Chapter 23.1, Problem 5R

To determine

To Explain: The variation in boiling and melting points as branching in hydrocarbon isomers increases.

Explanation of Solution

Introduction: The arrangement of carbon atoms in hydrocarbons affects the physical properties of the hydrocarbons.

As compared to straight chain isomers branched chain isomers have lower boiling points. The shape of the molecule tends to become spherical because of the branching. Thus with branching, surface area decreases. The intermolecular forces become weaker with decrease in surface area and can be overcome at a lower temperature. For example, the boiling points of n -pentane, iso-pentane and neo-pentane are 309, 301, and 282.5 K respectively.

With branching of chains, the melting points of hydrocarbons increase. The molecule tends to become spherical with the branching of chains and thus becomes more compact and symmetrical. In the crystal lattice these compact molecules are more closely packed. Hence, melting points increase with branching of chains.

Conclusion: With increase in branching of hydrocarbon isomers the boiling point decreases and the melting point increases.