Chapter 4, Problem 4.55P

Interpretation Introduction

Interpretation:

Which fatty acid out of behenic acid and erucic acid has a higher melting point is to be determined.

Concept introduction:

Melting point of a compound depends on the strength of the intermolecular interactions between its molecules. The stronger the interactions, higher the melting point.

The type and strength of the intermolecular interactions depends on the functional groups present in the molecules. Weak induced dipole-induced dipole interactions are present in all molecules. In addition, molecules with polar groups also have relatively stronger dipole-dipole interactions and/or hydrogen bonding.

When a molecule contains both polar and nonpolar groups, the relative strength of induced dipole-induced dipole interactions in the nonpolar part and the dipole-dipole interactions in the polar part depend on the length of the nonpolar part. If the nonpolar part consists of a long chain hydrocarbon, the induced dipole-induced dipole interactions can be stronger than the dipole-dipole interactions due to the polar part. The larger the interacting surface area, the stronger the induced dipole-induced dipole interactions. Long hydrocarbon chains have a large number of electrons, which increases the polarizability.

The surface area also depends on whether the chain is straight or branched or folded. If the number of carbon atoms is the same, a straight chain has the largest surface area, while a branched or folded chain has a relatively low surface area over which it can interact with other molecules. So straight chain compounds have stronger induced dipole-induced dipole interactions, and therefore, higher melting points than branched or folded chain compounds with the same number of carbon atoms.