Question

The correct stability order for conformation of ethylene glycol is (A) Anti \(>\) Gauche \(>\) Partially Eclipsed \(>\) Eclipsed (B) Gauche \(>\) Anti \(>\) Partially Eclipsed \(>\) Eclipsed (C) Eclipsed \(>\) Partially Eclipsed \(>\) Gauche \(>\) Eclipsed (D) Partially Eclipsed \(>\) Eclipsed \(>\) Gauche \(>\) Anti

Short answer

The correct stability order for the conformation of ethylene glycol is Anti > Gauche > Partially Eclipsed > Eclipsed, which corresponds to option (A). This order is due to the differences in steric hindrance and efficiency of intramolecular hydrogen bonding among the given conformations.

Step-by-step solution

Understanding Conformations

Conformations are different spatial arrangements of atoms that can be converted into one another by rotation around single bonds. In case of ethylene glycol, the relevant bonds for the rotation are the C-C and O-C bonds.

Understanding Anti Conformation

In anti conformation, the diol groups (-OH) of ethylene glycol are as far apart from each other as possible. This results in least steric hindrance and allows for efficient intramolecular hydrogen bonding, making it the most stable conformation.

Understanding Gauche Conformation

In the gauche conformation, the two diol groups (-OH) are next to each other. This causes a bit of a steric hindrance but still allows for efficient intramolecular hydrogen bonding.

Understanding Partially Eclipsed Conformation

In partially eclipsed conformation, the diol groups (-OH) are slightly overlapped due to which the sterics is larger than in the anti and gauche conformations and the efficient hydrogen bonding is also inhibited.

Understanding Eclipsed Conformation

In completely eclipsed conformation, the diol groups are maximally overlapped causing maximum steric hindrance and inhibition of any possible hydrogen bond, making it the least stable conformation. Based on the above understanding, the correct stability order for conformation of ethylene glycol is:

Identifying the Correct Option

Anti > Gauche > Partially Eclipsed > Eclipsed. Therefore, the correct answer is option (A).

Key concepts

Conformations in Organic Chemistry

To understand how molecules like ethylene glycol behave, we must explore the concept of conformations in organic chemistry. Conformations are the different spatial arrangements that molecules can adopt due to rotation around single bonds without breaking them. Imagine holding a book open and spinning just one page—each position of the page represents a different conformation.

These molecular arrangements are often visualized using Newman projections or sawhorse representations, portraying a molecule from a specific angle. For molecules with multiple atoms and bonds, such as ethylene glycol, there can be numerous potential conformations, each with its own level of stability.

The stability of each conformation is critical as it can influence the molecule's reactivity and interaction with other molecules. For instance, the most stable conformation will typically be the one that minimizes steric clashes and maximizes attractive interactions like hydrogen bonding within the molecule.

Steric Hindrance

Steric hindrance refers to the repulsion that results when the electron clouds of atoms or groups within a molecule are too close to each other. It’s like trying to walk through a crowded room—too many obstacles close to each other can impede the flow. In the context of ethylene glycol conformations, steric hindrance occurs when the hydroxyl groups (-OH) and other atoms are positioned in a way that they crowd each other.

Why is this important? Because the degree of steric hindrance affects a molecule's stability. When atoms are forced into uncomfortable proximities, the molecule’s energy rises, meaning less stability. A molecule will naturally favor a conformation that minimizes these repulsive interactions, thus avoiding scenarios where crowding is significant. Among the conformations of ethylene glycol, the anti conformation, with its widely spaced out atoms, faces minimal steric hindrance and is, therefore, the most stable.

Intramolecular Hydrogen Bonding

Why Hydrogen Bonds Matter

Hydrogen bonds are a type of attractive interaction that occurs when a hydrogen atom is covalently bound to a highly electronegative atom—most commonly oxygen, nitrogen, or fluorine—and then forms a weak bond with another electronegative atom. In ethylene glycol, this interaction can occur between the oxygen of one hydroxyl group and the hydrogen of the other.

Influence on Conformational Stability

Intramolecular hydrogen bonding greatly influences conformational stability. A conformation that allows for these hydrogen bonds will generally be more stable due to the attraction between these opposite parts of the molecule, like a magnet pulling its two ends together. On the flip side, conformations that limit or prevent hydrogen bonding, such as the eclipsed conformation in ethylene glycol, are less stable. Simply put, the ability to form hydrogen bonds can give a molecule its preferred (most stable) shape, allowing it to exist comfortably with minimal energy.