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My current assumption is that partial hydrogenation requires a polyunsaturated fat, because partial implies at least one but not all, which means there must be at least two double bonds in the fat, hence polyunsaturated, right?

Assuming that is correct:

My current understanding is that when you bubble hydrogen (using some heavy metal catalyst) through a vegetable oil, somehow this results in transforming polyunsaturated fats to unhealthy forms of unsaturated fats called trans fats.

What exactly is happening, here? I initially assumed that adding hydrogen to the fat would do little more than making double-bonded carbons into single bonds and then attaching two hydrogen atoms to the now-free valence electrons.

But apparently it also removes cis-kinks and makes them into straight trans-lines, which doesn't change the number of hydrogens but rather changes the shape.

So what's going on here exactly? What do we say partial hydrogenation is actually doing to the polyunsaturated fat?

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You actually don't need a polyunsaturated fat to be partially hydrogenated. The "partial" refers to the macroscopic view of the fat, rather than a per-molecule basis. That is, if you have, say, one mole of monounsaturated fats and hydrogenate the double bond in half a mole of them, then you've partially hydrogenated the original mole of fat. Taken individually, each molecule has either been hydrogenated or it's still monounsaturated - no single molecule has been "partially hydrogenated". However, the mole taken as a whole has only been partially hydrogenated - 50% of the double bonds that were originally present have been hydrogenated.

Regarding trans fats, the important thing to realize is that (parts of) the hydrogenation reaction is reversible. The addition of molecular hydrogen is not concerted. Each hydrogen is added in a step-wise fashion (roughly). It's in that one-hydrogen added, one-to-go stage where the reaction can reverse course and go back to a double bond. But prior to reforming, the catalyst has broken the double bond and you have free rotation around it. Which way it's put back together is a matter of chance.

So you start with a fat which has all cis bonds. (Because the stereo and regio-selective enzymes that make them only make cis fats.) You then start the hydrogenation process. Each time you add hydrogens some goes to completion, and you end up with saturated bonds. But a fraction of the reactions get part way and then reverse. But as the hydrogenation catalyst is not stereo- and regio-selective, it's just as happy reversing to a trans bond as it is to a cis bond. So it's those bonds - the ones which go part way and then reverse - which end up as the trans fat in the final product.

The reason why fully hydrogenated fats don't have this issue is because they're produced with an excess of hydrogen. Yes, some of the double bonds react part way and then reverse to trans fats, but the catalyst is not stereo- and regio-selective, and it's able to convert the trans bonds to saturated bonds just as easily as the cis bonds. All the trans fats that get formed in the reaction are subsequently destroyed by the complete hydrogenation.

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  • $\begingroup$ How does one ensure that only (for example) half of each molecule is hydrogenated? I thought it was all processed in giant vats where everything mixes together, rather than specifically bombarding certain sections of individual molecules with hydrogen. $\endgroup$ – user27186 Feb 25 '16 at 20:32
  • $\begingroup$ @user27186 You don't hydrogenate half of each molecule, you hydrogenate half the quantity of molecules ... I'll rephrase. $\endgroup$ – R.M. Feb 25 '16 at 20:39
  • $\begingroup$ Maybe I misread. So you're saying that partial hydrogenation means that if we have a vat of unsaturated fat in, say, liquid form, then we fully hydrogenate a fraction of the vat? As opposed to hydrogenating all of it and getting a vat of solid saturated fat? $\endgroup$ – user27186 Feb 25 '16 at 20:39
  • $\begingroup$ "Taken individually, each molecule has either been hydrogenated or it's still monounsaturated" Do you mean just unsaturated (since a polyunsaturated fat can be used too?) $\endgroup$ – user27186 Feb 25 '16 at 20:50
  • $\begingroup$ Also what you're saying seems to contradict this spot in this video here: youtube.com/watch?v=6zU0atqAL4s&t=2m6s which implies that partial hydrogenation does refer to individual molecules as being "partially hydrogenated" due to only some of the double bonds being broken and replaced with hydrogen. $\endgroup$ – user27186 Feb 25 '16 at 21:14
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Vegetable oils which have been partially hydrogenated, are now partially saturated so the melting point increases to the point where a solid is present at room temperature. The degree of hydrogenation of unsaturated oils controls the final consistency of the product.

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    $\begingroup$ Welcome to Chemistry.SE! Take the tour to get familiar with this site. Mathematical expressions and equations can be formatted using $\LaTeX$ syntax. While it may seem strange at first we do not use salutations in our Questions and Answers. Please use the upvote button to say thanks, or leave a comment how the post can be improved. $\endgroup$ – Martin - マーチン Feb 26 '16 at 6:11

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