From: Norwid-Rasmus Behrnd (behrndnr$##$yahoo.de)
Date: Fri Nov 28 2008 - 08:43:42 EST
Dear Ohad Ilovich,
it need not be the only reason why people do so, but part of my experience is the following:
In the work-up of HWE reactions, I subsequently washed my dissolved crude product with aqueous NaOH, followed by brine. The reasoning for the former is that the phosphates, stoechiometric byproduct of the HWE reaction, are good soluble in water; especially in lye. I speculate washing with an aqueous solution being already saturated with NaCl would be less effective for this purpose. However, the second wash with brine allowed to dry the organic phase (not altering the pH-value) well before drying with MgSO4, for example, so that less of the drying agent was needed. That's a trick mentioned in the handbook by Loewenthal/Zass ``Guide for the Perplexed Organic Experimentalist'' I became used to.
Addition of salts however might alter the solubility of organic substances (for example of bigger proteins, or colloids with surface charges) towards an increase or decrease; and cheap NaCl as a 1:1 electrolyte is perhaps in most cases innocent in this aspect.
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