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What is the conformation of the cyclohexane ring constituting [2.2.1]-bicycloheptane? Is it boat or twisted boat? Is that conformation abundant in other bicyclic systems containing a cyclohexane ring?

enter image description here

EDIT: From Modern Organic Synthesis by Zweifel I took the following image: enter image description here

which is in contradiction with current answers. Can anyone account for this observation?

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    $\begingroup$ Please be so kind and mark the edits in the question, so it is obvious how our argumentation builds up. $\endgroup$
    – ssavec
    Feb 15, 2016 at 14:56
  • $\begingroup$ @ssavec No, definitely not. Please never mark any edits whatsoever, that is what the edit history is for. A question should always stand as is, not as a chronological build up. The extension that is now introduced does hardly make any sense to me, as I don't know what are the current answers, and I don't know how it even relates to the first question. $\endgroup$ Mar 3, 2016 at 9:26
  • $\begingroup$ @Martin-マーチン You got the point. The added question does not relate to the original one at all. By substantially modyfing the question, OP rendered the existing answer (mine) look very incomplete. I don't think the question should become more complicated after it was answered. $\endgroup$
    – ssavec
    Mar 3, 2016 at 21:31
  • $\begingroup$ The question was: Conformation of six-membered ring constituting a bridged compound. I provided [2.2.1]-bicycloheptane as an example with the follow-up questions: Is it boat or twisted boat? Is that conformation abundant in OTHER bicyclic systems containing a cyclohexane ring?" I've got the answer for [2.2.1]-bicycloheptane, it is boat, but what about other systems. I provided an example where boat isn't the most stable conformation. The correct conformation for just [2.2.1]-bicycloheptane doesn't answer my question. $\endgroup$
    – EJC
    Mar 3, 2016 at 22:45
  • $\begingroup$ @ssavec I agree with that. Once an answer is given, it is better to ask a new question and provide a link for context. $\endgroup$ Mar 4, 2016 at 12:01

3 Answers 3

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According to structure determined by electron diffraction, norbornane is present in bridged boat conformation. http://pubs.acs.org/doi/abs/10.1021/ja01014a032, see Figure.

Figure 10 of the article

Norbornane and its derivatives are often used for studying effect of equatorial and axial substitution of cyclohexane and is considered rigid. Therefore, I don't expect significant population of the second conformer you sketched.

Edit: For the [2.2.2]bicyclooctane (not the lactone mentioned in question), both conformers are known in crystalographic database.

Bear in mind, that bicyclooctane constist of three hypothetical cyclohexanes, possibly equivalent by symmetry.

For any compound of interest, you should find enough data in crystallographic database, but I'd expect all reasonable conformations to exist.

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  • $\begingroup$ That's not the full answer. Is that conformation abundant in other bicyclic systems containing a cyclohexane ring like bicyclo[2.2.2]octane? $\endgroup$
    – EJC
    Feb 15, 2016 at 14:22
  • $\begingroup$ Which one is lower in energy? $\endgroup$
    – EJC
    Feb 15, 2016 at 22:01
  • $\begingroup$ I need solid evidence because there are contradictory answers here. $\endgroup$
    – EJC
    Feb 15, 2016 at 22:02
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    $\begingroup$ @Marko If you need solid evidence, than find it yourself. $\endgroup$
    – ssavec
    Feb 16, 2016 at 9:15
  • $\begingroup$ I need for one of these answers so I can accept it. Geoff said that the most stable conformation for bicyclo[2.2.2]octane is twist-boat, XuMuk said that it is boat and you didn't give a definite answer on that matter. $\endgroup$
    – EJC
    Feb 16, 2016 at 10:48
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Norbornane, as mentioned by the other answers forces the cyclohexane ring into a prototypical "boat" conformation. The bridging carbon creates two five-membered rings in the low-energy envelope or "pucker" conformation:

enter image description here

Put simply, the 1-carbon bridging system creates a very rigid system. While there are many eclipsed C-H interactions, changing conformation would require stretching bonds or angles - distortions that are much higher in energy.

In larger bicycloalkanes, like bicyclo[2.2.2]octane, there is now more conformational flexibility to adopt a twist-boat conformation:

enter image description here

Notice that from the bridgehead carbons, there are many eclipsing interactions. In Avogadro, I can watch this all-boat conformation twist into the twist-boat:

enter image description here

While it's important to do conformational analysis for every molecule (small interactions can change the low-energy conformer), I would guess most of the larger bicycloalkanes to adopt twist-boat configurations.

