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Wikipedia says the following about paraldehyde:

Theoretically, four stereoisomeric structures are possible. The structures (1) and (2) are known as cis- and trans-paraldehyde. The structures (3) (a conformer of (2)) and (4) (a conformer of (1)) don't exist for steric reasons.

Stereoisomers of paraldehyde according to Wikipedia.

If this is correct, it seems that the boat conformer is not theoretically possible. I would expect the boat conformer to be possible but be much less common than the chair conformer (as is the case with glucose and cyclohexane for example), but that does not seem to be the case.

Wikipedia says that conformers 3 and 4 don't exist for steric reasons, however they are still theoretically possible. To me this sounds like the nonexixtence of boat conformation is not just because of steric reasons, but there is a more fundamental reason for that.

So, my questions are:

  1. Is it true that the boat conformer is theoretically impossible?

  2. If yes, why is it impossible and what makes paraldehyde different from glucose and cyclohexane in this regard?

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  • $\begingroup$ It's possible, they just ignored it because it's so minor. Every molecule has an infinite number of conformers, so if you have limited space you can't really talk about all of them. Most books on cyclohexane hardly talk about boat conformation either, except to note in passing that it exists. $\endgroup$
    – orthocresol
    May 9, 2017 at 14:00
  • $\begingroup$ The boat conformer is a transition state in any case. See for example the interconversion for cyclohexane on Wikipedia $\endgroup$
    – Martin - マーチン ♦
    May 9, 2017 at 14:06

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A $\ce{CH_3}$ group is quite big. Steric hindrance is the reason.

Its not theoretically impossible, but it is energetically a lot less favourable than (1) and (2).

You wouldn't find it in nature so to speak.

to 2: A $\ce{OH}$ group is sterically a lot smaller than a $\ce{CH_3}$ group.

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  • $\begingroup$ Thank you for your reply. I just edited my questions for further clarification. I steric hinderance is the main reason, why does Wikipedia list conformers 3 and 4 as theoretically possible but nonexixtent due to steric reasons, but does not say that the boat form as theoretically possible? $\endgroup$
    – ttsc
    May 9, 2017 at 14:02
  • $\begingroup$ It is theoretically possible, it just is energetically so high, it couldn't exist without external help $\endgroup$
    – user37142
    May 9, 2017 at 14:03
  • $\begingroup$ Is it energetically much higher than structures (3) and (4)? To me it seems that if sturcutre (2) were in boat form, there were no $\ce{CH_3}$ groups in close contact with each other. In (3) the $\ce{CH_3}$ groups seem to be much closer. $\endgroup$
    – ttsc
    May 9, 2017 at 14:10
  • $\begingroup$ Free electron pairs (on the oxiygens) have a quite substantial steric part. A boat conformer is always higher than its "neighboring" conformer and will change to it, it is, as Martin said, a transition state. $\endgroup$
    – user37142
    May 9, 2017 at 14:16

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