 |
| Resolution |
|
| Equal amount of dextro and laevo varieties are got when an optically active compound is synthesized in the lab. The result is a racemic mixture. The two forms called enantiomorphs can be separated from the racemic mixture. This process is called resolution. |
| |
| Fractional crystallization or fractional distillation |
| |
| Fractional crystallization or fractional distillation cannot bring about resolution. This is because the enantiomorphs have identical physical properties. |
| |
| Some of the methods used for resolution are |
| |
| a) Mechanical separation |
| |
| b) Biochemical separation |
| |
| c) Salt formation |
| |
| d) Selective adsorption. |
| |
| Mechanical separation |
| |
| When the (+) and (-) isomers or their salts form well defined crystals showing hemihedral faces, hand picking is used to separate them. Pasteur separated sodium ammonium racemate crystals by mechanical separation in 1848. |
| |
| Below a certain temperature the enantiomorphs crystallize separately. Above this temperature they crystallize as the racemic variety. The nature of the solvent affects the temperature. This method is not popular nowadays because it gives rise to experimental difficulties. |
| |
| Biochemical separation |
| |
| Pasteur was the first to try out this method in 1858. One of the optical isomers can be destroyed selectively at a faster rate by the action of bacteria, moulds or fungi in the racemic mixture solution. This is called selective assimilation. |
| |
| For e.g., when Pencillium glaucum(green mould) is grown in an ammonium racemate solution, it destroys the (+)-tartrate by assimilation. The (-) tartrate is unaffected. |
| |
| The disadvantages are: |
| |
| i) One isomer is completely lost. |
| |
| ii) Some other byproduct might be formed and therefore the sample might be difficult to purify. |
| |
| Salt formation |
| |
| Best of all the methods of resolution, Pasteur was again the first to use this in 1858. The principle is - "The conversion of the active constituents of the racemic mixture into Diastereomers (salts) of another optically active or base." |
| |
 |
| |
| Solubilities of the two salts are different and can be separated by fractional crystallization. To get the original active compounds, the salts should be hydrolyzed with inorganic acids or alkalis. Bases used for resolution are quinine, brucine, cinchonine and morphine. Similarly the acids used for resolution are tartaric acid and camphor sulphonic acid. |
| |
| Selective adsorption |
| |
| Optically active adsorbents can selectively adsorb some other optically active substances. For e.g., Henderson and Rule in 1939 used dextro-lactose as an adsorbent to separate active constituents of racemic camphor. |
| |
