Question

Which of the following alcohols are least soluble in water?
A. Ethanol
B. 1-propanol
C. 1-butanol
D. 1-pentanol

Answer 1

Hint: Alcohols are organic compounds which have hydroxyl groups $\left( { - {\text{OH}}} \right)$. Alcohols are polar compounds. Solubility is based on the hydroxyl group present. When alcohol is mixed with water, hydroxyl groups and hydrogen interact with each other.

Complete step by step answer:
Alcohols are classified into three types- primary alcohols, secondary alcohols and tertiary alcohols. They interact with themselves and with other polar compounds by dipole-dipole interactions. Dipole-dipole interaction is the attraction between the positive end of one dipole and the negative end of another. Generally alcohols are soluble in water. This is because the ${\text{OH}}$ group in alcohols helps to form hydrogen bonds with water molecules. Generally lower alcohols are more soluble in water. Solubility of alcohols is an indication of their ability to form hydrogen bonds. When water and alcohol are mixed, there forms hydrogen bonding between them. But in higher alcohols, they act almost as alkanes since the hydroxyl group is attached at the end and the alkyl group is hydrophobic in nature. So the chance of hydrogen bonding between the hydroxyl group and water is very low. So higher alcohols are less soluble in water.
In the given options, 1-pentanol is higher alcohol than others since it has five carbon. Thus it is least soluble in water.
So, the correct answer is option D.

Additional information Primary alcohols are alcohols in which ${\text{OH}}$ group is attached to the primary carbon atom. Secondary alcohols are alcohols in which ${\text{OH}}$ group is attached to a secondary carbon atom. Tertiary alcohols are the one in which ${\text{OH}}$ group is attached to tertiary carbon atoms.

Note:
Alcohols have high boiling point. This is also due to the intermolecular hydrogen bonding between the ${\text{OH}}$ group in the molecule. But boiling point increases with increase in the number of carbon atoms. While in branched molecules, boiling point is low.