Question

The most stable conformation of ethylene glycol is (a) anti (b) gauche (c) fully eclipsed (d) partially eclipsed

Short answer

The most stable conformation of ethylene glycol is (b) gauche due to intramolecular hydrogen bonding.

Step-by-step solution

Understand the Molecule

Ethylene glycol is a simple compound with the chemical formula \( \text{C}_2\text{H}_6\text{O}_2 \). It consists of two hydroxyl groups (-OH) attached to two carbon atoms in a chain.

Identify Possible Conformations

The conformation of a molecule like ethylene glycol depends on the spatial arrangement of its atoms. For ethylene glycol, consider the torsional angles created when rotating around the C-C bond. Conformations include anti, gauche, fully eclipsed, and partially eclipsed.

Evaluate Energy Levels

Anti conformations usually have lower energy due to minimal steric hindrance, as substituents on adjacent carbons are furthest apart. Gauche conformations have substituents closer, leading to slight steric interaction, while eclipsed conformations (fully or partially) often have the highest energy due to maximum steric clash.

Consider Ethylene Glycol's Steric Hindrance

In ethylene glycol, the interaction between hydroxyl groups is crucial. The most stable configuration minimizes these interactions. Gauche can be particularly stabilized by intramolecular hydrogen bonding in ethylene glycol, not possible in fully or partially eclipsed forms.

Determine Most Stable Conformation

Given these considerations, the gauche conformation of ethylene glycol is often stabilized by hydrogen bonding between the -OH groups, lowering its energy relative to other conformations.

Key concepts

Ethylene Glycol

Ethylene glycol is a crucial topic in conformational analysis due to its chemical simplicity yet interesting behavior. With the formula \( \text{C}_2\text{H}_6\text{O}_2 \), it is formed by two hydroxyl groups \((-OH)\) attached to a two-carbon backbone. Each carbon in the molecule is bonded to three additional hydrogens, creating a visually symmetric structure. The molecule offers insight into how different conformations affect stability.

In ethylene glycol, changes about the carbon-carbon single bond allow for rotational degrees of freedom. Such rotations alter the spatial arrangement of the atoms, resulting in different conformations like anti, gauche, fully eclipsed, and partially eclipsed. Recognizing these conformations is vital for predicting reactivity and interactions in larger, more complex molecules. Understanding the basics of these structures is a critical foundation when delving into organic chemistry.

Steric Hindrance

Steric hindrance is a concept where the physical presence of atoms in a molecule can interfere with or impede chemical reactions and bonding. In the context of ethylene glycol, this idea plays a significant role in the energy associated with each possible conformation.

The anti conformation is considered lowest in energy due to minimal steric hindrance. In this form, the hydroxyl groups are as far apart as possible, reducing repulsive forces. Other conformations like the fully eclipsed and partially eclipsed ones are higher in energy. In these, atoms are closer together, causing increased steric clash, as their electron clouds repel each other. These clashes raise the energy because disrupting their closeness necessitates the input of more energy.

Intramolecular Hydrogen Bonding

Intramolecular hydrogen bonding significantly impacts the stability and conformation of ethylene glycol. This type of hydrogen bond occurs within a single molecule rather than between different molecules. For ethylene glycol, the gauche conformation is favorably stabilized by such hydrogen bonding.

In the gauche conformation, the hydroxyl groups are close enough to allow the partial negative charge on the oxygen atom of one group to attract the partial positive charge on the hydrogen atom of the other group. This internal hydrogen bond lowers the energy of the conformation, making it more stable than what might be expected from steric considerations alone.

• Hydrogen bonding requires a hydrogen donor (O-H) and a hydrogen acceptor (O- from the other hydroxyl group).
• In ethylene glycol, this bonding can only occur in conformations such as gauche, where the groups are suitably aligned.
• This decrease in energy makes gauche a particularly intriguing conformation for practical applications and study.