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Is there any such thing as a twist chair conformation?

Also, the boat and the chair conformations are achiral, while the twist boat conformation of cyclohexane is chiral and dissymetric. What about the half chair conformation? Am I right in thinking that it is achiral because it has a plane of symmetry?

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  • $\begingroup$ What plane of symmetry does the half-chair have? It looks like it has an axis of symmetry, but no plane. $\endgroup$ – jerepierre Jun 16 '15 at 16:02
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Short answer, no. There is not a "twist-chair."

To convince yourself, it probably helps to make a physical model with a chemistry model kit. If you try to twist the chair, you can't do it without significantly moving at least one atom, in which case, the conformation is basically a "skew" or "twist boat." Try it.

The boat and chair conformations are indeed symmetric and achiral. The twist-boat or skew conformation has point group $D_{2}$ and is indeed chiral.

A normal half-chair will have an axis of symmetry and is chiral.

An envelope conformation will have a plane of symmetry and thus be achiral.

I've found that some of the best depictions of six-member-ring conformations come from pyranose sugars. Because of the hydroxyl groups and oxygen atoms, there are 38 distinct conformations (2 chairs, 6 boats, 6 skew-boats, 12 half-chairs, and 12 envelopes).

From Wikipedia:

enter image description here

Obviously, in less-substituted cyclohexane, there are fewer unique shapes, but it helps to see the different ring-puckering or pseudo-rotation shapes.

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  • $\begingroup$ My book gave the "envelop" as "half-chair" - so I thought the half chair was achiral. The book is normally reliable - I wonder why the mistake. Are four atoms arranged linearly in the half chair? Does that create immense strain - 180 degrees in place of 109.5? $\endgroup$ – Charles Jun 17 '15 at 2:26
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    $\begingroup$ @Charles No, the 4 atoms are not linear, but they are planar. The difference between the half-chair and the envelope is that the half-chair has one atom above and one atom below the plane of the ring, while the envelope has 5 planar atoms and one atom above the ring. $\endgroup$ – Geoff Hutchison Jun 17 '15 at 3:08
  • $\begingroup$ How are those 4 atoms arranged in the plane of the ring - how does the half chair look if you tilt it a bit from the angle you've shown it at? $\endgroup$ – Charles Jun 18 '15 at 12:11
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    $\begingroup$ Here's an image showing 1-2-5-6 plane. I can send a PDB file if you like. $\endgroup$ – Geoff Hutchison Jun 18 '15 at 22:11

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