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Chemistry 2131:
Organic Chemistry for the Life Sciences (3)

Stereochemistry and Optical Activity

• last day we ended up looking at the example of D-glucose, a molecule with 4 stereocentres. As mentioned the best way to depict these molecules is by a Fischer projection.
• the maximum number of different stereoisomers is 2ⁿ, where n is the number of stereocentres. Please note that that number is the maximum number, there are not always so many as we will see in a minute.
• as with molecules with a single stereocentre, molecules that are mirror images of each other are enantiomers. If the molecules are not mirror images, they are called diastereomers
• let's look at a simple example, 2,3,4-trihydroxybutanal, this is a 4 carbon sugar. There are 2 stereocentres on carbon atoms 2 and 3. Let's look at all of the possible stereoisomers. There are 4 maximum. When the hydroxyls are on the same side of the Fischer projection these are a pair of enantiomers called erythrose (2(R)3(R) and 2(S)3(S)). When they are on opposite sides, they are a pair of enantiomers called threose (2(R)3(S) and 2(S)3(R)).
• let's look now at a ore complicated but very useful example, tartaric acid (2,3-dihydroxybutanedioc acid). There are 2 stereocentres, thus a possible maximum of 4 stereoisomers
• let's start by looking at the molecule with the hydroxyls on opposite sides of the Fischer projections (2(R)3(R) and 2(S)3(S)). This is a pair of enantiomers. The isomers with the hydroxyls on the same side are also a pair of enantiomers (2(R)3(S) and 2(S)3(R)).
• or are they? If we rotate on in the plane of the paper they are identical. These compoundshave a plane of symmetry through the middle of the molecule, therefore they are not chiral, this molecule is achiral. Such an achiral molecule containing 2 or more stereocentres is called a meso compound
• so how many stereoisomers are there for tartaric acid? Only 3, a pair of enantiomers and a meso compound.

2. Stereocentres in Cyclic Molecules:

• before leaving the concept of multiple stereocentres, let's have a brief look at examples of cyclic molecules with stereocentres
• consider 2-methylcyclopentanol. We already know that two stereoisomers are possible for this molecule, cis and trans. Let's look at the cis ismoers for starters. There are two possibilities that are mirror images of each other, these are enantiomers.
• the trans isomer also has a pair of enantiomers. So, we have our maximum number of possible stereoisomers. Remember that cis versus trans are diastereomers.
• consider the situationif both groups are hydroxyls. We end up with a pair of enantiomers and a meso compound (it has a plane of symmetry)

3. Optical Activity:

• let's look at some of the properties of stereoisomers. I will come back to the example of tartaric acid, the one with 2 enantiomers and a meso compound. Let's look at the physical properties of these isomers