Question

Arrange the following compounds in increasing order of solubility in water:
     ${{\text{(}{{\text{C}}_{2}}{{\text{H}}_{5}}\text{)}}_{3}}\text{N}$, ${{\text{(}{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}}\text{)}}_{\text{2}}}\text{NH}$, ${{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}}\text{N}{{\text{H}}_{\text{2}}}$

Answer 1

Hint: The above three compounds belong to the class of organic compounds called amines. Amines when added to water, oxygen present in water forms hydrogen bonds with the hydrogen atoms attached to the nitrogen atom. The extent and strength of hydrogen bonds determines the solubility of the respective amine in water. Remember that the solubility of the amine is not dependent on its basicity in aqueous solutions.

Complete step by step answer:
Amines are classified as primary, secondary, or tertiary by the number of carbons bonded to the nitrogen atom. Primary amine has one carbon bonded to the nitrogen. Secondary amine has two carbons bonded to the nitrogen, and tertiary amine has three carbons bonded to the nitrogen.

Primary Amine:
\[{{R}_{{}}}-N{{H}_{2}}\]
Secondary Amine:
\[{{R}_{2}}-NH\]
Tertiary Amine:
\[{{R}_{\begin{smallmatrix}
 3 \\

\end{smallmatrix}}}-N\]

Where, R is an alkyl group.
When we compare primary, secondary and tertiary amines we find that:
- primary amine has two hydrogen atoms attached to nitrogen atom,
- secondary amine has one hydrogen atom attached to nitrogen atom,
- tertiary amine has no hydrogen attached to nitrogen atom

As mentioned in the hint, the extent of solubility depends on the strength of hydrogen bonds formed between amine and water molecules.
Since primary amine has the greatest number of hydrogen atoms followed by secondary and then tertiary amine, the extent of solubility will be of the same order i.e. primary amine will be most soluble and tertiary amine will be least soluble.

Therefore, the increasing order of solubility of the compounds is,
${{\text{(}{{\text{C}}_{2}}{{\text{H}}_{5}}\text{)}}_{3}}\text{N}$ < ${{\text{(}{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}}\text{)}}_{\text{2}}}\text{NH}$ < ${{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}}\text{N}{{\text{H}}_{\text{2}}}$

Note: The order of solubility is applicable only to the lower aliphatic amines. As the number of carbon atoms increase, the extent of hydrogen bonding is very low and is hence considered insoluble in water. This is the reason why aromatic amines and higher members of aliphatic amines are stored in organic solvents like carbon tetrachloride or carbon disulphide.