Learning paths

Learning paths

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Learning path

Full curriculum

Full curriculum
  1. Functional groups in organic molecules
  2. Constitutional isomerism and molecular connectivity
  3. VSEPR theory and molecular geometry
  4. Cahn-Ingold-Prelog priority rules for substituents
  5. Chirality, enantiomers and diastereomers
  6. Amino acid structure, chirality and side-chain chemistry
  7. Amino acid protonation, zwitterions and isoelectric point
  8. Peptide bonds and polypeptide backbone directionality
  9. The genetic code, codons and reading frames
  10. Biological nitrogen fixation and assimilation
  11. Monosaccharide structure, cyclization and anomers

Unit content

Chirality, enantiomers and diastereomers

A molecule is chiral when it is not superimposable on its mirror image. The two non-superimposable mirror-image forms are enantiomers.

A common source of chirality in organic molecules is a tetrahedral carbon attached to four different substituents. Such an atom is a stereogenic center or chirality center.

Consider lactic acid,

$$\mathrm{CH_3CH(OH)CO_2H}.$$

Its central carbon is bonded to H, OH, CH3 and CO2H—four different groups—so two mirror-image configurations are possible.

Enantiomers have the same connectivity and most ordinary bulk physical properties in an achiral environment, but they can interact differently with other chiral systems. This is especially important in biology because enzymes, receptors and proteins are themselves chiral.

R and S configuration

A tetrahedral stereogenic center can be assigned an R or S configuration after its four substituents have been ranked using the Cahn-Ingold-Prelog rules.

  1. Identify priorities 1 through 4.
  2. View the center with the lowest-priority group, 4, pointing away from you.
  3. Trace the direction 1 → 2 → 3.
  4. Clockwise gives R; counterclockwise gives S.

For example, if a stereogenic carbon is attached to Br, OH, CH3 and H, CIP priorities are

$$\mathrm{Br}>OH>CH_3>H.$$

With H directed away, the spatial direction from Br → OH → CH3 determines whether the center is R or S.

Wedge-and-dash notation records three-dimensional orientation in a flat drawing: a solid wedge projects toward the viewer, a hashed wedge projects behind the page and an ordinary line lies approximately in the page.

Diastereomers

Not all stereoisomers are mirror-image pairs. Diastereomers have the same connectivity but are stereoisomers that are not enantiomers. Geometric E/Z isomers are one class of diastereomers. Molecules with several stereogenic centers can also form diastereomeric pairs when some, but not all, configurations differ.

A molecule containing stereogenic centers is not automatically chiral as a whole. Internal symmetry can make some structures superimposable on their mirror images despite containing stereogenic centers.

The key hierarchy is

same molecular formula
├── different connectivity → constitutional isomers
└── same connectivity → stereoisomers
    ├── non-superimposable mirror images → enantiomers
    └── other stereoisomeric relationships → diastereomers

Chirality therefore describes a property of the complete three-dimensional molecule, while R/S labels specify the configuration of individual stereogenic centers.