Question

Draw Newman projections for the staggered and eclipsed conformations of 2 -methylpentane for rotation about the \(\mathrm{C} 2-\mathrm{C} 3\) bond. Which conformation is lowest in energy?

Short answer

The staggered conformation of 2-methylpentane on rotation about the C2-C3 bond is lowest in energy.

Step-by-step solution

Drawing Newman Projection for Staggered Conformation

Begin by identifying the C2-C3 bond. Draw a circle representing the front carbon atom (C2), and a point in the circle marking the back carbon atom (C3). Next, sketch three lines at a 120° angle to each other representing the hydrogen and methyl group bonded to the front carbon. Similarly, draw three lines from the back carbon, ensuring that each line aligns with the spaces between the lines from the front carbon (not on top of them). This gives the staggered conformation.

Drawing Newman Projection for Eclipsed Conformation

Similar to before, identify the C2-C3 bond and draw a circle representing the C2 atom and a point in the circle marking the C3 atom. Draw lines representing the hydrogen and methyl group bonded to the front carbon. However, in this case, draw the lines from the back carbon directly behind the front carbon's lines, giving the eclipsed conformation.

Determine Lowest Energy Conformation

Under normal conditions, the staggered conformation is lower in energy and thus more stable due to decreased steric strain. So, for 2-methylpentane on rotation about the C2-C3 bond, the staggered conformation is the lowest energy conformation.

Key concepts

Staggered Conformation

In organic chemistry, the staggered conformation describes a specific arrangement of atoms about a single bond. To visualize this, imagine looking straight down the C2-C3 bond of 2-methylpentane.

• Here, the bonds on the front carbon (C2) align so that none of them overlap with the bonds from the back carbon (C3).
• As a result, the groups attached to these carbons are spread out, minimizing interactions and making this a stable conformation.

Staggered conformation is important because it represents a state of lower energy due to reduced repulsion between electrons in bonds. This configuration is energetically favored and often encountered in straight-chain alkanes.

Eclipsed Conformation

The eclipsed conformation of a molecule is quite the opposite of the staggered form. In this arrangement, the atoms or groups directly overlap when viewed along the bond axis.

• In the context of 2-methylpentane, this involves the bonds on the front carbon lining up perfectly with the bonds on the back carbon.
• This leads to maximum steric hindrance and increased energy levels due to electron repulsion.

One might visualize eclipsed conformation as shadowing, where each bond and the attached group on the front carbon casts a shadow on the back side. This conformation is less stable compared to staggered due to steric strain, which makes it higher in energy.

C2-C3 Bond Rotation

Rotation around the C2-C3 bond in 2-methylpentane allows for transformations between different conformations, such as staggered and eclipsed.

• This bond rotation is possible due to the single bond characteristic, which permits free rotation.
• Though in theory, rotation seems continuous, there are specific rotational states corresponding to energy minima and maxima, such as staggered and eclipsed conformations.

Understanding C2-C3 rotation helps in predicting the behavior of molecules in different energy environments and plays a crucial role in conformational analysis.

2-Methylpentane Conformation

2-Methylpentane is a simple branched alkane, and analyzing its conformation provides insight into molecular flexibility and energetics.

• Drawing Newman projections helps visualize how the molecule's spatial arrangement affects its energy.
• Conformational analysis of this compound reveals how its methyl group interacts with its surroundings, which highlights steric and energetic considerations.

By examining its different conformations, you learn how structures adapt to adopt their most stable form, usually driven by a reduction in steric strain and energy optimization.

Steric Strain

Steric strain in molecular structures refers to the repulsion between electron clouds, often due to atoms or groups being pushed too close to one another.

• This is a significant factor influencing the stability of different conformations, such as in the eclipsed conformation of 2-methylpentane.
• Positions of atoms leading to overlap increase steric hindrance, thereby elevating energy levels of the molecule.

Reducing steric strain is crucial for achieving a stable and low-energy conformation. Staggered formations do just that by spreading groups as far apart as possible, thus minimizing repulsive interactions and lowering potential energy.