Problem 38 - A Complete Resource Book in Chemistry for JEE Main

Question

Intermolecular hydrogen bonding is strongest in (a) methylamine (b) phenol (c) formaldehyde (d) methanol

Step-by-Step Solution

Answer

Methanol (d) has the strongest intermolecular hydrogen bonding.

Step 1: Understand the Concept

Intermolecular hydrogen bonding occurs when hydrogen is bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine. The strength of hydrogen bonding depends on the presence and number of these electronegative atoms available for bonding.

Step 2: Analyze Each Option

- **Methylamine (CH₃NH₂)**: Contains a ext{N} bonded to ext{H}, which can participate in hydrogen bonding. - **Phenol (C₆H₅OH)**: Contains an ext{O-H} group, which is capable of strong hydrogen bonding. - **Formaldehyde (HCHO)**: Lacks an ext{O-H} or ext{N-H} group necessary for hydrogen bonding. - **Methanol (CH₃OH)**: Contains an ext{O-H} group allowing hydrogen bonding.

Step 3: Determine the Strength of Hydrogen Bonding

Both phenol and methanol can participate in hydrogen bonding through ext{O-H} groups, but methanol is smaller, allowing it to form more hydrogen bonds per molecule, making the bonds more effective.

Step 4: Select the Strongest Hydrogen Bonding

Given the analysis, methanol, with its smaller size and effective hydrogen bonding through ext{O-H} groups, exhibits the strongest intermolecular hydrogen bonding.

Key Concepts

Intermolecular Forces

Intermolecular forces are the glue that stick molecules together. These forces make a big difference in physical properties like melting and boiling points.
They occur between molecules, unlike intramolecular forces which occur within a molecule. There are several types of intermolecular forces, but hydrogen bonding is a very special one. It's the strongest and plays a crucial role, especially when hydrogen is attached to highly electronegative atoms such as nitrogen, oxygen, or fluorine creating a partial charge.

Hydrogen bonds are like extra strong magnets holding molecules tightly together. In the case of methanol and phenol from the exercise, both have OH groups allowing strong hydrogen bonds. These bonds help explain why some substances are liquid at room temperature while others are gases.

• Hydrogen Bonding: Occurs when H is bonded to N, O, or F.
• Dipole-dipole Interactions: Happen in polar molecules.
• London Dispersion Forces: Present in all molecules, but more significant in larger atoms/molecules.

Understanding these forces helps us explain many behaviors of chemicals and why even small molecules like methanol can pack a punch.

Molecular Structure

The shape and structure of a molecule greatly tell how it behaves and interacts. In our context, think about how a tiny change in a molecule can lead to massive differences in properties.
For instance, methanol and formaldehyde have similar sizes but behave entirely differently in terms of hydrogen bonding due to structural differences.

Let's take a closer look at the molecules in the exercise:

• Methylamine (CH₃NH₂): Has a nitrogen atom that can lead to hydrogen bonding because N is electronegative.
• Phenol (C₆H₅OH): A larger molecule with an OH group, good for strong hydrogen bonds but bulky.
• Formaldehyde (HCHO): Despite having O, lacks OH or NH, making its hydrogen bonding poor.
• Methanol (CH₃OH): Small, nimble, with an OH group ideal for strong, effective hydrogen bonding.

How these atoms are connected, and the shape they form, dictate how easily they can connect with others. With methanol, its small, simple structure allows more flexibility and more chances to form bonds.

Electronegativity

Electronegativity is like a game of tug-of-war between atoms for electrons. Some atoms, like oxygen, nitrogen, and fluorine, are very strong in this tug of war. They tend to pull electrons towards themselves, which leads to polar bonds.
This is super important for hydrogen bonding. Hydrogen, when bonded to these electronegative giants like O or N, gets slightly positive, while the electronegative partner gets slightly negative.

This means a polar molecule can act like a magnet with a north and south pole:

• Electronegative elements: O, N, and F are essential for forming strong hydrogen bonds.
• Polarity: Polar bonds in molecules like methanol lead to effective hydrogen bonding.

In comparing methanol and phenol, the presence of the OH group in each plays a role, but methanol's small size maximizes the effect of these polar interactions, unlike bulkier phenol.

Organic Compounds

Organic compounds are molecules mainly made of carbon and hydrogen. They're everywhere in our daily life – from the caffeine in your coffee to the gas in your tank.
In chemistry, organic means carbon-based, and there's a vast variety of these molecules.

Due to their versatile nature and various configurations, organic compounds can exhibit different properties based on their structure and functional groups:

• Carbon Chains and Rings: Provide the backbone for organic molecules, as seen in phenol.
• Functional Groups: Like OH in methanol and phenol that determine chemical behavior.
• Versatility: Organic compounds range from very simple to incredibly complex shapes and functions.

In the exercise, we see how the presence of a simple group like OH can profoundly affect the properties of an organic compound, making methanol a superb candidate for hydrogen bonding relative to formaldehyde, which lacks this group.