How They Work
To modify the extraction procedures in this chapter, use a posturasdeyogafaciles.comical reaction in the separatory funnel to alter the polarity and, therefore, the partitioning of the compound in aqueous and organic layers.It is a common practice to perform an acid-base reaction to convert some compounds from neutral to ionic form (or vice versa).
Let us consider the case of a mixture of benzoic acid and cyclohexane dissolved in an organic solvent such as ethyl acetate in a separatory funnel.Water washing may be attempted to separate the components, but benzoic acid is not very water-soluble due to the nonpolar aromatic ring and only small amounts will be extracted into the aqueous layer (Figure 4.54a).
A wash with a base such as (ce[NaOH]) will allow separation of a mixture of benzoic acid and cyclohexane.Its acidic nature means that benzoic acid can undergo a reaction with ce[NaOH] that can lead to the carboxylate salt sodium benzoate.
Ce[PhCO_2H] left( aq) in egin[array] [cccc ccc
The solubility properties of carboxylic acids differ considerably from those of their carboxylate salts.Due to its ionic nature, sodium salicylate is 350 times more soluble in water than salicylic acid (Figure 4.55), and it is insoluble in organic solvents like diethyl ether.
Acid-base extraction in a separatory funnel can be used to remove residual carboxylic acid from ester products. .A sodium bicarbonate solution also neutralizes the catalytic acid in this reaction.
As sodium bicarbonate is much weaker than (ceNaOH), it is preferred for use in this process; (ceNaOH) washing would be likely to cause the ester to hydrolyze.
Mixtures of Acids and Bases
.Other strategies can be applied here as well.
Extracting Carboxylic Acids vs. Phenols
The carboxylic acids obtained from organic layers can be converted to their more water-soluble salts by shaking them with basic solutions, as described previously.
egin[array] [cccccccccccc] ce[PhCO_2H] left(Aq
Phenols undergo a similar reaction (left) ce[PhOH]
The problem is that phenols are significantly less acidic than carboxylic acids, and are not acidic enough to react completely with (ce[NaHCO_3]), a weaker base.So, bicarbonate solutions can be used to separate phenols and carboxylic acids (Figure 4.58b).