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Chemistry (including Biochemistry)/Process of Dehydrogenation

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QUESTION: How is the dehydrogenation process done?

ANSWER: While the dehydrogenation could happen in any system where a double bond is formed by the removal of two protons, a total overview would require a lot of time.  Lets limit our discussion to a popular topic: the dehydrogenation of fatty acids. Here I will discuss one such in-body reaction.

Let us take an acid having the general form of
R-CH2-CH2-C(=O)-OH

In a body, a catalytic enzyme acyl CoA dehydrogenase will attach to the acid end of the molecule to make the active complex
R-CH2-CH2-C(=O)-CoA

The fatty acid dehydrogenase (FAD) then acts on a proton adjacent to the -C(=O)- group, called an alpha proton, and then a proton from the next carbon over, called the beta proton.  Generally this reaction is written, with reconstitution of the acid after CoA leaves:

R-CH2-CH2-C(=O)-CoA  +  FAD --> R-CH=CH-C(=O)-CoA + FADH2 --> R-CH=CH-C(=O)-OH + CoA + FADH2

A thing to note is that there are some counter ions around, but they are not immediately needed to get the general idea of what is going on.

As an aside, there are different CoA dehydrogenases for different size acids, with one for short (~5 carbon), medium (~9 carbon) and long (13+ carbon) fatty acids.

There are many, many metal catalyzed industrial reactions for dehyrogenase reactions where alkanes are turned into alkenes, and just a little bit of poking around should turn some of these up (http://chemwiki.ucdavis.edu, for example section 16.5).

I hope this helps.

Take care.

---------- FOLLOW-UP ----------

QUESTION: That really sums up a lot for  me on how that works now!
Thank you for that!!
I have one more question; while doing the dehydrogenation process I was reading how selenium can be used as a catalyst.

My question is; what's the difference say using selenium or selenium dioxide? Especially with organic chemistry; it would target a specific part of a molecule?

Answer
There are two different things going on when you compare the pure species to oxides in catalysis - they apply to dehydrogenation, but are also true in general.

[1]  First, you have to look at the oxidation state.  Depending on if an atom is neutral, +/-1 or +/-more, it will have a different binding affinity to certain molecules.  You will note that in the previously discussed reaction that CoA pushes in at the acid side of the molecule.  Similarly, a metal catalyst has to have the correct charge, size and available space around it to get at the same region that the CoA occupies. So, typically this means that there is a particular combination of size-charge-ligand that can work as a catalyst.  Remembering that a catalyst has to be regenerated, this special combination needs to be just right for slipping in and also just right to leave after the double bond is formed.

[2]  There is a thing in metallo-organic chemistry (and inorganic chemistry in general) called the coordination geometry.  What this does is dictate at what angle and how many bonds can be formed with a metal species.  Often most importantly, it will show you where lone electron pairs are - which typically attack partially positive carbons like the one on the acid end of a fatty acid.  Typically, as the oxidation state changes, so to does the rules governing how many bonds it can form and at what angles it can form them.  This is particularly critical in catalysis in which often a catalytic species will need to hold on to a couple atoms simultaneously and at the correct angles.  

While I don't know anything specific about Se catalysis of dehydrogenation, I do know that the above are important for almost inorganic catalysts.   If you google images of Selenium bonding geometries, a bunch come up.  In all likelihood, neutral Se and charged Se do not have at all the same effect on dehydrogenation.

On a final note, because everything has to be 'just so' in terms of binding affinity and geometry, nature uses proteins to get the job done by using a large structure to hold the site in place, force the reaction and regenerate the catalyst.  Which I think is pretty cool.

Chemistry (including Biochemistry)

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Dr. Jeffery Raymond

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Materials chemistry. Materials science. Spectroscopy. Polymer science. Physical Chemistry. General Physics. Technical writing. General Applied Mathematics. Nanomaterials. Optoelectronic Behavior. Science Policy.

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Teaching: General Inorganic Chemistry I & II, Organic Chemistry I & II, Physical Chemistry I, Polymeric Materials, General Physics I, Calculus I & II
My prior experience includes the United States Army and three years as a development chemist in industry. Currently I am the Assistant Director of the Laboratory for Synthetic Biological Interactions. All told, 13 years of experience in research, development and science education.

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Texas A&M University, American Chemical Society, POLY-ACS, SPIE

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Journal of the American Chemical Society, Nanoletters, Journal of Physical Chemistry C, Journal of Physical Chemistry Letters, Ultramicroscopy Proceedings of SPIE, Proceedings of MRS, Polymer News, Chemical and Engineering News, Nano Letters, Small, Chemistry.org, Angewandte

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PhD Macromolecular Science and Engineering (Photophysics/Nanomaterials Concentration), MS Materials Science, BS Chemistry and Physics, Graduate Certificate in Science Policy, AAS Chemical Technology, AAS Engineering Technology

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