Question

Which of the following compounds has the lowest boiling point?

Answer 1

Answer : The correct answer is option A : Diethyl ether

Explanation :

The boiling point of a compound depends on the intermolecular forces (IMF) of attractions present among its molecules.

Stronger the IMF, more difficult it is to separate the molecules. And hence the compound shows higher boiling point.

The most common types of IMF are

a) Hydrogen bonding : Hydrogen bonding occurs when a compound has hydrogen atom directly attached to strongly electro-negative atoms like O, N and F

Hydrogen bondings are the strongest IMF. Therefore compounds that have hydrogen bondings show higher boiling point.

In the given examples, 2-butanol and 4-octanol both have -OH group where H is directly attached to highly electro-negative O atom. As a result hydrogen bonding is present in these compounds.

Therefore they show higher boiling point.

b) Dipole interactions : These are seen in case of polar compounds.

c) London dispersion forces : These are present in all the compounds but they are predominant in case of non polar compounds.

Both diethyl ether and diphenyl ether predominantly show London dispersion forces. Since these forces are weaker as compared to other IMF, the molecules having london dispersion tend to have lower boiling points.

But the magnitude of dispersion forces increases as the molecular weight of the compound increases.

Therefore diphenyl ether which has a molecular weight of 170 g/mol has much stronger london dispersion forces as compared to diethyl ether which has a molecular weight of 74 g/mol

From above discussion, we can conclude that diethyl ether has the weakest intermolecular forces of attraction. Hence it has the lowest boiling point.