Problem 44 - A Complete Resource Book in Chemistry for JEE Main

Question

The boiling point of alcohol is high due to (a) hydrogen bonding (b) large size of alcohol (c) presence of \(-\) OH group (d) high molecular weight

Step-by-Step Solution

Answer

The high boiling point of alcohol is due to hydrogen bonding (option a).

Step 1: Understanding the Question

The exercise asks us to identify why the boiling point of alcohols is high compared to other compounds. We need to consider the properties and structure of alcohols.

Step 2: Analyzing Hydrogen Bonding

Alcohols contain an ext{-OH} group, which means they are capable of forming hydrogen bonds with each other. Hydrogen bonding is a strong type of intermolecular force, which typically results in higher boiling points.

Step 3: Evaluating Size and Weight

Large size (option b) and high molecular weight (option d) are not the primary reasons for high boiling points in alcohols, as these factors contribute more to their physical state and density rather than directly affecting the boiling point.

Step 4: Considering Functional Group Presence

The presence of the ext{-OH} group (option c) in alcohols leads to hydrogen bonding; however, option c by itself does not address the reason for the high boiling point, as it's the hydrogen bonds formed due to this group that matter.

Step 5: Conclusion

Among the given options, hydrogen bonding (option a) is the main reason why alcohols have a high boiling point. The ext{-OH} group in alcohols allows for the formation of strong hydrogen bonds, which significantly increase the boiling point.

Key Concepts

Boiling Point

The boiling point of a substance is the temperature at which it changes from a liquid to a gas. When a liquid heats up, its molecules gain energy and move faster. At the boiling point, the energy is sufficient to overcome the attractive forces holding the molecules together in a liquid state. Hence, the liquid turns into a gas.
Boiling points can vary depending on different factors:

• Type of intermolecular forces present
• Molecular size
• Molecular weight

Why is the boiling point important? Understanding boiling points helps us predict how a substance behaves under heat. It provides insight into the strength of the forces between molecules in a liquid. Substances with strong intermolecular forces tend to have higher boiling points because more energy is needed to separate the molecules.
A good example is the comparison between alcohols and alkanes of similar molecular weight. Alcohols typically have higher boiling points due to their ability to form hydrogen bonds.

Alcohols

Alcohols are a group of organic compounds characterized by the presence of an (-OH) group, known as a hydroxyl group. This unique functional group is responsible for many of the properties that alcohols display, including their boiling points.
Alcohol Structure and Properties

• The -OH group allows alcohol molecules to form hydrogen bonds with each other and with water molecules.
• Alcohols are typically polar, facilitating their interactions with water and other polar substances.
• The carbon chain of alcohols can vary in length, affecting their solubility and boiling point.

The presence of the -OH group in alcohols is crucial because it enables the formation of hydrogen bonds, which are much stronger than other dipole-dipole interactions. This explains why alcohols generally have higher boiling points compared to other organic compounds of similar size and mass.

Intermolecular Forces

Intermolecular forces are the forces that occur between molecules. They are responsible for the physical properties of substances, such as their boiling and melting points. One of the strongest kinds of intermolecular forces is hydrogen bonding.
Types of Intermolecular Forces

• Dipole-dipole interactions: Occur between molecules with permanent dipoles.
• London dispersion forces: Weak forces present in all molecules, regardless of polarity.
• Hydrogen bonds: Strong dipole-dipole interactions occurring specifically in molecules with H atoms bonded to N, O, or F.

Hydrogen bonds are responsible for the high boiling points of alcohols. The (-OH) group in alcohols forms hydrogen bonds that require more energy to break. Consequently, these bonds increase the boiling point significantly. Thus, understanding intermolecular forces like hydrogen bonding is key to explaining why certain substances, such as alcohols, behave the way they do under different conditions.