Problem 29 - A Complete Resource Book in Chemistry for JEE Main

Question

H-bond is strongest in (a) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) (b) \(\mathrm{H}-\mathrm{F}\) (c) \(\mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\)

Step-by-Step Solution

Answer

The strongest H-bond is in  ext{H-F}.

Step 1: Understand Hydrogen Bonding

Hydrogen bonding is a type of strong dipole-dipole attraction between molecules. It occurs when hydrogen is bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine. This makes it form a strong attraction to lone pairs on electronegative atoms in other molecules.

Step 2: Identify Presence of Hydrogen Bonding in Each Option

Examine each compound to identify potential hydrogen bonding: (a) Ethanol  ext{C}_{2} ext{H}_{5} ext{OH} has O-H groups which form hydrogen bonds, (b) Hydrogen fluoride  ext{H-F} can form strong hydrogen bonds due to the highly electronegative fluorine, (c) Water  ext{H}_2 ext{O} has O-H bonds which also form hydrogen bonds, (d) Acetone  ext{CH}_{3} ext{COCH}_{3} lacks a direct hydrogen bond donor (O-H or N-H).

Step 3: Determine Bond Strength Based on Electronegativity

Hydrogen bonds are stronger when formed with more electronegative atoms. Fluorine is the most electronegative element, followed by oxygen. Therefore, hydrogen bonds in  ext{H-F} are stronger than those in water or ethanol.

Step 4: Compare Hydrogen Bonding Across the Options

 ext{H-F} forms the strongest hydrogen bond because fluorine's higher electronegativity compared to oxygen in  ext{H}_{2} ext{O} and  ext{C}_{2} ext{H}_{5} ext{OH} results in stronger hydrogen bonding.  ext{CH}_{3} ext{COCH}_{3} doesn't engage in hydrogen bonding.

Key Concepts

Electronegativity

Electronegativity is a key concept in chemistry that describes the ability of an atom to attract electrons towards itself. Think of it as a measure of how "hungry" an atom is for electrons.
This property is crucial when it comes to understanding chemical bonding. In molecules where atoms with different electronegativities are bonded, an uneven distribution of electron density is created.
Electrons tend to spend more time around the more electronegative atom, leading to something called a dipole.When it comes to hydrogen bonding, electronegativity is extremely important. Hydrogen bonds form when a hydrogen atom is covalently bonded to a highly electronegative atom, such as:

• Fluorine
• Oxygen
• Nitrogen

These elements are among the most electronegative, making them prime candidates for forming hydrogen bonds. Fluorine, in particular, is the most electronegative element, which explains why hydrogen bonds in \( \text{H-F} \) are exceptionally strong. This property's significance helps to clarify why certain molecules exhibit stronger hydrogen bonding based on the participating atoms' electronegativity levels.

Dipole-Dipole Attraction

Dipole-dipole attraction is a type of intermolecular force that arises from the interaction of polar molecules. These are molecules where there is a distinct separation of positive and negative electrical charges.
The molecule ends with unequal electron sharing, known as dipoles.Within dipole-dipole interactions, the positive end of one polar molecule attracts the negative end of another, creating a bond.
Hydrogen bonds are a special type of dipole-dipole attraction, but much stronger due to hydrogen's participation with highly electronegative atoms.Here's why dipole-dipole attractions are important:

• They affect boiling and melting points of substances.
• They are crucial in determining molecular structures.
• They influence solubility in different solvents.

Understanding dipole interactions is essential to grasping how molecules behave in different environments. In our original exercise, hydrogen bonding is the strongest form of dipole-dipole interaction,
especially evident in the strong hydrogen bonding seen in molecules like \( \text{H-F} \) due to fluorine's high electronegativity.

Hydrogen Bond Strength

Hydrogen bond strength is influenced by several factors, electronegativity being one of the most important. The more electronegative the atom bonded to hydrogen, the stronger the bond.
Hydrogen bonds are significantly stronger than regular dipole-dipole forces, but they are not as strong as covalent or ionic bonds.Several factors impact hydrogen bond strength:

• Electronegativity of the atom bonded to hydrogen: Stronger in bonds with fluorine than with oxygen or nitrogen.
• Number of hydrogen bonds: Compounds like water can form multiple hydrogen bonds, enhancing stability.
• Distance between molecules: Shorter distances strengthen hydrogen bonding.

In the context of the original exercise, \( \text{H-F} \) manifests the strongest hydrogen bond because fluorine's high electronegativity makes a very strong attraction with hydrogen.
This explains why \( \text{H-F} \) is deemed to exhibit the strongest hydrogen bonding compared to others like water (\( \text{H}_2\text{O} \)) or ethanol (\( \text{C}_2\text{H}_5\text{OH} \)).Understanding these aspects can help in predicting the physical properties of substances where hydrogen bonding plays a significant role.