Question

what is the theory behind acid-base separations (i.e. how does it work and how can it be used to purify an impure organic compound)?

Answer 1

Acid-base extraction is a liquid liquid extraction used to seperate organic compounds from a mixture of compounds based on the difference in the acid-base properties (ability of organic compound to add or lose proton easily with respect to other constituents in the mixture).

HOW DOES IT WORK?

This method makes use of the "like dissolves like rule" i.e. organic compounds (neutral) are soluble in organic solvents and ionic compounds (charged organic compounds) are soluble in water. Suppose we have a mixture of two neutral organic compounds A and B. A neither accepts nor loses protons easily i.e. is a weak acid as well as base while B is a strong acid and loses proton easily. We will add mixture to a test tube containing a mixture of organic solvent (ether) and water. Both A and B being neutral molecules will be present in the organic solvent and to seperate A and B we first add any organic base like aniline to the organic solvent. Aniline will take up proton from B and thus B will become a positively charged species. Being a charged specie B will be soluble in water along with anilinium ion and A remains soluble in the organic layer. Layers can be seperated and A can be recovered easily from organic solvent by evaporating the solvent. For B we again make a mixture of ether and water and add any mineral acid (like HCl) to water. B will get protonated. Neutral B will solubalise in ether and can be recovered by seperating and evaporating the ether layer.

TO PURIFY AN IMPURE ORGANIC COMPOUND

As explained above, any organic compound having impurities with different acidic or basic properties with respect to the compound can be purified using this method. If either the organic compound or the impurities are more acidic/ basic with respect to each other, the above mentioned procedure can be applied and the impurity can be removed from the organic compound. Thus, we get a purified organic compound.