Problem 81 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

The dihedral angle between two methyl groups in partially eclipsed conformation of n-butane is (a) \(180^{\circ}\) (b) \(120^{\circ}\) (c) \(90^{\circ}\) (d) \(109^{\circ} 28^{\prime}\)

Step-by-Step Solution

Answer

The dihedral angle is 120°.

Step 1: Understanding Dihedral Angles

Dihedral angle is the angle between two planes. In organic chemistry, when discussing conformations, this angle is specifically between substituents on adjacent carbons.

Step 2: Identifying Partially Eclipsed Conformation

In n-butane, a partially eclipsed conformation occurs when the methyl groups have a dihedral angle less than that of staggered (which is 180°) but more than fully eclipsed (which is 0°).

Step 3: Determining the Dihedral Angle

The partially eclipsed conformation typically has a dihedral angle of 120°. This is because while staggered is the most relaxed (180°), the first step toward eclipsed, which is the least stable, moves to 120° in butane.

Key Concepts

n-butane conformations

In organic chemistry, understanding the different conformations of a molecule like n-butane is crucial. N-butane, a simple alkane with four carbon atoms, can rotate around its central C-C bond, creating various spatial arrangements known as conformations. These rotations lead to different potential energy levels and stability. Let's explore the most commonly discussed conformations of n-butane:

• Staggered Conformation: This is the most stable form. In this orientation, the carbon-hydrogen bonds on adjacent carbons are offset from each other. It minimizes repulsions and maximizes stability.
• Eclipsed Conformation: Here, the bonds are aligned, increasing repulsion. This makes it less stable than the staggered form.
• Gauche Conformation: This is a type of staggered conformation where the two methyl groups (CH₃) are 60° apart, causing some steric strain, but it is still more stable than eclipsed forms.
• Partially Eclipsed Conformation: This occurs when the dihedral angle between the two methyl groups is approximately 120°, which is less stable than staggered but more stable than fully eclipsed conformations.

Learning about these various conformations helps in understanding the dynamic nature of molecules and how they impact chemical reactions and interactions.

partially eclipsed conformation

The partially eclipsed conformation of n-butane is an interesting and important concept in chemistry. It occurs when the dihedral angle between two adjacent methyl groups is about 120°. This conformation is partway between the staggered and fully eclipsed conformations and is crucial when studying molecular interactions.

• Transition State: The partially eclipsed conformation can be considered a transition state during the rotation of the molecule. It's a key step as n-butane swings between the more stable staggered forms.
• Energy Considerations: The partially eclipsed conformation possesses higher energy compared to staggered forms because of increased steric strain, but it is not as energetically unfavourable as the fully eclipsed conformation.

Recognizing this conformation helps in predicting reaction pathways and the stability of transient species in various chemical processes

conformation stability in alkanes

Alkanes like n-butane can adopt a variety of conformations, each with differing levels of stability. The stability in conformations is primarily influenced by the repulsion between bonds and the torsional strain due to eclipsing interactions. For an alkane:

• Staggered Conformation: This is the most stable due to minimized torsional strain. Adjacent bonds are as far apart as possible, which translates to lower potential energy.
• Partially Eclipsed and Fully Eclipsed: As the molecule rotates, partially eclipsed (120°) and fully eclipsed (0°) conformations appear. Both cause increased torsional strain. Fully eclipsed is the least stable, as it experiences maximum strain.
• Torsional and Steric Effects: Factors such as torsional strain (from eclipsing C-H bonds) and steric hindrance (from bulky groups like methyl groups coming close) influence each conformation’s energy levels.

Understanding and analyzing these stability differences can greatly aid in predicting chemical behaviors and reaction mechanisms.