Question

Increasing order of stability among the three main conformations (eclipse, anti, gauche) of 2 fluoroethanol is (a) eclipse, gauche, anti (b) gauche, eclipse, anti (c) eclipse, anti, gauche (d) anti, gauche, eclipse

Short answer

The order is (a) eclipse, gauche, anti.

Step-by-step solution

Understanding Conformations

In organic chemistry, the stability of molecules can be analyzed through their conformations. Conformations are different arrangements of atoms that result from the rotation around a single bond without breaking any bonds. The three main conformations of interest in this problem are eclipse, anti, and gauche.

Eclipse Conformation

In the eclipse conformation, atoms or groups on adjacent carbon atoms are aligned with each other, leading to maximum steric hindrance and electronic repulsion. This configuration leads to the least stability due to the increased torsional strain.

Anti Conformation

In the anti conformation, two groups are positioned opposite to each other at a 180-degree angle, reducing electronic repulsion and steric hindrance. Because it minimizes these repulsive forces, it is considered the most stable conformation.

Gauche Conformation

In the gauche conformation, the groups are staggered but positioned at a 60-degree angle, which results in intermediate stability. The gauche conformation is less stable than the anti conformation but more stable than the eclipse due to lower steric and electronic repulsion compared to eclipse but higher than anti.

Determining Stability Order

With the most stability associated with anti, intermediate stability with gauche, and the least stability with eclipse, the increasing order of stability is defined as: eclipse, gauche, anti.

Key concepts

Eclipse Conformation

The eclipse conformation occurs when atoms or groups attached to adjacent carbon atoms align with each other. This conformation is marked by a direct overlap, which leads to maximum steric hindrance. Steric hindrance refers to the resistance that results from the atoms or groups being too close together. This alignment also increases electronic repulsion, as negatively charged electron clouds from these atoms interfere with each other.

As a result, the eclipse conformation harbors a lot of torsional strain. Torsional strain is the name given to the repulsion felt when atoms or groups are forced into close proximity during rotation around a bond. Because of these factors, the eclipse conformation is considered to be the least stable of the three main conformations.

• Maximum steric hindrance occurs in the eclipse conformation.
• High electronic repulsion adds to torsional strain.
• Eclipse conformation is the least stable arrangement due to these effects.

Anti Conformation

Anti conformation is the term used for an arrangement where two substituent groups on adjacent carbon atoms are positioned opposite each other. This arrangement results in a 180-degree angle between them. By being as far apart as possible, the substituents experience minimal steric hindrance. Additionally, the electronic repulsion between the groups is also reduced.

As a result, the anti conformation is the most stable conformation due to lower energy associated with minimized repulsive interactions. In many instances, molecules will naturally adopt the anti conformation as it enables them to exist in a state of greater stability.

• Substituents are positioned at a 180-degree angle in anti conformation.
• Minimal steric and electronic repulsion occur.
• Anti conformation is the most stable form.

Gauche Conformation

Gauche conformation describes a scenario where the substituent groups occupy positions at a 60-degree angle relative to each other. While these groups are staggered, meaning they are not directly overlapping, they are still close enough to each other to introduce some steric hindrance and electronic repulsion.

The gauche position offers intermediate stability. It’s more stable than the eclipse due to reduced steric and torsional interactions but is less stable than the anti arrangement because the groups are not as far apart as they could be. This intermediate nature makes the gauche conformation versatile in representing certain molecular interactions but has its own limitations in terms of stability.

• Gauche conformation has a 60-degree angle between groups.
• Staggered arrangement minimizes overlap but not completely.
• It offers intermediate stability compared to other conformations.