Chapter 7, Problem 7.1P

Interpretation Introduction

Interpretation:

A model of chair cyclohexane corresponding to the leftmost model in the given figure is to be drawn. The two reasons corresponding to the fact that the half-chair conformation is less stable than the chair or twist-boat conformation is to be stated.

Concept introduction:

The conformation of cyclohexane is a type of $3-\text{D}$ shape of cyclohexane molecule. There are total four types of significant conformations of cyclohexane exists which are chair conformation, half-chair conformation, boat conformation and twist-boat conformation. The switching of cyclohexane molecule is very easy between the four conformations.

Answer to Problem 7.1P

A model of chair cyclohexane corresponding to the leftmost model in the given figure is shown below.

The half-chair conformation is less stable than the chair or twist-boat conformation because half-chair conformation has maximum ring strain and the presence of angle strain in half-chair conformation of cyclohexane molecule.

Explanation of Solution

The given leftmost figure of chair conformation of cyclohexane is shown as,

Figure 1

The model of chair conformation of cyclohexane corresponding to the leftmost model is shown as,

Figure 2

If the carbon-4 in order to locate carbons 2-5 in a common plane then the above model chair conformation of cyclohexane is shown as,

Figure 3

Thus, the raising of carbon-4 in order to locate carbons 2-5 in a common plane forms the half-chair conformation of cyclohexane molecule. This half-chair conformation of cyclohexane molecule is a transition state for the conversion into the chair and twist boat conformations.

The two reasons which explain that the half-chair conformation is less stable than the chair or twist-boat conformation are as follows.

• The ring strain in half chair conformation is more as compared to the chair or twist-boat conformation.

• The angle strain of $120°$ and torsional strain is present in half chair conformation of cyclohexane. These two strains exist in the half chair conformation because of the two fully eclipsed carbon-carbon bonds and two partially eclipsed carbon-carbon bonds.

Conclusion

A model of chair cyclohexane corresponding to the leftmost model in the given figure has been shown above. The two reasons corresponding to the fact that the half-chair conformation is less stable than the chair or twist-boat conformations has been stated above.