Question

a) Draw Newman projections of the gauche and the anti conformations of 1,2 -ethanediol. b) Why might the gauche conformation be expected to be the more stable of the two? c) Do you think that gauche is also the most stable conformation of 1,2 -dimethoxyethane? Explain.

Short answer

Question: Discuss the difference in stability between the gauche and anti conformations of 1,2-ethanediol and 1,2-dimethoxyethane, and explain the factors contributing to this difference. Answer: The gauche conformation of 1,2-ethanediol is more stable due to the presence of hydrogen bonding between the two hydroxyl groups, which provides additional stability compared to the anti conformation where no hydrogen bonding occurs. On the other hand, in 1,2-dimethoxyethane, there is no opportunity for hydrogen bonding, so the stability relies on the steric effects and torsional strain from the rotation about the C-C bond. In this case, the anti conformation might be more stable as it has less steric hindrance and torsional strain compared to the gauche conformation.

Step-by-step solution

Introducing Newman Projections and Conformations

Newman projections are a way to represent conformations of molecules along a specific bond by viewing the molecule along that bond. There are two types of conformations to focus on - gauche and anti. Gauche conformations have a dihedral angle between two groups of about 60 degrees, while anti conformations have a dihedral angle of about 180 degrees.

Drawing Newman Projections for 1,2-ethanediol

1,2-ethanediol has the formula HO-CH2-CH2-OH. We will draw the Newman projections along the C-C bond for both gauche and anti conformations. #gauche conformation# - Place the front carbon (left-most) with its two hydrogen atoms and the hydroxyl group (OH) around it. - Then place the back carbon (right-most) with its two hydrogen atoms and hydroxyl group around it, such that the angle between the two hydroxyl groups is about 60 degrees. #anti conformation# - Place the front carbon (left-most) with its two hydrogen atoms and hydroxyl group (OH) around it. - Then place the back carbon (right-most) with its two hydrogen atoms and hydroxyl group around it, such that the angle between the two hydroxyl groups is about 180 degrees. Now that we have the Newman projections for both conformations of 1,2-ethanediol, we can proceed to answer the next question.

Explaining the Stability of Gauche Conformation

The gauche conformation might be expected to be more stable due to the presence of hydrogen bonding between the two hydroxyl groups. This intermolecular force makes the gauche conformation more stable compared to the anti conformation, where no hydrogen bonding occurs.

Examining 1,2-dimethoxyethane and Its Stability

1,2-dimethoxyethane has the formula CH3-O-CH2-CH2-O-CH3. Let's examine the gauche conformation and determine its stability. In 1,2-dimethoxyethane, there are no hydroxyl groups, which means there is no opportunity for hydrogen bonding to stabilize the gauche conformation. The stability of the given molecule relies on the steric effects and torsional strain from the rotation about the C-C bond. In this case, the anti conformation might be more stable as it has less steric hindrance and torsional strain compared to the gauche conformation.

Key concepts

Gauche Conformation

When we talk about the gauche conformation, we're referring to a specific arrangement of atoms in a molecule when observed through a Newman projection. Here, the dihedral angle between substituents on adjacent carbon atoms remains at roughly 60 degrees. Why is this important? Gauche conformation can have implications on the stability of a molecule. For example, in molecules like 1,2-ethanediol, the gauche conformation allows for hydrogen bonding between the hydroxyl groups. This interaction provides additional stability to the molecule compared to when the molecule is in its anti conformation. Stability arises because hydrogen bonds are a type of attractive force that helps hold parts of the molecule together more firmly. Remember:

• Gauche conformation has a 60-degree angle.
• It's often stabilized by hydrogen bonding in specific molecules.

Anti Conformation

The anti conformation in a Newman projection is characterized by a dihedral angle of 180 degrees between substituents on adjacent carbon atoms. This essentially places the groups directly opposite each other, minimizing steric interactions. In terms of molecular stability, the anti conformation is typically more stable than the gauche conformation if no additional interactions like hydrogen bonding are in effect. In essence, anti conformation reduces steric hindrance, where groups are spread apart, minimizing repulsion. This concept's usefulness can be seen when analyzing molecules like 1,2-dimethoxyethane. With no hydrogen bonding possibilities, steric and torsional strain are minimized in the anti conformation, often making it more stable than the gauche variant. Key points:

• Anti conformation has a 180-degree angle.
• Generally minimizes steric hindrance.

1,2-ethanediol

1,2-Ethanediol, with the chemical structure HO-CH2-CH2-OH, is a simple organic compound better known as ethylene glycol. This molecule is particularly interesting when studying conformational structures using Newman projections because of the two hydroxyl (OH) groups. These hydroxyl groups enable hydrogen bonding, which is a key factor that contributes to the stability of the gauche conformation in 1,2-ethanediol. In contrast, when this molecule is viewed in its anti conformation, the opportunity for hydrogen bonding is absent, often resulting in less stability. This demonstrates how the presence and positioning of functional groups in a molecule can significantly affect its preferred conformation and overall stability. Consider:

• Hydroxyl groups enable hydrogen bonding.
• Gauche conformation is often more stable due to these interactions.

1,2-dimethoxyethane

1,2-Dimethoxyethane, with the chemical formula CH3-O-CH2-CH2-O-CH3, offers a different perspective on molecular conformations. This molecule does not have hydroxyl groups, replacing them with methoxy groups (OCH3). In 1,2-dimethoxyethane, the lack of hydrogen bonding opportunities means that the stabilizing effect found in 1,2-ethanediol in the gauche conformation is absent. Here, the anti conformation is generally more stable due to reduced steric hindrance and torsional strain. This means the substituents are aligned in a way that minimizes repulsion, leading to less energetic strain on the molecule. Recap:

• No hydrogen bonding due to methoxy groups.
• Anti conformation often more favorable due to less steric hindrance.