Wednesday, December 14, 2011

Process Wednesday: Claisen's alkali

It's time again for another one of Kilomentor's questions. He asks of a potential process chemist:
24. What is Claisen’s alkali? For what separations can it be used?
Well, according to Purification of Laboratory Chemicals (a useful text if there ever was one (and it's free right now to Division of Organic Chemistry members!)), it's a water/methanol solution of KOH. As for its uses, it's probably Kilomentor himself who should answer that question:
Phenols may be separable from neutral substances by liquid/liquid extraction with aq. base, if the molecular weight is not too high. This is not a guaranteed success because phenols are only weak acids and the alkali phenolate, particularly as the molecular weight increases, may simply be water insoluble. Because the free phenol in this situation is lipophilic, the phenolate in the presence of both water and an organic phase may substantially hydrolyse back to sodium hydroxide and the free phenol. the neutral phenol “happily” jumps into the organic layer. For example, if a 10 ml. solution of 0.01 mol of 2,4-dimethylphenol is reacted with one equivalent of alkali in water and is then shaken with 20 ml of ethyl ether for about 10 minutes, the amount of the phenol found in the ether is 43% and the water is strongly basic. The amount extracted depends upon the ratio of alkali to phenol, the ratio of the phases, and the particular organic solvent used. In the case of 2-isopropyl-5-methyl-phenol (thymol) the amounts extracted by different solvents under the above conditions are: ether, 88; benzene, 38; carbon tetrachloride, 25; and pet. ether 22 percent. 
In the extreme case of di-ortho substituted phenols there is steric hindrance to the solvation shell that is needed around the oxygen anion, which makes the anion formation energetically disfavoured. With di-ortho phenols, even when the molecular weight is rather low- the phenol will not dissolve in aqueous sodium hydroxide. For that reason such species were called cryptophenols in the days before spectroscopic testing, because these phenols did not give the characteristic qualitative test for a phenol. Cryptophenols can be dissolved in methanolic-KOH called Claisen’s alkali. 
So I suspect that Claisen's alkali can be used in the extraction of phenols and other somewhat acidic functional groups (carboxylic acids) from fairly non-polar solvents? I confess, I'm not completely sure on this one.  

1 comment:

  1. Interesting as have used this technique on several occasions but did not learn/apply "Claisen alkali" terminology as actually believed it was based on principles of the Williamson Ether Synthesis which employ same/similar reagents to generate a phenolate that then reacts with an aklyl halide. I endorse recommendation on book Purification of Lab Chemicals as valuable text as even though no longer in lab its a great source for basic physical properties/general how to approach info for reports and sourcing questions (although I have older 3rd addition that does not list Claisen alkali).