Return to Degrees of Unsaturation

Alkenes are obviously not fully saturated, they are missing at least two hydrogens. An obvious example of unsaturation in everyday life is in fats. You have all heard about saturated and unsaturated fats. The difference is simple (in principle): unsaturated fats have carbon-carbon double bonds, saturated fats don't. Unsaturated fats have more degrees of unsaturation than saturated fats. The term "fat" refers to a great number of possible chemical structures but we will not discuss these details here.

Here is an equation showing a representitive (simple) unsaturated fat (on the upper right) and a saturated fat (on the lower left). The double bonds in the unsaturated fat cause "kinks" in the otherwise regular structure of the carbon chain. As a consequence, unsaturated fat molecules can not pack together very well, and have a much higher tendency to be liquids. Saturated fats can pack together better and tend to be solid.

why don't saturated fats have zero degrees of unsaturation?

Plants tend to be the sources of unsaturated fats (hence vegetable oils), and animals tend to produce more saturated fats (hence butter!). Margerines are made from vegetable oils (and lots of other stuff), by conversion of the double bonds into single bonds. This is done in a hydrogenation reaction (hence the term hydrogenated fats), whereby a hydrogen atom is added to each end of the double bond (removing the unsaturation) using a catalyst. This is one of the reactions of alkenes we will cover later in the semester. This reaction is shown schematically above.

why doesn't the hydrogen add to the carbon-oxygen double bonds?

Natural unsaturated fats tend to have the double bonds in a cis- configuration, see below. Polyunstaturated fats have several double bonds that can be reduced (i.e. have hydrogen added to them). Partially hydrogenated fats have had some but not all of the double bonds reduced. This partial reduction can result in the formation of some trans-double bonds. Because trans- unsaturated fats are not common in nature, these fats are not metabolized by your body in the same way as "natural" fats.

There is a lot of debate about whether partially hydrogentated fats are harmful for this reason. I am not going to get into all of the nutritional issues and debate (not all very reasoned!) about fats. If you are interested, The American Heart Association may be a good place to start.

When I worked in industry, I had an Egyptian colleague (incidentally a brilliant chemist) who told me about butter made from buffalo milk that was used in Egypt. It was apparantly so full of saturated fats (and presumbly it did not contain a large variety of unsaturated fats) that not only was it a solid, but it would actually crystallize. Presumably it would not be considered to be very healthful today (!), however, it did have one very interesting property. Because it was crystalline, it did not go rancid very quickly. Remember that organic chemical are important both because complex structures are easily formed AND because the molecules are soluble. When a compound is in a crystal, it is difficult for it to get involved in reactions, only the small number of molecules on the surface are likely to react at a reasonable rate. Butter goes rancid as a result of a chemical reaction. I don't know what it is, but it presumably involves an oxidation with air. The most obvious place to do a reaction in a fat is at the carbon-carbon double bonds (the electrons in the double bonds are higher in energy, therefore more reactive. A fully saturated fat such as in buffalo milk butter has NO carbon-carbon double bonds to react! So, if you live in a hot country with no refrigeration, buffalo butter has a few advantages :)