Draw the Newman structure for the most stable conformation of 1-bromopropane considering rotation about the C1-C2 bond.
The Correct Answer and Explanation is :
For 1-bromopropane (CH3CH2CH2Br), when considering the most stable conformation around the C1-C2 bond, the Newman projection focuses on minimizing steric hindrance by arranging substituents to maximize spatial separation. The most stable conformation typically is the one that minimizes torsional and steric strain.
Determining the Most Stable Conformation
- Identify the Substituents: On the front carbon (C1), there are three hydrogen atoms. On the rear carbon (C2), there is one bromine atom (Br), one hydrogen atom, and a methyl group (CH3).
- Consider Steric Hindrance: Bromine is a large substituent compared to hydrogen and the methyl group. Therefore, positioning the bromine atom to avoid the largest groups on the front carbon minimizes repulsive interactions.
- Choose the Conformation: The most stable conformation for 1-bromopropane is the anti conformation, where the bromine atom on the rear carbon is positioned 180 degrees opposite to the methyl group on the front carbon. This arrangement minimizes the steric repulsion because the bulky bromine atom and the methyl group are as far apart as possible.
Explanation
The anti conformation is favored because it reduces the torsional strain caused by electron cloud repulsion between adjacent bonds and minimizes the steric strain from the bulky substituents. In the Newman projection looking down the C1-C2 bond, placing Br and the CH3 group in opposite directions (anti to each other) achieves this spatial arrangement most effectively.
This conformation reduces overlap between electron clouds of the C-H and C-Br bonds compared to other conformations where these groups might be closer (e.g., gauche or eclipsed). The anti conformation also aligns less bulky substituents (hydrogens) opposite each other, further reducing repulsive interactions and resulting in a lower energy state for the molecule.
Newman Projection Drawing
The Newman projection for the most stable conformation of 1-bromopropane looking down the C1-C2 bond in the anti position would show:
- The front carbon (C1) with three hydrogen atoms spread around it.
- The rear carbon (C2) with a Br atom directly opposite one of the hydrogens on the front carbon, a hydrogen atom, and a methyl group (CH3).
Let’s generate an image of this Newman projection.
Here is the Newman projection for the most stable conformation of 1-bromopropane looking down the C1-C2 bond. This image depicts the anti conformation, where the bromine atom is placed directly opposite to the methyl group on the rear carbon, minimizing steric hindrance for a lower energy state.
