Problem 107 - General Chemistry: Principles and Modern Applications

Question

Draw a Newman projection for the conformation of lowest energy for viewing 2 -methylhexane along the \(\mathrm{C} 2-\mathrm{C} 3\) bond.

Step-by-Step Solution

Answer

The Newman projection of lowest energy for 2-methylhexane along the C2-C3 bond is a staggered conformation with the methyl group and hydrogen atoms evenly distributed around the circle.

Step 1: Understanding Newman Projections

In a Newman projection, we look at a molecule from one end. The molecule should be visualized such that the bond of interest is along the sight line. The carbon atom in front appears as a dot and the three groups attached are shown radiating out from this center. The carbon atom in the back is represented as a circle and the three attached groups are also shown radiating from the center but in the orientation they appear from the front carbon perspective.

Step 2: Drawing Molecule Structure

Draw the skeletal structure of 2-methylhexane. This contains 7 carbon atoms, a methyl group at the second carbon, and the rest have hydrogen atoms attached. This will help to provide a visual before drawing the Newman projection.

Step 3: Newman Projection

Draw the Newman projection along the C2-C3 bond. The front carbon (C2) of the Newman projection will have the methyl group (CH3) and two hydrogen atoms attached. The back carbon (C3) has three hydrogen atoms attached. The lowest energy conformation, known as the staggered conformation, has the hydrogen atoms and methyl group distributed evenly around the circle, reducing the molecular strain.

Key Concepts

2-Methylhexane

2-Methylhexane is an organic compound belonging to the alkane family. It consists of a six-carbon chain with a single carbon branching off the second carbon atom, forming a methyl group. This gives it a total of seven carbon atoms.
Understanding the structure of 2-Methylhexane is crucial for visualizing its conformations. It involves:

• Recognizing the main hexane chain and locating the methyl group at the second carbon.
• Keeping track of attached hydrogen atoms, which balance out the one-carbon central units typical of alkanes.
• Remembering that its molecular formula is C₇H₁₆.

Being able to draw and interpret the structure of 2-Methylhexane helps in various chemical analyses, including predicting how it might interact with other molecules due to steric factors or potential reaction sites.

Molecular Conformation

Molecular conformation refers to the three-dimensional arrangement of atoms in a molecule. Molecules like 2-Methylhexane can adopt different shapes, or conformations, without breaking any bonds. This is due to free rotation around single bonds.
Key points to understand about molecular conformation include:

• Conformations result from rotations around carbon-carbon single bonds, leading to different spatial arrangements.
• The energy levels associated with different conformations vary, determining their stability.
• The shape of a molecule in a particular conformation affects its physical and chemical properties.

In the case of 2-Methylhexane, focusing on the rotation around the C2-C3 bond gives insights into its potential conformations and helps predict which conformation will be most energetically favorable.

Staggered Conformation

In the field of molecular conformation, staggered conformation is the term used to describe a specific spatial orientation where atoms or groups are positioned with optimal spacing across a bond. This arrangement minimizes repulsive interactions and is thereby more energetically favorable.
For molecules like 2-Methylhexane:

• In the staggered conformation, the atoms or groups around the C2-C3 bond are distributed in such a way that steric hindrance is at its lowest.
• The staggered arrangement is typically shown in a Newman projection as having 60-degree angles between substituents when viewed along the axis of a bond.
• This conformation reduces potential energy compared to the eclipsed conformation, where groups are aligned.

The staggered conformation generally enables the molecule to exist in a state of lower energy, which is more stable, making it a common point of reference for analyzing molecular shapes and behaviors.