# Why is the boat form of paraldehyde not theoretically possible?

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.

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?

• 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. – orthocresol May 9 '17 at 14:00
• The boat conformer is a transition state in any case. See for example the interconversion for cyclohexane on Wikipedia – Martin - マーチン May 9 '17 at 14:06

A $\ce{CH_3}$ group is quite big. Steric hindrance is the reason.
to 2: A $\ce{OH}$ group is sterically a lot smaller than a $\ce{CH_3}$ group.
• 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. – ttsc May 9 '17 at 14:10