Back in July 2012 we finally got a polarimeter at RMIT. How had we been doing chiral syntheses up until that point? Good question! We are now carrying out several chiral synthesis projects.
Polarimeter
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I hadn't done any chiral syntheses up to that point (largely as we had no polarimeter). I'd just returned from America where I had purchased (S)-Naproxen sodium in a shop called 'DollarTree'. This actually cost US$1.07 due to state tax. I found a method to test optical rotation of (S)-Naproxen by extraction from over the counter pills. This was largely meant for undergraduates, but I carried it out in order to learn some polarimetry. The procedure was as follows:
3 tablets of naproxen sodium (220 mg per tablet) were placed in MeOH (15 mL) and swirled. When the outer blue coating began to loosen and peel away, the white centres of each tablet were quickly removed from the MeOH and placed into a separate flask. They were then dissolved in fresh MeOH (20 mL), which took about 20 min at RT. The solution was passed through filter paper to remove tablet components such as TiO2 and micro crystalline cellulose. The MeOH was removed by rotavap to get clean white crystals of naproxen sodium. To make the freebase, the naproxen sodium was then rinsed into a sep funnel with hydrochloric acid and ethyl acetate (or other water immiscible solvent). The naproxen sodium anion is now protonated and no longer ionic so resides in the organic phase. The HCl becomes NaCl and lives in the aqueous phase. The organic phase was then dried with MgSO4, passed through filter paper and rotavaped to get clean white crystals of naproxen that were ready for optical rotation tests.
British Pharmacopedia 1988 gives specific optical rotation of (S)-Naproxen as 63.0° to 68.5°, when measured at 4% w/v in UV grade chloroform. As we had no UV grade chloroform, I measured the extracted freebase in UV grade DCM to get a specific optical rotation of 63.9°. I therefore declared my naproxen from 'DollarTree' to be the pure S enantiomer. This was quite preferable to the R enantiomer, which is said to be a liver toxin.
British Pharmacopedia 1988 gives specific optical rotation of (S)-Naproxen as 63.0° to 68.5°, when measured at 4% w/v in UV grade chloroform. As we had no UV grade chloroform, I measured the extracted freebase in UV grade DCM to get a specific optical rotation of 63.9°. I therefore declared my naproxen from 'DollarTree' to be the pure S enantiomer. This was quite preferable to the R enantiomer, which is said to be a liver toxin.
