Organic Chem 101, Part III

Okay, so today in the lab I'm doing what is known as a Hydrogenation reaction. This is smiply what it the name says it is. I'm adding hydrogen to the molecule. Simple right?

See sometimes if you read the label of some comercial products such as oils, butters and lards, they will list them as partially saturated, or partially saturated fatty acids. Which just means that they have been partially hydrogenated. Cool eh?

http://www.chemguide.co.uk/organicprops/alkenes/hydrogenation.html

So what does all this actually mean?

Well first something that is Unsaturated, is a molecule that has a double bond (see the diagram below), and something that is Saturated has no double bonds (again see diagram below). Why do they use the terms Saturated and Unsaturated. It's just a term that refers to whether each carbon molecule is maxed out on the number of bonds that it has. An Unsaturated Carbon is only bonded to 3 other atoms, whereas a Saturated molecule is bonded to 4 other atoms.

So, how does one go about adding hydrogen to an Unsaturated system?

Fairly simply actually. What you do is you take your Unsaturated molecule, dissolve it up in Methanol or Ethanol (Ethanol is less flamable and less likely to result in an explosion or fire) and you add your catalyst, which is usually Palladium absorbed on charcoal (or carbon). Usually you only need a few mole percent of catalyst for the reaction to proceed. Then you put it under an atmosphere of hydrogen and stir. The usual way of putting it under a hydrogen atmosphere in a Chemistry lab is to fill a ballon full of hydrogen, and attach it to your flask. This will cause the flask to fill with hydrogen and allow for your reaction to take place. These reactions are usually fairly clean, so the only purification required (assuming you go into the reaction with clean material) is to just filter the solution through celite (crushed fossilized sea creatures).




What actually happens at a molecular level?

Well, what happens is that the Palladium usually co-ordinates to the double bond in a pi-allyl fashion, and also co-ordinates to one molecule of hydrogen. The hydrogen then adds across the double bond, giving rise to your unsaturated system.

So what does it mean when something is partially hydrogenated?

Well it means just that, only some of the double bonds in the molecule have been hydrogenated, therefore it is not fully saturated, but it's not still in it's fully unsaturated form.

So, any questions? Do you feel any smarter?