The traditional method for resolving a racemic mixture into its enantiomers is to use an enantiomeri — Organic Chemistry Chemistry Question
Resolution of (±)-α-Methylbenzylamine and Determination of the Optical Purity
The traditional method for resolving a racemic mixture into its enantiomers is to use an enantiomerically pure natural product that bonds with the compound to be resolved. The enantiomers in the racemic mixture bond with the optically pure resolving agent to form two diastereomers. The diastereomers are separated, and then the resolving agent is cleaved from the separated enantiomers. The optical purity of a compound is defined as the ratio of its optical rotation to the rotation of a pure enantiomer.
A racemic mixture of –methylbenzylamine is readily resolved by (R,R)–(+)–tartaric acid. The resulting (S)–(–)––methylbenzylaminium (R,R)–(+)–tartrate salt, SRR–salt, has a lower solubility than its diastereomeric counter part, (R)–(+)––methylbenzylaminium (R,R)–(+)–tartrate salt, RRR–salt. The SRR salt is induced to crystallize, whereas the RRR salt stays in solution. The crystals are removed by filtration and purified, and (S)–(–)––methylbenzylamine is regenerated by treatment with a base.
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Procedure and Questions:
In an Erlenmeyer flask (250 cm3) are placed (R,R)–(+)–tartaric acid (7.8 g, 52.0 mmol) and methanol (125 cm3). The mixture is heated on a hot plate until the solution is nearly boiling. A racemic mixture of –methylbenzylamine (6.25 g, 51.6 mmol) is added slowly over a period of 5 minutes to the solution. (Caution: at this step, the mixture is very likely to froth and boil over) Stopper the flask and let it stand overnight (18 hours).
Formation of prismatic crystals indicates a complete resolution of enantiomers, whereas impure isomers will appear in needles. Needles should be dissolved by careful heating, and crystallized again on cooling slowly. A seed of prismatic crystal can be added to induce the recrystallization.
The crystals are filtered through a Büchner funnel, and rinsed with a few portions of cold methanol. The crystals are transferred to a pre-weighed Erlenmeyer flask (50 cm3), and purged with a stream of nitrogen. The dry crystals are weighed, and the yield is calculated. The crystals in the flask are treated with water (25 cm3), and 50% aqueous sodium hydroxide solution (4 cm3) is added slowly. The mixture is extracted with 10 cm3 of methylene chloride for three times using a separatory funnel. The organic layers from each extraction are combined in a stoppered flask, and dried over anhydrous sodium sulfate (1.0 g) for about 10 minutes. The dried solution is decanted into a round–bottom flask (50 cm3), and methylene chloride is removed by rotary evaporation. The residual –methylbenzylamine is weighed, and the yield is calculated. Every effort should be taken to avoid prolonged exposure of the amine to air. Transfer the –methylbenzylamine into a polarimeter tube cell, and measure its optical rotation. The reported specific rotation of (S)–(–)––methylbenzylamine is [α]D23 = –40.3° (neat). Calculate the percentage for each of the enantiomers in the resolved sample.