Problem 94 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

In benzene molecule carbon atoms are inclined at an angle of: |BHU 2000] (a) \(60^{\circ}\) (b) \(109^{\circ}, 28\) (c) \(120^{\circ}\) (d) \(180^{\circ}\)

Step-by-Step Solution

Answer

In a benzene molecule, carbon atoms are inclined at an angle of \(120^{\circ}\).

Step 1: Understand the Benzene Structure

Benzene is a planar cyclic molecule with the formula \(C_6H_6\). It consists of a ring of six carbon atoms with alternating double bonds, creating a symmetrical hexagonal shape.

Step 2: Determine the Bond Angles in Benzene

Due to its planar and symmetrical structure, each carbon atom in benzene forms angles with adjacent carbon atoms. In a regular hexagon, each interior angle should be \(120^{\circ}\).

Step 3: Conclusion from Structure

The symmetry and planarity of the benzene ring ensure that the bond angles between adjacent carbon atoms are \(120^{\circ}\), consistent with the geometry of a regular hexagon.

Key Concepts

Planar Cyclic Molecule

A key feature of benzene’s molecular structure is that it forms a planar cyclic molecule. This means that all six carbon atoms and the hydrogen atoms attached to them lie on the same plane. The term "cyclic" hints at the formation of a closed ring. This planarity is not just about aesthetics; it allows for the delocalization of electrons across the molecule. These electrons are spread out evenly across the planar structure, lending the benzene ring its stability.

• Aromatic Nature: The even spread of electrons gives rise to benzene's aromatic properties, making it less reactive than linear alkanes or alkenes.
• Flat Structure: Because all atoms are in a single plane, benzene can easily stack its molecules, similar to sheets of graphene, adding further stability to compounds in which it is present.

This unique planarity helps benzene to engage in important reactions, making it a fundamental compound in various chemical processes.

Hexagonal Shape

Another prominent characteristic of benzene is its hexagonal shape. Imagine a perfect hexagon – all sides equal and all angles equal too. Benzene naturally aligns into this geometric form because of its six carbon atoms linked in a symmetrical arrangement. This setup gives benzene a balanced and uniform appearance.

• Symmetry: The uniform hexagonal structure means that benzene is symmetric, contributing to its chemical stability.
• Equal Bond Lengths: Despite having alternating single and double bonds, the electrons in benzene are delocalized, resulting in equal bond lengths throughout the molecule.

The hexagonal shape of benzene is not only important for its appearance but also influences its reactivity and interaction with other molecules.

Bond Angles in Benzene

A tedious yet fascinating detail about the benzene molecule is its bond angles, which are always consistent due to its hexagonal geometry. In benzene, the bond angles between adjacent carbon atoms are each precisely 120 degrees. This is typical of the interior angles of a regular hexagon, as you may recall from basic geometry.

• Geometric Consistency: With all bond angles at 120 degrees, benzene maintains a perfect equilibrium, which strengthens the molecule.
• Planar Support: The 120-degree angles ensure that benzene remains planar, which is crucial for its delocalized electron system.

Understanding these bond angles is essential for recognizing why benzene exhibits such remarkable stability and why it is less prone to reactions that would disrupt its structure.