Increased substitution around a bond leads to increased strain. Take the four substituted butanes listed below, for example. For each compound, sight along the C2-C3 bond and draw Newman projections of the most stable and least stable conformations. Use the data in Table to assign strain energy values to each conformation. Which of the eight conformations is most strained?
Which is least strained?
(a) 2-Methylbutane
(b) 2, 2-Dimethylbutane
(c) 2, 3-Dimethylbutane
(d) 2, 2, 3-Trimethylbutane
The Correct Answer and Explanation is :
It seems I’m unable to access the image you referenced directly. However, I can still guide you through the problem with a detailed explanation of Newman projections and the strain energies associated with them.
General Concept of Newman Projections:
A Newman projection is a way to visualize the conformational arrangement of atoms or groups around a bond. The most stable conformation (anti) is when the bulky groups are as far apart as possible, while the least stable conformation (eclipsed) occurs when bulky groups are as close to each other as possible.
The energy associated with a conformation depends on the steric strain (repulsion between atoms or groups) and torsional strain (due to eclipsing bonds).
Approach for Each Compound:
1. 2-Methylbutane:
- Most stable conformation: The anti-conformation with the methyl group on C2 and the hydrogens on C3 opposite each other. This conformation minimizes steric strain.
- Least stable conformation: The eclipsed conformation with the methyl group on C2 eclipsing the hydrogen atoms on C3, causing significant torsional strain.
- Strain energy: The most stable conformation has minimal strain, while the least stable one has the maximum steric and torsional strain.
2. 2, 2-Dimethylbutane:
- Most stable conformation: The anti-conformation where the two methyl groups are on opposite sides, minimizing repulsion.
- Least stable conformation: The eclipsed conformation where the two bulky methyl groups are aligned, causing both steric and torsional strain.
- Strain energy: The more substituted nature of this compound leads to higher strain in the eclipsed conformation.
3. 2, 3-Dimethylbutane:
- Most stable conformation: The anti-conformation with the two methyl groups positioned opposite each other.
- Least stable conformation: The eclipsed conformation where one methyl group on C2 and the other on C3 are aligned, creating substantial strain.
- Strain energy: This molecule exhibits strain due to the close proximity of two substituted groups on adjacent carbons.
4. 2, 2, 3-Trimethylbutane:
- Most stable conformation: The anti-conformation with all three methyl groups positioned opposite to each other.
- Least stable conformation: The eclipsed conformation, where multiple bulky groups are aligned, causing high strain due to steric clashes.
- Strain energy: This molecule has the highest strain due to the three bulky substituents.
Summary:
- The most strained conformation is likely to be 2, 2, 3-trimethylbutane in its eclipsed conformation, due to the high steric strain from three bulky methyl groups being aligned.
- The least strained conformation would be the anti conformation of 2-methylbutane, as it has the fewest substituents, causing the least strain overall.
These Newman projections allow you to understand how steric hindrance and torsional strain increase as more substituents are added around the bond. The increased substitution leads to more strain, especially in eclipsed conformations.