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

(d) \(\mathrm{IV}<\mathrm{I}<\mathrm{II}<\mathrm{III}\).

Step 1: Identify the Compounds

First, let's identify the given compounds: (I) 1,2-dihydroxy benzene is commonly known as catechol. (II) 1,3-dihydroxy benzene is known as resorcinol. (III) 1,4-dihydroxy benzene is known as hydroquinone. (IV) Hydroxy benzene is simply phenol.

Step 2: Understand Boiling Point Influences

Boiling points are influenced by molecular interactions. In these compounds, hydrogen bonding plays a crucial role. Compounds with more hydroxyl groups and optimal hydrogen bonding will have higher boiling points. For dihydroxy benzenes, the more symmetrical the hydroxyl groups, the stronger the intermolecular hydrogen bonding, leading to higher boiling points.

Step 3: Arrange Based on Hydrogen Bonding

Hydroquinone (III) has the hydroxy groups para, allowing efficient hydrogen bonding, so it has the highest boiling point. Resorcinol (II), with meta hydroxyl groups, forms less efficient hydrogen bonds than hydroquinone but more than catechol, which has ortho positions (I). Lastly, phenol (IV) has only one hydroxyl group, resulting in the lowest hydrogen bonding and hence the lowest boiling point.

Step 4: Rank the Compounds

Rank the compounds based on the analysis of hydrogen bonding: Phenol (IV) < Catechol (I) < Resorcinol (II) < Hydroquinone (III). This corresponds to increasing boiling points.

Key Concepts

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction that plays a significant role in determining the boiling points of organic compounds. It occurs when a hydrogen atom is directly bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine. This bond results in a significant attraction between molecules, requiring more energy to break. In dihydroxy benzenes, each hydroxyl group contains a hydrogen atom that can form hydrogen bonds with nearby molecules. The position and number of hydroxyl groups on the benzene ring can influence the strength and efficiency of hydrogen bonding. The more optimal the arrangement for hydrogen bonding, the higher the boiling point will be. Compounds with more and stronger hydrogen bonds generally have higher boiling points due to the added energy needed to overcome these interactions.

Molecular Interactions

The boiling point of a compound is heavily influenced by molecular interactions. Among these interactions, hydrogen bonding is particularly important for dihydroxy benzenes. However, other interactions also play a role.

• Van der Waals forces: These are weak forces present in all molecules, including dihydroxy benzenes. While weaker than hydrogen bonds, they still contribute to the boiling point.
• Dipole-dipole interactions: In polar molecules, partially positive and negative ends attract each other. This attraction is significant in dihydroxy benzenes where the hydroxyl groups create polarity.

The combined strength of these interactions determines the overall boiling point. In dihydroxy benzenes, effective hydrogen bonding combined with other molecular interactions leads to a higher boiling point.

Phenol

Phenol, also known as hydroxybenzene, is a simple compound consisting of a benzene ring and a single hydroxyl group. This structure makes it an excellent example of how hydrogen bonding affects boiling points. Because phenol only has one hydroxyl group, its ability to form hydrogen bonds is limited compared to its dihydroxy counterparts. Consequently, phenol has a lower boiling point than the dihydroxy benzenes due to less effective intermolecular forces. It's important to recognize that while phenol has fewer opportunities for hydrogen bonding, it still forms hydrogen bonds due to its hydroxyl group. However, the strength and number cannot match compounds with two hydroxyl groups.

Catechol

Catechol, also known as 1,2-dihydroxy benzene, features two ortho-positioned hydroxyl groups. This positioning affects the intramolecular and intermolecular interactions, contributing to its boiling point properties. Ortho-positioning allows for intramolecular hydrogen bonding, which can sometimes restrict effective intermolecular hydrogen bonds. This tendency results in catechol having a lower boiling point than other dihydroxy benzenes with different arrangements of hydroxyl groups. Nevertheless, catechol still forms more hydrogen bonds than phenol due to having two hydroxyl groups, thus having a higher boiling point than phenol.

Resorcinol

Resorcinol, or 1,3-dihydroxy benzene, has its hydroxyl groups in the meta position on the benzene ring. This configuration impacts hydrogen bonding differently than ortho and para positions. The meta arrangement allows for more extensive intermolecular hydrogen bonding compared to catechol. Resorcinol cannot form significant intramolecular hydrogen bonds, so it directs most bonding potential toward forming intermolecular hydrogen bonds. As a result, resorcinol typically has a higher boiling point than catechol, although it remains less than that of hydroquinone. Its boiling point reflects the balance between intermolecular and potential intramolecular interactions based on its structure.

Hydroquinone

Hydroquinone, also known as 1,4-dihydroxy benzene, takes advantage of the para-positioning of its hydroxyl groups. This arrangement represents an optimal structure for effective hydrogen bonding. In the para position, both hydroxyl groups are perfectly positioned to engage in efficient hydrogen bonding with adjacent molecules. This unique spatial arrangement leads to stronger intermolecular hydrogen bonds compared to other dihydroxy benzene isomers. Hydroquinone's structure enables it to have the highest boiling point among the dihydroxy benzenes discussed here. The strength and efficiency of its hydrogen bonds are key contributors to its enhanced boiling point, making it distinctive in terms of thermal properties.