Nomenclature of Enantiomers

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One type of steorisomerism is that of optical isomers, also called enantiomers. Enantiomers are non-superimposable mirror-image pairs of stereoisomers, that is, a chiral molecule and its mirror image. This poses as problem for naming the compounds. Two enantiomers have the same atoms connected in the same way, so that following IUPAC nomenclature rules will give both compounds the same name. How can we distinguish the two mirror-images? This depends on the geometry of the molecule.


Naming Octahedral Enantiomers

The most common source of chirality in an octahedral complex is a tris(bidentate) set of ligands about a central atom. The three chelating ligands form a helix about the chiral centre. The D-isomer exhibits a right-handed or clockwise helix, and the L-isomer has a left-handed or counter-clockwise helix.

Δ-[Cr(en)3]3+ Λ-[Cr(en)3]3+


Naming Tetrahedral Enantiomers

Four different groups in tetrahedral coordination about a central atom leads to chirality. This is the most common origin of chirality in organic chemistry, but applies to tetrahedral coordination complexes as well. A system of nomenclature to distinguish the two groups must somehow describe the three-dimensional arrangement of those four groups about the chiral centre. This can be accomplished by ranking the four groups in decreasing order of priority, such that group a has the highest priority, followed by groups b, c, and d in decreasing order. The molecule is turned so that the group with the lowest rank (d) is behind the carbon atom, and the other three ranked groups form a wheel about the central atom. The ranking either forms a clockwise or counter-clockwise circle, tracing a path from a to b to c. If the path is clockwise, then the molecule is the R (rectus, right) enantiomer; if the path is counter-clockwise, the molecule is the S (sinister, left) enantiomer.

Place the lowest-priority group d behind the carbon.
Other groups form a clockwise path from a to b to c.
This is the R isomer.
  Place the lowest-priority group d behind the carbon.
Other groups form a counter-clockwise path from a to b to c.
This is the S isomer.

Cahn-Ingold-Prelog Rules

  1. Substituents are ranked according the atomic number of the atom connected to the chiral carbon, such that the atom with the higher atomic number gets higher priority. As a result, a hydrogen group is always lowest priority.
    Example: Cl (Z = 17) > N (Z = 7) > C (Z = 6) > H (Z = 1).

  2. If two substituents connected to the chiral centre have the same priority, the next atoms connected to the first atom are also ranked, until a point of difference in the substituent groups is found. The group with atoms of greater atomic number at that point has higher rank.

    Example: -NCCH3 is ranked below -N2, because the second N atom in the second group gives this group higher priority. The presence of additional atoms farther down the chain of the first group is irrelevant, since they appear after the first point of difference.

  3. Double and triple bonds are ranked as if the atom were connected to two or three different atoms of the same atomic number.

    Example: -OC(=O)CH3 is treated as if the first carbon were connected to two more oxygen atoms, one for each bond in the C=O group. The atoms in red are not actually present, but are imagined for the purposes of assigning priority to the group.

Practice Examples: R/S Nomenclature

For each of the chiral molecules below, determine whether the compound is the R or S isomer, as follows:

  1. Find the chiral carbon and identify and rank the four different groups around it
  2. Manipulate the molecule so that the lowest ranking group is at the back of the molecule.
  3. Determine whether the high-to-low ranking order forms a clockwise (R) or counter-clockwise (S) loop about the carbon.

  1. Rankings: -Cl > -OH > -NC5H5 > -CN
  2. CN is positioned in back
  3. Cl > OH > NC5H5 wheel is clockwise: R
  1. Rankings: -ONO > -OH > -NH3 > -NH2
  2. NH2 is positioned in back
  3. ONO > OH > NH3 wheel is clockwise: R
  1. Rankings: -Br > -Cl > -SCN > -NCCH3
  2. NCCH3 is positioned in back
  3. Br > Cl > SCN wheel is counter-clockwise: S

This page is maintained and copyright by W. Stephen McNeil at UBC Okanagan. .