This page looks plain and unstyled because you're using a non-standard compliant browser. To see it in its best form, please upgrade to a browser that supports web standards. It's free and painless.
kilomentor | 06 October, 2007 11:15
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. Kilomentor has an article about Claisen’s Alkali.
Phase Switching Hydrolysis
In some situations another trick can be employed to separate a weak phenol or cryptophenol from a non-phenol. Suppose for example you are trying to separate two carboxylic acid esters that differ only because one has a free phenol and the other a phenol ether. If one puts the mixture into a two phase mixture of say toluene and water, adds sodium hydroxide to the water and stirs the phases gently, then after some time the phenolic ester will be found transferred to the aqueous base phase, where the ester has hydrolysed to the carboxylate, while the ether-ester is untouched in the toluene phase.
I have used this trick several times. It works because the free phenol increases the solubility of its ester substrate slightly in the water and once in the water, its ester is quickly hydrolysed. As the sodium carboxylate it is stuck quantitatively in the water. The ether -ster on the other hand is essentially insoluble in the water. It cannot “see” the alkali because the stirring is gentle and there is little interface so it remains unreacted in the toluene. Conditions for the separation can be optimized by adjusting the organic solvent, the stirring and the temperature of the two phase mixture.
Although I have not tried the method with any combinations other than phenol-esters and ether-esters, other functional groups might be useful to replace the phenol by creating this initial small water solubility. Perhaps thiol, primary and secondary sulfonamide, imide, terminal acetylene, alpha unsubstituted alkyl nitro or dithiane might work. Any compound that can act as a weak acid in aqueous alkali has a good chance to succeed.
| « | August 2008 | » | ||||
|---|---|---|---|---|---|---|
| Su | Mo | Tu | We | Th | Fr | Sa |
| 1 | 2 | |||||
| 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| 10 | 11 | 12 | 13 | 14 | 15 | 16 |
| 17 | 18 | 19 | 20 | 21 | 22 | 23 |
| 24 | 25 | 26 | 27 | 28 | 29 | 30 |
| 31 | ||||||