Question

Oil of wintergreen, or methyl salicylate, has the following structure:

Methyl 4-hydroxybenzoate is another molecule with exactly the same molecular formula; it has the following structure:

Account for the large difference in the melting points of the two substances.

Short answer

Methyl 4-hydroxybenzoate has hydrogen bonding as its principal intermolecular force, but methyl salicylate does not. Because it takes more heat energy to break these bonds/forces, stronger IM forces result in a higher melting point.

Step-by-step solution

Definition of hydrogen bonding

The electrostatic attraction between polar molecules in which hydrogen is bonded to an electronegative atom, such as oxygen, nitrogen, or fluorine, is known as a hydrogen bond. This is not the same as a covalent bond in which electrons are shared. This is caused by an attraction between the positive and negative poles of charged atoms.

Effect of intermolecular forces on atoms

The melting point is determined by the amount of energy required to overcome the forces that hold molecules together in a lattice. The stronger the intermolecular interactions, the more energy is required, so the melting point is higher.

Explanation

The melting point refers to the temperature at which the solid and liquid forms of a pure substance can exist in balance. When a solid is heated, its temperature rises until it reaches the melting point. More heat will transform the solid into a liquid without changing the temperature.

Methyl4-hydroxybenzoate has hydrogen bonding as its principal intermolecular force, but methyl salicylate does not have hydrogen bonding. Hence, it takes more heat energy to break these bonds/forces. Hence, stronger intermolecular forces result in a higher melting point.