Hydrogenation of imines to amines by $\ce{H2}$ on $\ce{Pd/C}$ has been used e.g. in the synthesis of Pumiliotoxin. By contrast, carbonyl groups are rarely reduced in this way, which I though was due to the fact that if a carbonyl is adsorbed, the interaction is through O’s lone pairs, rather than C=O $\pi$. However, the lone pair on the imine N should be of an even higher energy than in carbonyl and so should adsorb more easily. Why then does the hydrogenation of imines happen successfully?

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    $\begingroup$ I feel like the polarity of the bond might play a role, but not sure $\endgroup$ Nov 8, 2017 at 11:20
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    $\begingroup$ The HOMO should be the $\ce{C=X}$ bond either way … $\endgroup$
    – Jan
    Nov 8, 2017 at 12:25
  • $\begingroup$ @Jan So why does the adsorption of carbonyls occur at O? And does it actually? $\endgroup$ Nov 8, 2017 at 12:27
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    $\begingroup$ @substitutedbutadiene I wouldn’t be sure, I would have to look through the literature. Heterogenous catalysis is really not my strong point … ^^' $\endgroup$
    – Jan
    Nov 8, 2017 at 12:30
  • $\begingroup$ Depending what you look for and where you get the information, I was under the impression the typical C=O is around 100kJ/mol stronger than the typical C=N bond. Hence, the latter is easier to break. Stronger adsorption is backed up by the fact that the resultant amines, in certain circumstances, can poison catalysts, whereas alcohols do not tend to. $\endgroup$
    – Beerhunter
    Nov 11, 2017 at 19:32


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