Question

Arrange these compounds in order of increasing boiling point. (a) 1 -butanol, butane, diethylether (b) hexane, 1 -hexanol, dipropylether

Short answer

Question: Arrange the given compounds in the two sets in order of increasing boiling point and justify your answer. Answer: The order of increasing boiling point for set (a) is: butane < diethylether < 1-butanol, and for set (b): hexane < dipropylether < 1-hexanol. This order is determined by comparing the intermolecular forces present in each compound, with stronger forces leading to a higher boiling point. Hydrogen bonding has the strongest influence on boiling points, followed by dipole-dipole interactions and London dispersion forces.

Step-by-step solution

Comparing Intermolecular Forces in Set (a) Compounds

First, we need to determine the types of intermolecular forces present in each compound in set (a): 1. 1-butanol: Due to the presence of an -OH group, 1-butanol has hydrogen bonding, dipole-dipole interactions, and London dispersion forces. 2. Butane: Being a nonpolar molecule, butane only has London dispersion forces. 3. Diethylether: Due to the presence of a polarized C-O bond, diethylether has dipole-dipole interactions and London dispersion forces, but no hydrogen bonding.

Ordering Set (a) Compounds by Boiling Point

Now that we know the types of intermolecular forces in each compound, let's arrange them in order of increasing boiling point: 1. Butane (only London dispersion forces) 2. Diethylether (dipole-dipole interactions and London dispersion forces) 3. 1-butanol (hydrogen bonding, dipole-dipole interactions, and London dispersion forces) Hence, the order of increasing boiling point for set (a) is: butane < diethylether < 1-butanol.

Comparing Intermolecular Forces in Set (b) Compounds

Moving on to set (b), we need to determine the types of intermolecular forces present in each compound: 1. Hexane: Being a nonpolar molecule, hexane only has London dispersion forces. 2. 1-hexanol: Due to the presence of an -OH group, 1-hexanol has hydrogen bonding, dipole-dipole interactions, and London dispersion forces. 3. Dipropylether: Due to the presence of a polarized C-O bond, dipropylether has dipole-dipole interactions and London dispersion forces, but no hydrogen bonding.

Ordering Set (b) Compounds by Boiling Point

Now that we know the types of intermolecular forces in each compound, let's arrange them in order of increasing boiling point: 1. Hexane (only London dispersion forces) 2. Dipropylether (dipole-dipole interactions and London dispersion forces) 3. 1-hexanol (hydrogen bonding, dipole-dipole interactions, and London dispersion forces) Hence, the order of increasing boiling point for set (b) is: hexane < dipropylether < 1-hexanol.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction that occur between molecules. They play a crucial role in determining a substance's physical properties like boiling points and melting points. These forces are weaker than the bonds holding atoms together within a molecule, but they significantly impact how substances behave under different conditions.

The strength of these forces dictates the amount of energy required to separate the molecules and, hence, the boiling point. Typically, the stronger the intermolecular forces, the higher the boiling point, because more energy is needed to break these interactions. Various types of intermolecular forces include hydrogen bonding, London dispersion forces, and dipole-dipole interactions. Each of these forces is unique and affects molecular interactions differently.

Hydrogen Bonding

Hydrogen bonds are a type of strong dipole-dipole interaction that occur when hydrogen is covalently bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine. These bonds are among the strongest types of intermolecular forces due to the significant difference in electronegativity between hydrogen and these atoms.

When a hydrogen bond forms, it creates an attraction between the hydrogen of one molecule and the electronegative atom of another molecule. This interaction is stronger than ordinary dipole-dipole forces, leading to higher boiling points for substances where hydrogen bonding is prevalent.

For example, in both 1-butanol and 1-hexanol, the -OH group allows for hydrogen bonding, which contributes to a higher boiling point compared to substances without hydrogen bonds.

London Dispersion Forces

London dispersion forces, also known as van der Waals forces, are the weakest type of intermolecular force. These forces arise from temporary fluctuations in electron density within molecules, leading to instantaneous dipoles. Even nonpolar molecules, like butane and hexane, can exhibit London dispersion forces.

Despite being the weakest, these forces can become significant in heavier atoms and molecules due to the increased probability of temporary dipoles forming. In the case of nonpolar molecules, London dispersion forces are the primary interaction in keeping the molecules together.

These forces are responsible for the boiling point differences among nonpolar substances. The larger the molecular weight or surface area, the stronger the London dispersion forces, resulting in higher boiling points.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between molecules possessing permanent dipoles, meaning they have regions of partial positive and negative charge. These forces are stronger than London dispersion forces but weaker than hydrogen bonds.

In polar molecules like diethyl ether and dipropylether, due to the presence of polar bonds (such as C-O), dipole-dipole interactions play a significant role. They cause these molecules to align with each other so that opposite charges are near each other, increasing the boiling points relative to nonpolar molecules.

The presence of permanent dipoles means these substances will generally have higher boiling points than those only experiencing London dispersion forces. Overall, this interaction adds to the array of intermolecular forces, thus influencing boiling points in varying degrees.