Conformations of butane

Question

Would these be valid conformations of butane? I tried matching these up with the group but my drawings all seem rotated versus the book.

I'm fairly sure my anti conformer is correct, along with my gauche conformer. I'm not too sure about the two eclipsed conformers; from what I understand, there are two eclipsed conformations - one with strong torsional strain resulting from the methyl group interacting with the other methyl group, and the other resulting from the methyl groups interacting with the hydrogens, right?

Also, can we consider the dihedral angle to be the angle between the methyl groups?

enter image description here

Another problem is that what I've drawn doesn't seem to match with the book's energy diagram too. I'd expect the anti-conformer to be the lowest in energy, but I'd also expect a difference in E for the two eclipsed conformations. One should have more torsional strain from placing the two methyl groups next to each other and the other, less, because the methyl groups are eclipsing the comparatively small hydrogen.

Just to be clear, your drawn figures are for butane and the book's figure is for propane. — jerepierre, May 23 '14 at 15:54

Martin - マーチン · Answer 1 · 2014-05-27T10:59:11.733

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Some years ago I did a (relaxed) rotational scan of n-butane for the purpose of showing that fatty acids are all-trans. That is also why I found the graph in Awesome's answer some kind of disturbing - it should form a sinus like wave. I include this graph here:

energy diagram of the rotation of n-butane

Edited out the error noticed by ron. Thanks again.

edited May 27 '14 at 10:59

answered May 23 '14 at 07:46

Martin - マーチン

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score 5 · Accepted Answer · answered May 23 '14 at 03:19

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You have got them correct. Just your diagrams in eclipsed should have very close bonds. In your third diagram, one $\ce H$ is a bit far for another.

Wikipedia provides a nice description for this.

A diagram too :

enter image description here

You are correct.

answered May 23 '14 at 03:19

evil999man

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I've edited my text to reflect my beef with the solution manual. – Dissenter May 23 '14 at 03:20
Thank you though for catching the problem with the hydrogens! You're right, they are just a tad bit far apart! – Dissenter May 23 '14 at 03:21
Thank you. Do you know why the diagram in the book provides the wrong graph? – Dissenter May 23 '14 at 03:24
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@Dissenter Because it is of propane, not butane, – evil999man May 23 '14 at 03:26
Ah, good catch again! – Dissenter May 23 '14 at 03:37
That diagram seems very odd to me... (as in qualitatively right, but hand-drawn or something.) – Martin - マーチン May 23 '14 at 04:37
@Martin So what is your point? – evil999man May 23 '14 at 04:39
Which diagram, Martin? – Dissenter May 23 '14 at 06:19

Conformations of butane

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