Draw 2,3-dimethylpentane in its most and least stable conformation using Newman Projections: Use C2 and C3 as your front and back carbons in the projection. Be sure to label which is the most and which is the least stable confirmation; Indicate which type(s) of strain are present in each (and where in each):
The Correct Answer and Explanation is:
Here are the Newman projections for 2,3-dimethylpentane.
Least Stable Conformation
- Strain Types Present:
- Torsional Strain: Present between all three pairs of eclipsing bonds. This is the inherent energy cost of aligning bonds.
- Steric Strain: Most significant between the large eclipsing methyl (CH3) and ethyl (C2H5) groups. Their electron clouds are forced into close proximity, causing strong repulsion.
Most Stable Conformation
- Strain Type Present:
- Gauche Strain (a type of steric strain): Present between the front methyl group and the back ethyl group. It is also present between the other front methyl group and the back methyl group. These groups are adjacent (60 degrees apart), causing some electron cloud repulsion.
Explanation
Conformational analysis of 2,3-dimethylpentane along the C2-C3 bond reveals how atomic arrangement affects molecular stability. The groups attached to the front carbon (C2) are a hydrogen (H) and two methyl (CH3) groups. The back carbon (C3) holds a hydrogen, a methyl group, and a bulkier ethyl (C2H5) group. The stability of a conformation is determined by the amount of strain present, primarily torsional and steric strain.
The least stable conformation is the one with maximum energy and strain. This occurs in an eclipsed arrangement where the dihedral angle between bonds on the front and back carbons is zero degrees. To maximize instability, the largest groups are aligned. In this case, the front methyl group eclipses the back ethyl group. This arrangement creates two major types of strain. Torsional strain arises from the repulsion between the electrons in the eclipsing C-H and C-C bonds. More significantly, severe steric strain results from the large ethyl and methyl groups being forced into the same physical space, causing their electron clouds to repel each other strongly.
Conversely, the most stable conformation is the one that minimizes energy and strain. This is achieved in a staggered arrangement, where the groups on the back carbon are positioned 180 degrees (anti) or 60 degrees (gauche) relative to the groups on the front. To achieve maximum stability, the largest group on the front carbon (a methyl) is positioned anti, or 180 degrees away, from the largest group on the back carbon (the ethyl). This anti-periplanar arrangement places the bulkiest substituents as far apart as possible, effectively eliminating the most severe steric clashes. While this conformation is the most stable, it is not entirely strain free. It still contains residual steric strain from gauche interactions, which occur when the remaining bulky groups (methyl/methyl and methyl/ethyl pairs) are positioned 60 degrees apart.
