Question

Draw Newman projections for the staggered conformations of ethyl chloride \(\left(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{Cl}\right)\) and 1,2 -dichloroethane \(\left(\mathrm{Cl}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{Cl}\right)\). In the second case, there are two staggered conformations of different energy. Can you estimate which is more stable?

Short answer

The anti conformation of 1,2-dichloroethane is more stable because the chlorine atoms are further apart, reducing steric hindrance compared to the gauche conformation.

Step-by-step solution

Understanding the Newman Projection

A Newman projection is a way to visualize the conformation of a molecule from a perspective where one looks along the axis of a carbon-carbon bond. We'll focus on the front carbon and see the substituents positioned 120° apart due to the tetrahedral arrangement of the sp3 carbons.

Newman Projection for Ethyl Chloride

For ethyl chloride \(\text{(CH}_3\text{-CH}_2\text{-Cl})\), consider looking down the \(\text{C-C}\) bond. Place the ethyl group (\(\text{-CH}_2\text{Cl})\) at the back and the front carbon as \(C_1 \) with \(H\) atoms. The staggered form will have front-facing hydrogen atoms evenly spaced.

Drawing Staggered Conformations for Ethyl Chloride

Draw a circle to represent the front carbon \(C_1\). Attach three hydrogen atoms at 120° angles. For the second carbon \(C_2\) behind, draw a Cl, H, and \(\text{-CH}_3\) (methyl group) at staggered positions relative to the front carbon.

Newman Projection for 1,2-Dichloroethane

For 1,2-dichloroethane \(\text{Cl-CH}_2\text{-CH}_2\text{-Cl}\), consider looking down one of the C-C bonds. Label front carbon as \(C_1 \) with its substituents (one Cl and two H atoms). The back carbon \(C_2\) will have a Cl in one of the staggered positions as well.

Drawing Staggered Conformations for 1,2-Dichloroethane

There are two staggered conformations due to rotation about the C-C bond: anti and gauche. In the anti conformation, the chlorines (Cl) are on opposite sides (180° apart); in the gauche conformation, they are adjacent (60° apart). Draw both conformations.

Determining Stability of the Conformations

The anti conformation is typically more stable because the large chlorine atoms are far apart, minimizing steric hindrance. The gauche conformation is less stable due to the closer proximity of the chlorine atoms causing repulsion.

Key concepts

Staggered Conformation

A staggered conformation occurs when the atoms or groups attached to two adjacent carbons are positioned 60° apart, as seen in a Newman projection. This contrasts with the eclipsed conformation, where groups are directly aligned, increasing repulsion. In staggered conformations, the groups are spaced as far as possible, minimizing steric hindrance. Thus, staggered conformations are generally more stable and lower in energy compared to eclipsed conformations.

Steric Hindrance

Steric hindrance refers to the repulsion between atoms or groups that are forced into close proximity. This phenomenon arises when larger substituents try to occupy the same space, leading to higher energy and less stable conformations. In Newman projections, we prefer staggered conformations partly due to reduced steric hindrance, as groups are spread out, minimizing these repulsions.

Molecular Stability

Molecular stability in terms of conformation mainly hinges on minimizing repulsive interactions, such as those caused by steric hindrance. More stable conformations have lower energy states. For example, in Newman projections, staggered conformations are more stable because they minimize electron-electron repulsion between neighboring groups. Conversely, eclipsed conformations are higher in energy due to the increased repulsive forces.

Ethyl Chloride Conformation

Ethyl chloride \((CH_3-CH_2-Cl)\) when observed through a Newman projection along the C-C bond, presents several possible conformations. One can draw its staggered conformation by placing the hydrogens 120° apart. The back carbon \((CH_2Cl)\) should have the substituents (Cl, H, and CH_3) also spaced out evenly. This arrangement ensures minimized repulsion and better stability, showing why staggered forms are often the most favorable.

1,2-Dichloroethane Conformation

For 1,2-dichloroethane \((Cl-CH_2-CH_2-Cl)\), the Newman projection along the central C-C bond reveals two staggered conformations: anti and gauche. In the anti conformation, the chlorine atoms are opposite each other, reducing repulsion and thus providing greater stability. In the gauche conformation, the chlorine atoms are 60° apart, leading to increased steric hindrance and making this conformation less stable. Therefore, anti is typically more stable than gauche due to the minimized repulsive interactions.