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  • $\begingroup$ So which conformation is the more stable one for bicyclo[2.2.2]octane? $\endgroup$
    – EJC
    Feb 15, 2016 at 20:18
  • $\begingroup$ It's twist-boat - you can see the twist in the final image (of bicycle[2.2.2]octane). $\endgroup$ Feb 15, 2016 at 20:22
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    $\begingroup$ Relative energy of hydrocarbon conformers is reasonably tricky question, involving among others long range weak interactions, dynamic effects and symmetry. That's why I (as computational chemist) tried to avoid answer based on calculations. BTW, how large is the energy difference of those? $\endgroup$
    – ssavec
    Feb 15, 2016 at 20:43
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Your molecule is "bicyclo[2.2.1]heptane", also called "norbornane".

And it looks like: Simple 3d of bicycloheptane

Coordinates:

C    -0.336900000000      0.382800000000      0.560000000000
C     1.186600000000      0.194300000000      0.678100000000
C     1.458300000000     -0.983300000000     -0.277900000000
C     0.054500000000     -1.313600000000     -0.817600000000
C    -0.397400000000     -0.196100000000     -1.775400000000
C    -0.669200000000      0.981300000000     -0.819300000000
C    -0.819800000000     -1.054000000000      0.403500000000
H    -0.777400000000      0.930600000000      1.394100000000
H     1.468700000000     -0.067200000000      1.698900000000
H     1.738100000000      1.093400000000      0.400200000000
H     1.874300000000     -1.831700000000      0.267200000000
H     2.157900000000     -0.722600000000     -1.072900000000
H    -0.028300000000     -2.315100000000     -1.241800000000
H    -1.309100000000     -0.480900000000     -2.302600000000
H     0.360200000000      0.039800000000     -2.523700000000
H    -1.716400000000      1.283000000000     -0.870400000000
H    -0.059000000000      1.855400000000     -1.049900000000
H    -1.889000000000     -1.133000000000      0.198000000000
H    -0.570700000000     -1.688700000000      1.255400000000

Could you draw the 3d of twisted boat? (it definitely is not the minimal energy conformer)

I can try to calculate the energy of the conformers by quantum-chemistry, but there is a problem - minimisation leads to the boat conformation.

This is MO of norbornane: enter image description here

This is 3d structure of 2-Oxabicyclo(2.2.2)octan-3-one ((1s,4s)-2-oxabicyclo[2.2.2]octan-3-one): enter image description here

and xyz coordinates:

C     0.863100000000      0.683400000000      0.628700000000
C     1.100200000000     -0.847300000000      0.586700000000
C    -0.188300000000     -1.567200000000      0.104800000000
C    -1.274300000000     -0.508200000000     -0.211400000000
C    -0.760200000000      0.456300000000     -1.309800000000
C     0.533400000000      1.154500000000     -0.810100000000
C    -0.391200000000      0.927800000000      1.487000000000
O    -0.385000000000      1.631600000000      2.492800000000
O    -1.472000000000      0.267900000000      0.979400000000
H     1.721200000000      1.214600000000      1.041700000000
H     1.383200000000     -1.208500000000      1.576700000000
H     1.934600000000     -1.081400000000     -0.074900000000
H    -0.549400000000     -2.249900000000      0.875300000000
H     0.019000000000     -2.174800000000     -0.776900000000
H    -2.210400000000     -0.981200000000     -0.509400000000
H    -1.525500000000      1.199400000000     -1.538800000000
H    -0.568800000000     -0.088500000000     -2.235000000000
H     0.404800000000      2.237900000000     -0.827600000000
H     1.365600000000      0.933900000000     -1.479200000000

I don't know what authors call for "twist boat", but for compounds, you are asking about, there is definitely only one conformer!

For visualisation these geometries you can use a lot of software - chemcraft, VMD, avogadro, pymol...

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  • $\begingroup$ That's not the full answer. Is that conformation abundant in other bicyclic systems containing a cyclohexane ring like bicyclo[2.2.2]octane? $\endgroup$
    – EJC
    Feb 15, 2016 at 14:22
  • $\begingroup$ @Marko This conformation is very solid, as said ssavec - population of other conformations is almost zero. $\endgroup$
    – XuMuK
    Feb 15, 2016 at 14:45
  • $\begingroup$ I will edit my question to prove my point. $\endgroup$
    – EJC
    Feb 15, 2016 at 14:52
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    $\begingroup$ @Marko your link is totally irrelevant for the lactone issue. $\endgroup$
    – ssavec
    Feb 15, 2016 at 20:46
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    $\begingroup$ @Lucian09474 I have used two softwares: chemcraft, it can be obtained very cheap for academic, also, there is in the Internet very old version, but without any limitations (chemcraft freeware now is very limited, but for fast and smooth pictures is on of the best and easiest). MO were calculated by Jaguar - programme of the Schrodinger -. It costs not a little money. Also, there are a lot of other, not so bad drawing/rendering software: VMD, vega zz, Mercury... $\endgroup$
    – XuMuK
    Feb 20, 2016 at 22:21

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