Describing the configuration of a chiral centre.

•	How do chemists explain which enantiomer they are talking about? One way is to use a set of rules to assign a letter R or S , to describe the configuration of groups at a chiral centre.

•	Once again, display the structure of alanine (refcode: LALNIN23).

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First, look at the four atoms directly attached to the stereogenic centre and assign priorities in order of decreasing atomic number. The group with the highest atomic number is ranked first, the lowest atomic number is ranked fourth. If two or more of the atoms are identical, then we assign priorities by assessing the atoms attached to those atoms, continuing on as necessary until a difference is found.

•	So, we assign priority 1 to the NH3 group. Priorities 2 and 3 will be assigned to the CO2 and CH3 groups respectively since the CO2 group carries oxygen atoms whereas the CH3 only carries hydrogen atoms. Finally, priority 4 is assigned to the hydrogen atom.

•	Now, orientate the molecule in the display so that the lowest priority substituent (the hydrogen) is pointed away from you. The hydrogen should be almost eclipsed by the chiral carbon atom.

•	Next, mentally trace a path from substituent priority 1 (NH3) to 2 (CO2) to 3 (CH3). If we are moving in a clockwise direction, then we assign the label R to the chiral centre; if we move in an anticlockwise direction, we assign the label S.

 

•	What is the configuration of our alanine molecule? Is it the (S)-alanine, or (R)-alanine enantiomer?

•	Some further examples of chiral molecules are provided. Identify the chiral centre in each of the following molecules and assign their configuration using R and S notation:

•	Carvone (spearmint oil) (refcode: RERXIV)

•	Adrenaline (refcode: ADRENL)

•	Ibuprofen (refcode: JEKNOC10).