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.
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?).
If you have learned about half of the following terms, go directly to the Question.
Else, please click the terms which you have not yet learned, and review them.
Polarized light 偏光
Optical activity 光学活性
Asymmetric carbon atom 不斉炭素原子
Dextro (Levo) rotatory 右（左）旋性
Specific rotation 比旋光度
Optical resolution 光学分割
Fisher projection フィッシャー投影図
Relative configuration 相対立体配置
DL nomenclature DL表示
Absolute configuration 絶対立体配置
RS nomenclature RS表示
Now, you may read through Summary to ensure the point, or go to Questions to test yourself.
5.1 Optical activity
5.3 Racemic form optical resolution
5.4 Fischer projection
5.5 The relative configuration
5.6 Absolute configuration -R, S nomenclature-
Go to index page (International Version)
Optically active substance: a substance that can rotate
the plane of polarized light (as solid or as a solution)
Dextrorotatory: the property to rotate the plane of polarized light clockwise (to the right side)
Levorotatory: the property to rotate the plane of polarized light counterclockwise (to the left side)
Specific rotation: the magnitude of rotation. (+) indicates dextrorotatory, and (-) indicates levorotatory.
go to 5.1 Optical activity
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.
Center of chirality (asymmetric carbon atom)
Axis of chirality
Plane of chirality
go to 5.2 Chirality
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
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).
5.4 Fischer Projection
(+)-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.
5.5 Relative configuration
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.
S-configuration：If an arrow connecting three ligands L-M-S runs counterclockwise, the enantiomer is called S.
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R ，S 一convention