Question

In the boat conformation of cyclohexane, the most destabilizing interaction is (a) Eclipsing (b) Flagpole-flagpole (c) 1,3 -diaxial (d) 1,3 -diequatorial

Short answer

Flagpole-flagpole interaction (b) is the most destabilizing in boat conformation.

Step-by-step solution

Identify Boat Conformation

In organic chemistry, cyclohexane can exist in several conformations. The boat conformation is one of these, characterized by its 'boat-like' structure with two carbons sticking up at one end. This is distinctly different from the chair conformation which is more stable.

Understand Destabilizing Interactions

Destabilizing interactions in cyclohexane can include eclipsing interactions, steric hindrance from nearby groups, or interactions between groups in axial positions. In the boat conformation specifically, there are two hydrogens at the 'poles' or ends of the boat, called the flagpole hydrogens, which can interact with each other.

Recognize Key Destabilization in Boat Conformation

In the boat conformation of cyclohexane, the flagpole hydrogens on carbons 1 and 4 can interact negatively due to their proximity, leading to increased steric strain. This specific interaction is commonly considered to be the most destabilizing feature of the boat conformation.

Analyze Given Options

- **(a) Eclipsing:** Eclipsing occurs mainly in planar or near-planar conformations and is less significant in the context of axial positions. - **(b) Flagpole-flagpole:** This refers to the steric hindrance between the flagpole hydrogens in the boat conformation. - **(c) 1,3-diaxial:** Interaction concerns chair conformations involving axial groups. - **(d) 1,3-diequatorial:** Generally offers less steric strain as equatorial positions are more staggered in stable conformations.

Conclude Most Destabilizing Interaction

Considering the specific issues in the boat conformation, the flagpole-flagpole interaction is indeed the most destabilizing due to its steric strain, which makes answer (b) the correct choice.

Key concepts

Steric Strain in Cyclohexane

Steric strain occurs when atoms are pushed closer together than their preferred distances, causing repulsion. This happens in cyclohexane due to the arrangement of its atoms in space. In the boat conformation, steric strain is significantly pronounced.
This is mainly due to the flagpole hydrogens or the hydrogens located at the ends of the boat-shaped structure. These hydrogens are forced closer together than is comfortable, leading to a rise in energy and instability of the molecule. In simpler terms, imagine two people trying to fit through a narrow doorway at the same time. They push against each other, causing discomfort. This is similar to what happens with the atoms in cyclohexane when steric strain is present. It's the molecular equivalent of being cramped!
The less space and more interactions between groups, the higher the steric strain will be, which can lead to destabilization of the molecule.

Destabilizing Interactions in Organic Chemistry

In organic chemistry, destabilizing interactions are forces that increase the internal energy of a molecule, making it less stable. Such interactions are often due to atoms or groups being too close, exerting forces on one another. Cyclohexane in its boat conformation presents a perfect example of this. You can visualize the destabilizing interactions through:

• Flagpole-flagpole interaction: This occurs between the two hydrogens positioned at the top ends of the 'boat'. They are squeezed too close together, causing unwanted repulsion.
• Eclipsing interactions: When bonds or atoms are aligned with each other, as in the case of some adjacent carbon-hydrogen pairs in cyclohexane, they create tension in the conformation due to increased electron cloud overlap.

Each of these contributes to cyclohexane's instability in the boat form, making it less favored compared to more stable conformations like the chair.

Eclipsing vs Steric Hindrance

Eclipsing and steric hindrance are both phenomena that contribute to molecular instability, but they arise from different principles. - **Eclipsing**: Involves atoms or bonds aligned such that they overlap in terms of their electron clouds, leading to a rise in energy. This is like having two shadows overlap with each other, blocking light and causing a dark spot. It's a less significant factor in cyclohexane's boat conformation where alignment is not as pronounced as in planar structures. - **Steric Hindrance**: Occurs when bulky groups within a molecule interfere with each other's rotations or movements. It's like trying to fit large puzzle pieces too close together, causing interference. In cyclohexane's boat conformation, steric hindrance is more pronounced due to flagpole-flagpole interactions where ample space is not available. While both can cause destabilization, in the case of cyclohexane's boat conformation, steric hindrance, specifically flagpole-flagpole interaction, is considered the main source of instability.