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I encountered a question which mentioned the skew position during discussion of the conformers of cyclohexane (chair/boat).

Here is the question (grammatical errors have been emended):

Which of the following statements is/are wrong about the greater stability of the chair form over the boat form?
(A) In the chair conformation, all $\ce{C-H}$ bonds in adjacent carbons are in the skew position
(B) In the boat conformations, there are four skew interactions and two eclipsed interactions
(C) The boat form has angle strain
(D) In the boat conformation, there are three skew interactions and three eclipsed interactions

It talks about skew positions. What does that mean? Is a skew position a staggered form? Or does it mean an axial position, or an equitorial position?

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  • $\begingroup$ Please do not forget that the boat conformation is a transition state, it is not a minimum on the potential energy surface. Some people might sell this fact as angle strain. $\endgroup$ Sep 15, 2021 at 22:02

2 Answers 2

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It also appeared that the term skew position (or symmetrical skew) was used to refer to the chair conformation in some early research papers. I found two papers published in the early 1960s where the term was used in that way.

...with the growing interest in the family of cyclohexane conformations which include the boat and the symmetrical skew as extreme cases...

The symmetrical skew cyclohexane with neither bond length nor bond angle distortion, alternatively referred to as the stretched conformation or the "halfrotated" form, is shown by the numerical analysis to have dihedral angles of 33.1, 33.1, and 70.67", and not the 30, 30, 60" values quoted, for instance, by Klyne and Prelog (1960).

The skew conformation has the minimum energy of the flexible form...

References

Angyal, S. J., & Hoskinson, R. M. (1962). 578. Cyclitols. Part XIII. The conformation of cyclic diketals: the skew conformation. Journal of the Chemical Society (Resumed), 2991-2995.

Bottomley, G. A., & Jefferies, P. R. (1961). The Dihedral Angles of Cyclohexane. Australian Journal of Chemistry, 14(4), 657-659.

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I googled "conformation skew staggered" and found:

The infinite number of possible intermediate conformations between the two extreme conformations are referred to as skew conformations. In these conformations, hydrogen atoms are closer than in staggered but away than in eclipsed conformation. enter image description here

Source: https://knowledgebin.org/entry/stereochemistry-7/

Then I googled "torsion angles boat chair conformation" and found:

enter image description here

Source: Libretexts

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