Trans-hydrogenation of alkenes

Both this and that question seem to indicate that the hydrogenation of alkenes to alkanes or alkynes to alkenes is always a syn process.

However since hydrogenation is not a concerted process, I do not see why we could not have a non-negligible (although small) portion of the trans product.

Following this reaction mechanism for syn additions :

before the penultimate step (the addition of the second hydrogen), can't the molecule simply rotate around the $\ce{C-C}$ axis (I expect this should be possible with small enough alkenes) ?

The Wikibooks article on alkenes does indicate that typically the hydrogenation of alkenes leads to only $98\%$ of the syn product, but I have been unable to find more precise information elsewhere.

• Yes, the trans product probably will be formed in small quantities. Given the very small size of a hydrogen atom I would guess that the second hydrogen transfer step is very fast, so rotation is unlikely, given that it will be sterically hindered.
– bon
Jul 7 '16 at 18:54
• Is there some experimental data with concrete examples, and which maybe shows how it evolves with the size of the alkene ? I haven't been able to find any @bon Jul 7 '16 at 18:55
• I don't have the time or resources to search for it now but maybe someone else will turn up something.
– bon
Jul 7 '16 at 19:09