Animated Solution for Chemistry - Organic Chemistry: Among the following four aromatic compounds, which one will have the lowest melting point?

Unmasking Intermolecular Forces

Have you ever wondered why some substances melt in the palm of your hand while others require the intense heat of a furnace? The secret lies hidden in the microscopic world of molecules and the invisible forces that bind them together. When we are asked to compare the melting points of different organic compounds, we are essentially being asked to judge a microscopic tug-of-war.

In this problem, we are presented with four distinct aromatic compounds and tasked with finding the one with the lowest melting point. To solve this, we must evaluate the strength of the intermolecular forces present in each molecule.

Before we dive into the specific molecules, let's establish the ground rules. The melting point of a molecular solid is directly proportional to the strength of its intermolecular forces. The stronger the molecules hold onto each other, the more thermal energy (heat) is required to break the crystal lattice and turn the solid into a liquid.

There are three primary types of intermolecular forces we need to consider, ranked from strongest to weakest: 1. Hydrogen Bonding: The heavyweight champion. This occurs when a hydrogen atom is directly bonded to a highly electronegative atom like Oxygen, Nitrogen, or Fluorine. 2. Dipole-Dipole Interactions: The middleweight. These forces exist between polar molecules where the positive end of one molecule attracts the negative end of another. 3. London Dispersion Forces: The lightweight. These are temporary, induced dipole interactions that exist in all molecules, but they are the only forces present in non-polar molecules.

Let's put our four candidates under the microscope.

Compound (a): Phthalic Acid This molecule is armed with two carboxylic acid ($-COOH$) groups. These groups are notorious for forming extensive and incredibly strong intermolecular hydrogen bonds. Because the molecules are locked together so tightly, phthalic acid boasts a very high melting point.

Compound (d): 2-Naphthol Similar to phthalic acid, 2-naphthol features a hydroxyl ($-OH$) group attached to its aromatic rings. This allows it to participate in intermolecular hydrogen bonding. While perhaps not as extensively networked as a dicarboxylic acid, these hydrogen bonds still ensure a relatively high melting point.

Compound (c): 1,4-dimethyl-9,10-anthraquinone This molecule is a giant. It lacks the $-OH$ groups necessary for hydrogen bonding, but it is far from weak. It possesses polar carbonyl ($C=O$) groups that create significant dipole-dipole interactions. Furthermore, its massive size and large surface area mean that its London dispersion forces are exceptionally strong. The combination of these forces results in a high melting point.

Compound (b): Naphthalene Finally, we arrive at naphthalene. Unlike the others, naphthalene is a pure hydrocarbon. It consists entirely of carbon and hydrogen atoms, making it a completely non-polar molecule. It has no $-OH$ groups for hydrogen bonding and no polar $C=O$ groups for dipole-dipole interactions.

The only forces holding naphthalene molecules together in a solid state are the weak London dispersion forces. Because these forces are so easily overcome by thermal energy, naphthalene requires the least amount of heat to melt.

Therefore, naphthalene has the lowest melting point among the four compounds, melting at a modest $\sim 80^{\circ }C$.