Question

Which of the following is least soluble in water?
(A). ${{\text{C}}_{2}}{{\text{H}}_{6}}$
(B). $\text{C}{{\text{H}}_{3}}\text{OH}$
(C). $\text{C}{{\text{H}}_{3}}\text{N}{{\text{H}}_{2}}$
(D). ${{\text{C}}_{6}}{{\text{H}}_{5}}\text{OH}$

Answer 1

- Hint: The solubility of any compound in water depends on whether the compound is polar or not. The polarity of the compound will help in the formation of hydrogen bonding between molecules of a substance. We can also check which compound electronegative atom is present or not.

Complete step-by-step solution -
We all have read the concept that “Like dissolves like” and water is a highly polar solvent. Therefore, for any compound to become soluble in water, it should be polar or have hydrogen bonding in it.
But non-polar (covalent) solvents like gasoline, oil, fat and saturated compounds like ethane are insoluble in polar solvents. Likewise, predominantly non-polar (covalent) compounds are soluble in non-polar solvents, but not in polar solvents such as water.
Also, compounds like Methanol ($\text{C}{{\text{H}}_{3}}\text{OH}$),$\text{C}{{\text{H}}_{3}}\text{N}{{\text{H}}_{2}}$and ${{\text{C}}_{6}}{{\text{H}}_{5}}\text{OH}$, the presence of electronegative atom leads to the formation of hydrogen bonding within the molecules and the water molecule leading to higher solubility in water.
However, ${{\text{C}}_{2}}{{\text{H}}_{6}}$ parent chain only consists of carbon and hydrogen whose electronegativity is almost similar and come under the category of non-polar aliphatic chains. Therefore, it is highly non-polar whereas water is a polar solvent due to the presence of electronegative oxygen atoms. And as per the solubility theory, like dissolve like, ${{\text{C}}_{2}}{{\text{H}}_{6}}$ is insoluble in water.

Hence, the correct option is (a). ${{\text{C}}_{2}}{{\text{H}}_{6}}$

Note: We should also know in which conditions hydrogen bonding can take place. They are-
The molecule should be polar which means it should contain a highly electronegative atom like O, N and F that are linked to hydrogen atom. Higher the electronegativity, higher is the polarization within the molecule.
The distance between the hydrogen atom and electronegative atom should be small and in appropriate proximity should be thee. Also, electronegative atoms should be small, from the first period for greater electrostatic attraction.