{H}_{3}})}_{2}}NH$ (Dimethylamine), as it has two hydrogen atoms attached to the nitrogen.
Now, polarity depends on the electronegativity of the element, and electronegativity of oxygen is more than that of nitrogen. Thus, oxygen will form stronger intermolecular hydrogen bonds even though it only has one hydrogen atom attached to it.
Therefore, ${{C}_{2}}{{H}_{5}}OH$ (Ethanol) has the highest boiling point then comes ${{C}_{2}}{{H}_{5}}N{{H}_{2}}$ (Ethylamine) and ${{(C{{H}_{3}})}_{2}}NH$ (Dimethylamine) has the lowest boiling point amongst the three.
The correct order is given by, ${{C}_{2}}{{H}_{5}}OH$ (Ethanol) > ${{C}_{2}}{{H}_{5}}N{{H}_{2}}$ (Ethylamine) > ${{(C{{H}_{3}})}_{2}}NH$ (Dimethylamine).

Note:
The electronegativity of nitrogen in amines is 3.0, while that of oxygen in alcohol is 3.6, thus oxygen forms stronger hydrogen bonds. Primary and secondary amines form hydrogen bonds, while tertiary amines cannot form intermolecular hydrogen bonds as they do not have any hydrogen atom attached to the nitrogen atom.