Question

Which one of the following is the correct order of boiling point?
A.Chloroethane < ethane < ethanol
B.Ethane < chloroethane < ethanol
C.Ethanol < ethane < Chloroethane
D.Ethane > Chloroethane > ethanol

Answer 1

Boiling point reflects the strength of forces between molecules in the following order; ionic > H-bonding > dipole-dipole interaction > Vander Waal forces. This shows that boiling point increases with the polarity of a molecule as ionic bonding between cation and anion is highly polarised. Thus, compounds with ionic bonds have the highest boiling point.

Complete step-by-step solution:
Boiling point of an organic compound is the measure of the volatility of that compound. Less volatile compound has a higher boiling point. It tells about their physical properties, structural characteristics and identification of the compound. A liquid boils when its vapour pressure becomes equal to the atmospheric pressure.
The factors that affect boiling point of an organic compound are:
1.Strength of intermolecular forces: intermolecular forces such as ionic, hydrogen bonding, dipole-dipole interaction, Vander Waal forces have a great influence depending on the functional group present. Their order is ionic > H-bonding > dipole-dipole interaction > Vander Waal forces.
2.Length of carbon chain: boiling point increases with increasing carbon chain due to increase in the force of attraction between molecules.
3.Branching: branching decreases the surface area, thereby decreasing the attractive force between the molecules and so, the boiling point decreases.
4.Polarity: polarity of the molecule is determined by the functional group present in it such as halogens. These are electronegative elements, when bonded to alkyl groups increases the polarity and so the boiling point.
As per our question, the boiling point comparison can be made by using the factor 1 and 4.
Ethane $({C_2}{H_6})$ contains a non polar functional group. The only attraction between ethane molecules is weak Van Der Waals Dispersion forces. Therefore, it has lowest boiling point of all.
Chloroethane $({C_2}{H_5}Cl)$ contains an electronegative element chlorine and provides polarity to the molecule. Dipole-dipole forces and dispersion forces are also present. So, it has a higher boiling point than that of ethane.
Ethanol $({C_2}{H_5}OH)$ has the highest boiling point due to the presence of hydrogen bonding in the molecule. The hydroxyl group present in ethanol accounts for strong intermolecular bonding and it becomes hard to break that force, thus the boiling point rises.
Hence, the correct option is (B).

Note: There comes one more side factor on which the boiling point depends. It is the molecular weight of a compound. Larger molecules create Vander Waal forces as they have more electrons and therefore, they have higher boiling point as compared to small molecules.