Before going on to talk about single enantiomers of chiral molecules in more detail, we need to explain how chemists describe which enantiomer they’re talking about. We can, of course, just draw a diagram, showing which groups go into the plane of the paper and which groups come out of the plane of the paper. This is best for complicated molecules. Alternatively, we can use the following set of rules to assign a letter, R or S, to describe the configuration of groups at a chiral centre in the molecule. Here again is the enantiomer of alanine you get if you extract alanine from living things.

1. Assign a priority number (1–4) to each substituent at the chiral centre. Atoms with higher atomic numbers get higher priority. Alanine’s chiral centre carries one N atom (atomic number 7), two C atoms (atomic number 6), and one H atom (atomic number 1). So, we assign priority 1 to the NH₂ group, because N has the highest atomic number. Priorities 2 and 3 will be assigned to the CO₂H and the CH₃ groups, and priority 4 to the hydrogen atom; but we need a way of deciding which of CO2H and CH3 takes priority over the other. If two (or more) of the atoms attached to the chiral centre are identical, then we assign priorities to these two by assessing the atoms attached to those atoms. In this case, one of the carbon atoms carries oxygen atoms (atomic number 8) and one carries only hydrogen atoms (atomic number 1). So CO2H is higher priority that CH₃; in other words, CO₂H gets priority 2 and CH₃ priority 3.

2. Arrange the molecule so that the lowest priority substituent is pointing away from you. In our example, naturally extracted alanine, H is priority 4, so we need to look at the molecule with the H atom pointing into the paper, like this.

3. Mentally move from substituent priority 1 to 2 to 3. If you are moving in a clockwise manner, assign the label R to the chiral centre; if you are moving in an anticlockwise manner, assign the label S to the chiral centre. A good way of visualizing this is to imagine turning a steering wheel in the direction of the numbering. If you are turning your car to the right, you have R; if you are turning to the left you have S. For our molecule of natural alanine, if we move from NH₂ (1) to CO₂H (2) to CH₃ (3) we’re going anticlockwise (turning to the left), so we call this enantiomer (S)-alanine. You can try working the other way, from the configurational label to the structure. Take lactic acid as an example. Lactic acid is produced by bacterial action on milk; it’s also pro duced in your muscles when they have to work with an insufficient supply of oxygen, such as during bursts of vigorous exercise. Lactic acid produced by fermentation is often racemic, although certain species of bacteria produce solely (R)-lactic acid. On the other hand, lactic acid produced by anaerobic respiration in muscles has the S configuration. As a brief exercise, try drawing the three-dimensional structure of (R)-lactic acid. You may find this easier if you draw both enantiomers first and then assign a label to each. You should have drawn:

Remember that, if we had made lactic acid in the laboratory from simple achiral starting materials, we would have got a racemic mixture of (R)- and (S)-lactic acid. Reactions in living systems can produce enantiomerically pure compounds because they make use of enzymes, themselves enantiomerically pure compounds of (S)-amino acids.

مواضيع ذات صلة

Absolute and relative stereochemistry

Diastereoisomers can be chiral or achiral

Dia stereoisomers are stereoisomers that are not enantiomers

The rotation of plane-polarized light is known as optical activity

Many chiral molecules are present in nature as single enantiomers

Stereogenic centres

Chiral molecules have no plane of symmetry

Some compounds can exist as a pair of mirror-image forms

Coupling in the proton NMR spectrum Nearby hydrogen nuclei interact and give multiple peaks

Exchange of acidic protons is revealed in proton NMR spectra

Protons on heteroatoms have more variable shifts than protons on carbon

Non-aldehyde protons in the aldehyde region: pyridines