Question

Ethanol and dimethyl ether form a pair of functional isomers. The boiling point of ethanol is higher than that of dimethyl ether due to the presence of (a) H-bonding in dimethyl ether (b) H-bonding in ethanol (c) \(\mathrm{CH}_{3}\) group in ethanol (d) \(\mathrm{CH}_{3}\) group in dimethyl ether

Short answer

Option (b): H-bonding in ethanol.

Step-by-step solution

Understanding Functional Isomers

Ethanol and dimethyl ether are functional isomers, meaning they have the same molecular formula, _2H_6O, but different structures and functional groups. Ethanol contains an -OH (hydroxyl group) making it an alcohol, while dimethyl ether contains an -O- (ether group), classifying it as an ether.

Role of Hydrogen Bonding

In ethanol, the -OH group allows the formation of hydrogen bonds between ethanol molecules. Hydrogen bonds are significant because they are strong dipole-dipole attractions that occur between an electronegative atom (Oxygen in this case) and a hydrogen atom bonded to another electronegative atom.

Boiling Point and Intermolecular Forces

The boiling point of a substance is related to the strength of its intermolecular forces. The stronger the intermolecular forces, the higher the boiling point. Hydrogen bonding in ethanol is a strong intermolecular force, leading to a higher boiling point compared to dimethyl ether, which lacks hydrogen bonding.

Confirming the Correct Option

Since the higher boiling point of ethanol is due to the presence of hydrogen bonds, we identify option (b) as the correct answer, which specifies the presence of H-bonding in ethanol as responsible for its higher boiling point.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a special type of intermolecular force that plays a crucial role in the behavior of certain molecules. It occurs when a hydrogen atom, covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine, experiences attraction to another electronegative atom in a nearby molecule. This forms a "bridge" known as a hydrogen bond.

Hydrogen bonds are stronger than regular dipole-dipole attractions but weaker than covalent or ionic bonds. They significantly impact the properties of substances, especially those containing -OH or -NH groups. For example, in ethanol, hydrogen bonding occurs between the hydrogen of the -OH group and the oxygen of another ethanol molecule. This substantial interaction requires additional energy to break, contributing to a higher boiling point.

These bonds greatly affect physical properties such as boiling and melting points. Substances with hydrogen bonding generally have higher boiling points than similar molecules without hydrogen bonding because more energy is needed to overcome these strong attractive interactions.

Intermolecular Forces

Intermolecular forces are the forces of attraction between molecules. They dictate the physical properties of substances, such as boiling and melting points, viscosity, and solubility. There are several types of intermolecular forces, each varying in strength and impact on a substance's characteristics.

• London Dispersion Forces: These are the weakest types of intermolecular forces, present in all molecules. They arise due to momentary fluctuations in electron distribution within molecules, creating temporary dipoles.
• Dipole-Dipole Interactions: These occur in polar molecules where permanent dipoles attract each other. This type of interaction is stronger than London dispersion forces.
• Hydrogen Bonding: As discussed, this is a special, stronger form of dipole-dipole interaction occurring in molecules with a hydrogen atom bonded to an electronegative atom.

The strength and type of these forces explain why ethanol has a higher boiling point than dimethyl ether. Ethanol experiences hydrogen bonding, a strong intermolecular force, while dimethyl ether primarily exhibits dipole-dipole and London dispersion forces, which are weaker.

Boiling Point

The boiling point of a substance is the temperature at which its vapor pressure equals the atmospheric pressure, allowing the liquid to turn into vapor. It's a key physical property indicating intermolecular force strength present in a substance.

Substances with stronger intermolecular forces require more energy to separate the molecules and turn them into gas, resulting in higher boiling points. For instance, ethanol has a higher boiling point than dimethyl ether because ethanol molecules are linked by hydrogen bonds. These strong forces need more energy to be broken compared to the weaker dipole-dipole interactions and London dispersion forces in dimethyl ether.

• Dependence on Molecular Interaction: The more substantial the intermolecular forces, like hydrogen bonds in ethanol, the higher the boiling point.
• Influence of Molecular Structure: Even if two substances have the same molecular formula, like ethanol and dimethyl ether, structural differences can lead to different boiling points due to the presence or absence of hydrogen bonding.

Understanding these concepts helps explain why functional isomers, although having the same chemical composition, can exhibit varying physical properties.