Question

Which of the following hydrocarbons have the highest boiling point?
(A) $C{H_4}$
(B) ${C_3}{H_8}$
(C) ${C_4}{H_{10}}$
(D) ${C_5}{H_{12}}$
(E) ${C_6}{H_{14}}$

Answer 1

In this question we are discussing the boiling point of hydrocarbons mainly for alkanes .
Saturated hydrocarbons having only single bonds in the carbon chain are termed as alkanes.

Complete step by step answer:
The boiling point of a substance is the temperature at which vapour pressure of a liquid is equals the pressure surrounding the liquid and the liquid changes into a vapour.(Vapour pressure is determined by kinetic energy of the molecules)
Alkanes are not very reactive and have little biological activity; all alkanes are colourless and odourless non-polar compounds .Alkane molecules are nonpolar, they are insoluble in water, which is a polar solvent, but are soluble in nonpolar and slightly polar solvents. Consequently, alkanes are used as solvents for organic substances of low polarity, such as fats, oils, and waxes.
Boiling point Principle:
The molecules which are bonded to each other strongly through strong intermolecular forces cannot move freely easily and therefore, they do not escape the liquid as they do not achieve the kinetic energy required to escape. Therefore, molecules which have strong intermolecular forces will have high boiling point. This is because more kinetic energy will be required to break the intermolecular bonds so that individual molecules may escape the liquid as gases.

According to the question,
Boiling point $\propto$ Surface area $\propto$ $\dfrac{1}{{branching}}$
For a high boiling point, the maximum number of carbon atoms required in the compound and have large surface area.
So, the highest boiling point is Pentane (${C_5}{H_{12}}$). Hence, option (D) is Correct.

So, the correct answer is Option D.

Note: Alkynes have higher boiling points than alkanes or alkenes, because the electric field of an alkyne, with its increased number of weakly held pi electrons, is more easily distorted, producing stronger attractive forces between molecules.