In the analysis of the product mixture of a new catalytic hydrogenation reaction I'm trying out I am getting this side product molecule (in low quantities; less than 1% w/w).

enter image description here

Is this a likely / explainable side product? What would be a potential mechanism?

I'm skeptical because the C8 increases to a C9. I assumed C-C bond making wasn't a very common occurrence in this sort of reaction? Or is it?

The reaction conditions are fairly mild: 10 barg & 80 C. Somehow I associated this sort of hydrogenation with functional group identity changes & not Carbon-backbone changes.

Or am I wrong?

More info. Solvent is Methanol. pH is alkaline by small qty. of NaHCO3 initially added. Catalyst is Pd on C.

Edit: Supposing the MeOH solvent reacts with the starting material, would this be the reaction to expect?

enter image description here

Any mechanism to go from this alcohol-ether to the impurity I'm seeing?

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    $\begingroup$ With no conditions disclosed (catalyst, solvent, acidity etc) this is not a reasonably easy question. However, I would check purity of the reagent, since styrene used to make it could contain some 2-phenylpropylene, especially if it was technical grade. I would also check that the reaction mixture didn't have any sources of nucleophilic methyl synthone. $\endgroup$ – permeakra Oct 25 '16 at 6:07
  • $\begingroup$ Thanks @permeakra. Sorry, I forgot to add those. Solvent is Methanol. pH is alkaline by small qty. of NaHCO3 initially added. Catalyst is Pd on C. $\endgroup$ – curious_cat Oct 25 '16 at 6:09
  • $\begingroup$ @permeakra Does that info. help answer? $\endgroup$ – curious_cat Oct 25 '16 at 6:09
  • $\begingroup$ Your methanol adduct seems most likely to me. How are you characterizing the impurity? Mass spec? NMR? Other? $\endgroup$ – jerepierre Oct 25 '16 at 14:06
  • $\begingroup$ @jerepierre Thanks! Yes, by mass spec. But how does the adduct transform itself into the product I'm seeing? No doubt the skeletons match but how does the ether change into a C-C bond? Would hydrogenation do the trick? $\endgroup$ – curious_cat Oct 25 '16 at 16:55

First of all, I would check for impurities in styreneoxide. There is a good chance it was prepared from styrene having some 2-phenylpropylene in it, and this could account for your impurity.

Other than that, I would think among lines that hydrogenation is usually reversible, so above strong hydrogenation catalys phenylethanol is in equilibrium with corresponding aldehyde and methanol is in equilibrium with formaldehyde. Crotone condensation of said carbonyl compounds with subsequent hydrogenation may account for the impurity you see here.

  • $\begingroup$ Is alphamethyl styrene the same as 2-phenylpropylene? Then it is present as an impurity in styrene. Have to figure out what molecule it gets converted to at the styrene oxide stage. $\endgroup$ – curious_cat Oct 25 '16 at 11:43
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    $\begingroup$ @curious_cat yes, it is. The you are dealing with impurity produced from another impurity. $\endgroup$ – permeakra Oct 25 '16 at 15:55

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