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7-ResolutionF07 - Resolution of a Racemic Mixture Reading...

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Resolution of a Racemic Mixture Reading assignment: Review chapter 6 in Loudon. Pay special attention to sections 6.1, 6.3, 6.4, 6.6 and 6.8. Introduction In last week’s lab you were introduced to the basics of stereochemistry and hopefully feel more comfortable identifying the different kinds of isomers including enantiomers and diastereomers. One of the major differences between the two kinds of stereoisomers is that enantiomers have identical boiling points, melting points, densities, polarities, NMR spectra and solubilities 2 but differ in their interaction with plane polarized light and chiral environments, such as an enzyme active sites. Alternatively, diastereomers differ in all properties. 3 Standard separation techniques rely on differences in boiling point (distillation), solubility (recrystallization and extraction), and polarity (standard chromatography) and are therefore not appropriate for the separation of enantiomers. Enantiomeric resolution is one effective method that involves converting a racemic mixture of enantiomers into diastereomeric salts, which do have different physical properties and therefore can be separated by standard methods. Resolution of racemic phenylethylamine. The goal of today’s lab is to separate a racemic mixture of (R)- and (S)-phenylethylamine ( α - methylbenzylamine) and to isolate enantiomerically pure (S)-phenylethylamine (figure 3). (S)-Phenyleythlamine Ph CH 3 NH 2 H CH 3 Ph H H 2 N (R)-Phenylethylamine Figure 3. Enantiomers of phenylethylamine. As you can see from scheme 1, the racemic mixture of phenylethylamine is reacted with an enantiomerically pure carboxylic acid, (2R,3R)- tartaric acid. This acid was chosen because it is inexpensive and commercially available in high enantiomeric purity. The basic amine of phenylethylamine deprotonates the carboxylic acid of the tartaric acid. Diastereomeric salts are formed that differ only in the configuration of the phenylethylammonium portion. The (S)- phenylethylammonium (2R,3R)-tartrate salt is less soluble in methanol and will selectively precipitate while the (R)-phenylethylammonium (2R,3R)-tartrate salt remains in solution. Collection of the crystals completes the separation of isomers. The neutral (S)-phenylethylamine is recovered by deprotonation of the ammonium salt with aqueous sodium hydroxide followed by extraction into an organic solvent.
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H COOH OH H HO COOH CH 3 Ph H H 2 N (S)-1-Phenylethylamine COO- H HO COOH HO H Ph CH 3 NH 2 H Ph CH 3 NH 3 + H COO- H HO COOH HO H CH 3 Ph H + H 3 N (R)-1-Phenylethylamine RACEMIC MIXTURE + (2R,3R) - Tartaric acid Methanol (S)-1-phenylethylammonium (2R,3R) tartrate salt (R)-phenylethylammonium (2R,3R) tartrate salt
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7-ResolutionF07 - Resolution of a Racemic Mixture Reading...

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