Question

Which one of the following has the minimum boiling point:
A.1-butene
B.1-butyne
C.N-butane
D.Isobutane

Answer 1

Hint: To answer this question, you must recall the properties on which boiling points of various hydrocarbon molecules depend. The explanation of the varying boiling points can be given on the basis of Van der Waals force between the molecules.

Complete step by step answer:
The boiling points of unbranched hydrocarbon chains show a regular increase with increasing molecular weight. The increase is by 20 to 30 degree Celsius for addition of each carbon to the chain. Branching of the chain lowers the boiling point.
In unbranched chains, as the molecular weight increases, there is an increase in the molecular surface area. With increase in the surface area, the Van der Waals forces between molecules increase and more energy is required to separate the molecules from one another. Branching in the chain, makes a molecule more compact resulting in a decrease in the surface area. As a result, the strength of the Van der Waals forces operating between the molecules decreases. The boiling points of alkynes are slightly higher than corresponding alkenes, which further have a higher boiling point than alkanes. This is probably due to the reason that because of the presence of multiple bonds, the unsaturated hydrocarbons are relatively linear and hence their molecules are closely packed.
We can also say that due to the presence of pi electrons, the unsaturated molecules are slightly polar and thus have slightly higher boiling points.
Taking all the above points into consideration we can say that the isobutene has the lowest boiling point since it is a branched alkane.

The correct option is D.

Note:
The order of boiling points of hydrocarbons can be given as:
Alkynes > Alkenes > Alkanes > Branched alkanes
More is the branching, more is the decrease in its boiling point, it can be given as:
Primary>Secondary>Tertiary