Stereoisomers

As said in the last lesson, stereoisomers always have the same connectivity. However, due to their rotation in space they form isomers. Stereoisomers can be broken down further into more subgroups.

First are geometric isomers. Geometric isomers have atoms bonded in the same order but rotated differently. There are special naming conventions for these compounds depending on the rotation of the molecule.

If the cis- and trans- prefixes sound familiar, go to your fridge or pantry and take a look at a nutritional label. The amount of trans fat in a food is almost always noted on the nutritional label. This is because trans fatty acid have a much more negative impact on the body than cis fatty acids. Cis fat is found much more in nature, whereas trans fat is largely a result of more artificial foods. The reason why trans fats are worse for the body is that a fat digesting enzyme, known as lipase, is unable to breakdown trans-fatty acids and hence these build up in the blood causing significant damage and blockage to blood vessels. Watch this short video from the Mayo Clinic to learn more about trans fat.

So, looking back on the models of both molecules, a cis molecule has both groups on the same side of the double bond, whereas a trans molecule has them on opposite sides of the double bonds. Just because a molecule has a double or triple bond however does not mean that they have an isomer. A molecule like propene has a double bond but no geometric isomers because there is only one special group on the molecule.

Next are optical isomers. These differ in the placement of molecules around an atom. For these, 3D models are very beneficial. To understand optical isomers, there is a few vocab terms to learn. The first is superimposable. to superimpose means to be able to place on top on another exactly, with no overhang. if two molecules are superimposable, they are not isomers, but the same molecule. Objects that are not super imposable are called chiral. The center of a chiral compound (like the black atom in the still image below) is aptly called the chiral center. Any carbon with four different groups, and is a mirror image of one of its isomer, has chirality.

The R/S system is used to describe chiral compounds. Watch this video from khan academy to learn about it.

In order to describe chirality, remember these three steps

• Prioritize the four groups attached to the center (based on atomic number)

• Point the lowest priority group away from you

• Determine if the sequence 1-2-3 runs clockwise (R) or counterclockwise (S)

Molecules that are mirror images, but not superimposable are called enantiomers. The second image above displays a pair of enantiomers. If you hold your hands palms up, they are enantiomers, mirror images yet not able to be superimposed. If two objects are non mirrored images, then they are diastereomers. Diastereomers can differ at many different locations, meaning a compound can have many chiral centers. Watch this short video on diastereomers.

Take a look at the updated isomer flowchart below. When you're done, take the quiz.