Ch. 5 - Lactic acid


In this chapter you will firstly learn that compounds with an asymmetric carbon atom have a pair of enantiomers, and understand the relation between the enantiomerism and optical activity. You will also learn Fisher projection as an additional means to describe the three dimensional information on the two-dimensional paper.

Since we could not see a molecule directly, for many years we could only define the relative configuration of a molecule. The DL nomenclature, which is based on the stereochemistry of glyceraldehydes, is a consistent and logical system to define the relative configuration. Furthermore, you will learn RS nomenclature, which was introduced for expressing the absolute configuration, and understand the relation among these nomenclatures and the sequence rule.

Goal of this chapter

When you will have finished this chapter, you will be able to do followings:

  1. To distinguish chiral compounds, especially compounds with asymmetric carbon atom(s).
  2. To understand the method to separate racemates into optical active enantiomers.
  3. To draw a Fischer projection of a compound with one asymmetric carbon atom, and convert this projection into an equivalent Fisher projection.
  4. To compare the stereochemistry of a compound with one asymmetric carbon atom with that of glyceraldehydes, and determine whether the relative configuration of the given compound is D or L.
  5. To apply the sequence rule to the above compound and determine the absolution configuration (R or S?).

New terms and concepts

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Else, please click the terms which you have not yet learned, and review them.

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S5.1 Optical activity

S5.2 Specific rotation

Specific rotation: the magnitude of rotation. (+) indicates dextrorotatory, and (-) indicates levorotatory.

go to 5.1 Optical activity

S5.3 Chirality

Chiral molecule: related to its own mirror image in the way that your left hand is related to your right hand.

Achiral molecule: a molecule which is not chiral.


S5.4 Types of chirality

go to 5.2 Chirality

S5.5 Racemates and optical resolution

Racemates: a 1 : 1 mixture of a pair of enantiomers.

Optical resolution: separation of racemates into component enantiomers.

go to 5.3 Racemates and optical resolution

S5.6 Rules of Fischer projection

1) exchange: the exchange of even number (2n) gives the original, while the exchange of odd number (2n−1) gives the enantiomer (n is an integer).

2) rotation: a clockwise rotation by 2n x (π/2) gives the original, while a rotation of (2n−1) x (π/2) the enantiomer (n is an integer).

go to 5.4 Fischer Projection

S5.7 Relative configuration

(+)-glyceraldehyde 21 is the key compound of D-series.
(-)-glyceraldehyde 22 is the key compound of L-series.
Any compound derived from 21 without affecting its configuration belongs to D-series.
Any compound derived from 22 without affecting its configuration belongs to L-series.

go to 5.5 Relative configuration

S5.8 R, S nomenclature

When looking down the bond from the asymmetric carbon atom toward the ligand of lowest priority (s),
R-configuration:If an arrow connecting three ligands L-M-S runs clockwise, the enantiomer is called R.
-configuration:If an arrow connecting three ligands L-M-S runs counterclockwise, the enantiomer is called S.


S5.9 How to decide the configuration by R, S nomenclature

1) From Fischer projection : Convert Fischer projection so that the ligand of the lowest priority comes to the bottom. Follow the direction L > M > S.

If it is clockwise, the configuration is R;

if counterclockwise, it is S.

2) From the flying-wedge drawings or molecular model : the idea is the same.

go to 5.6 Absolute configuration一RS 一convention