Problem 100 - A Complete Resource Book in Chemistry for JEE Main

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

Step-by-Step Solution

Answer

Ethanol has a higher boiling point due to hydrogen bonding (option b).

Step 1: Understanding Functional Isomers

Functional isomers are compounds with the same molecular formula but different functional groups, leading to distinct chemical properties. Ethanol and dimethyl ether both have the formula \(C_2H_6O\), but ethanol contains an -OH group (alcohol) while dimethyl ether contains an -O- group (ether).

Step 2: Identifying Boiling Point Effects

Boiling point is influenced by intermolecular forces. Stronger forces result in higher boiling points because more energy is required to separate the molecules. We need to identify the presence of such forces in ethanol and dimethyl ether.

Step 3: Exploring Hydrogen Bonding

Hydrogen bonding is a strong intermolecular force occurring when hydrogen is bonded to electronegative atoms like oxygen or nitrogen. Ethanol, with its -OH group, is capable of forming hydrogen bonds, whereas dimethyl ether cannot due to its structural configuration.

Step 4: Drawing Conclusions on Boiling Points

Ethanol can form hydrogen bonds because of the -OH group, resulting in stronger intermolecular forces compared to dimethyl ether. Thus, ethanol has a higher boiling point than dimethyl ether due to hydrogen bonding.

Key Concepts

Hydrogen Bonding

Hydrogen bonding is a specific type of attractive force that occurs when a hydrogen atom is directly bonded to an electronegative atom, such as oxygen or nitrogen. This type of bonding is significantly stronger than the typical Van der Waals forces but weaker than covalent or ionic bonds.
In the case of ethanol, the presence of the -OH (hydroxyl) group allows for hydrogen bonding. This hydroxyl group consists of an oxygen atom, which is highly electronegative, bonded to a hydrogen atom. Because of this, hydrogen bonding can take place, as the hydrogen from one ethanol molecule is attracted to the oxygen from another molecule. This creates a network of strong intermolecular forces.
On the other hand, dimethyl ether lacks such a capability. Although it contains oxygen, the absence of a hydrogen atom directly bonded to oxygen means it cannot engage in hydrogen bonding. As a result, dimethyl ether exhibits weaker intermolecular forces compared to ethanol.

Intermolecular Forces

Intermolecular forces play a crucial role in determining the physical properties of substances like boiling points, melting points, and solubility. These forces are the attractions that occur between molecules, as opposed to intramolecular forces, which bind atoms within a molecule.

• Hydrogen Bonds: As previously described, these are strong interactions that occur specifically when hydrogen is bonded to nitrogen, oxygen, or fluorine.
• Dipole-Dipole Interactions: These happen between polar molecules, where positive and negative charges interact.
• Dispersion Forces (London Dispersion Forces): Present in all molecules, these forces arise from momentary variations in electron density, leading to temporary dipoles.

In ethanol, hydrogen bonding is the primary intramolecular force, greatly influencing its properties such as its higher boiling point. Dimethyl ether, however, primarily relies on dipole-dipole interactions and dispersion forces, which are weaker compared to hydrogen bonds. This significant difference in the types of intermolecular forces accounts for the variation in boiling points between ethanol and dimethyl ether.

Boiling Point

The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure surrounding the liquid, causing it to transition from liquid to gas. A key factor affecting the boiling point is the strength of the intermolecular forces present in the substance.
Stronger intermolecular forces result in higher boiling points because more energy is required to overcome these forces and allow molecules to escape into the gas phase. Conversely, weaker intermolecular forces mean that a lower amount of energy is sufficient to vaporize the liquid.
In the case of ethanol and dimethyl ether, these two compounds are functional isomers—meaning they have the same molecular formula ( C_2H_6O ) but different structures and functional groups. Ethanol has a hydroxyl group that forms hydrogen bonds, while dimethyl ether does not. This results in ethanol having a higher boiling point due to the additional energy needed to break the hydrogen bonds during the transition from liquid to gas.