A "stereoisomer" is a possible configuration of a carbon chain which contains one or more double-bonds. For each double-bond (depending on its location), there are usually two possible configurations, or stereoisomers. What do I mean by configuration? I mean that it may have a cis or trans configuration. In a cis configuration, the carbons linked to the double-bond are on the same side, but in a trans configuration, the carbons linked to the double-bond are on opposing sides. (This is why hydrogenated oils are sometimes called trans fats... but I'll explain that later.) In order to calculate the possible number of stereoisomers of a carbon chain, you use the formula 2n, where n represents the number of double-bonds.
Calculating number of possible stereoisomers:
Oleic Acid . CH3(CH2)7CH=CH(CH2)7COOH
1 double-bond, so 2 possible stereoisomers (21 = 2)
Linoleic Acid . CH3(CH2)4(CH=CHCH2)2(CH2)6COOH
2 double-bonds, so 4 possible stereoisomers (22 = 4)
Linolenic Acid . CH3CH2(CH=CHCH2)3(CH2)6COOH
3 double-bonds, so 8 possible stereoisomers (23 = 8)
It is important to note that naturally, nearly all bonds found in animal and plant fats will be cis configuration. Conjugated linoleic acid is an example of a fatty acid with a natural trans configuration. But the trans fats found in processed foods are not naturally configured. So we run into a problem here. Remember the first rule of biology: Form determines function. For each molecule that our bodies process, we require a similarly shaped "key" called an enzyme to "unlock" it. This is part of why we cannot digest trans fats created from hydrogenation (for which I will eventually blog to show the reaction equation and explain the process). We simply do not have the required enzymes to digest them.
