Do geometric stereoisomers matter? You bet. Here's one reason why:
All the things we hear about trans fats in our diets relate to the presence of trans-alkenes in the fats we eat. Nearly all (but not all) naturally occurring fats are cis. The majority of the trans fats we eat come from fats that have been manipulated by hydrogenation: an addition of hydrogen across the double bonds. If a particular alkene group does not get fully hydrogenated during this process, it can switch from the cis to trans orientation. New industrial processes are allowing us to hydrogenate fats in ways that avoid this problem.
The amount of unsaturation in dietary fats has traditionally been determined by finding the "iodine number" or "iodine value." This number is calculated by reacting iodine with the fats, until a purple color shows up. The presence of the iodine color indicates there are no more groups on the fat that will react with it. The reaction that normally consumes the iodine is an addition reaction (of X2, in this case I2) across the double bond.
Those of you lucky enough to be in Organic this fall will discuss this particular reaction type in class next Thursday. I can hardly wait!
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