Problem 129 - A Complete Resource Book in Chemistry for JEE Main

Question

(I) 1,2 -dihydroxy benzene (II) 1,3 -dihydroxy benzene (III) 1,4 -dihydroxy benzene (IV) Hydroxy benzene The increasing order of boiling points of above mentioned alcohols is (a) \(\mathrm{I}<\mathrm{II}<\mathrm{III}<\mathrm{IV}\) (b) \(\mathrm{I}<\mathrm{II}<\mathrm{IV}<\mathrm{III}\) (c) IV \(<\mathrm{I}<\mathrm{II}<\mathrm{III}\) (d) \(\mathrm{IV}<\mathrm{II}<\mathrm{I}<\mathrm{III}\)

Step-by-Step Solution

Answer

The correct order is (c) IV < I < II < III.

Step 1: Understand Hydroxy Benzene Compounds

Let's first define each compound. (I) 1,2-dihydroxy benzene is also known as catechol, (II) 1,3-dihydroxy benzene is resorcinol, (III) 1,4-dihydroxy benzene is hydroquinone, and (IV) hydroxy benzene is phenol. These are all benzene derivatives with hydroxyl groups (-OH) attached, influencing their boiling points.

Step 2: Assess Intermolecular Forces

Compounds with more hydroxyl groups typically have higher boiling points due to enhanced hydrogen bonding. Among the options, compound III (1,4-dihydroxy benzene, or hydroquinone) and II (1,3-dihydroxy benzene, or resorcinol) have two hydroxyl groups, which means they have stronger hydrogen bonding than compound IV (phenol).

Step 3: Compare Boiling Points of Isomers

Compounds I, II, and III are all dihydroxy benzenes, and their boiling points are influenced by the position of the -OH groups. Typically, hydroquinone (III) has the highest boiling point due to its symmetrical structure which enables strong hydrogen bonding, followed by resorcinol (II), and catechol (I) due to steric hindrance.

Step 4: Arrange in Increasing Order

Considering the above analyses, phenol (IV) having one -OH group is expected to have the lowest boiling point. Next, comparing catechol, resorcinol, and hydroquinone, catechol (I) has the lowest boiling point among the dihydroxy benzenes due to its ortho position and steric hindrance, followed by resorcinol (II), and the highest boiling point is that of hydroquinone (III) due to maximum symmetry and effective hydrogen bonding.

Key Concepts

Hydrogen Bonding

Hydrogen bonding is a crucial concept in understanding the boiling points of alcohols, such as dihydroxy benzenes. It is a type of intermolecular force that occurs when a hydrogen atom is attracted to a highly electronegative atom like oxygen, nitrogen, or fluorine. In the case of alcohols, the hydrogen in the hydroxyl group (-OH) often forms hydrogen bonds with the oxygen of neighboring molecules. This creates a strong attraction between the molecules.

In dihydroxy benzenes, the more hydroxyl groups present, the more opportunities for hydrogen bonding. This results in a stronger intermolecular force and, consequently, a higher boiling point. This is because it requires more energy to break these intermolecular interactions during the phase change from liquid to gas. Additionally, the position of the hydroxyl groups on the benzene ring affects the hydrogen bonding effectiveness, influencing the boiling points of isomers like catechol, resorcinol, and hydroquinone.

Benzene Derivatives

Benzene derivatives are compounds that have one or more substitutes replacing hydrogen atoms in the benzene ring structure. This includes the addition of functional groups like hydroxyl groups, which transform the simple benzene into various alcohols with different chemical properties.

These structural changes significantly affect physical properties like the boiling point. For instance, hydroxy benzene, known as phenol, contains a single benzene ring with a hydroxyl group. This single group allows for hydrogen bonding but to a lesser extent compared to dihydroxy derivatives.

• Catechol (1,2-dihydroxy benzene)
• Resorcinol (1,3-dihydroxy benzene)
• Hydroquinone (1,4-dihydroxy benzene)

These are all examples of benzene derivatives with two hydroxyl groups, increasing their hydrogen bonding potential and boiling points above phenol.

Dihydroxy Benzene

Dihydroxy benzene refers to benzene rings substituted with two hydroxyl groups. The positioning of these groups creates isomers with distinct physical properties. The three common isomers are catechol, resorcinol, and hydroquinone, each with different configurations:

• Catechol (1,2-dihydroxy benzene): The -OH groups are adjacent, or ortho, resulting in intramolecular hydrogen bonding and steric hindrance that reduces its boiling point relative to other isomers.
• Resorcinol (1,3-dihydroxy benzene): The -OH groups are in a meta position, allowing for moderate hydrogen bonding and a higher boiling point than catechol.
• Hydroquinone (1,4-dihydroxy benzene): The -OH groups are opposite, or para, which maximizes the molecule's symmetry and allows for efficient hydrogen bonding, resulting in the highest boiling point among the three.

The differences in boiling points illustrate how molecular structure and hydrogen bonding impact physical properties.