Adjusting the pH of a solution often provides an easy way to separate compounds. Separating a mixture of benzoic acid (PhCO₂H) and toluene (PhMe) is easy: dissolve the mixture in CH₂Cl₂, add aqueous NaOH, shake the mixture of solutions, and separate the layers. The CH₂Cl₂ layer contains all the toluene. The aqueous layer contains the sodium salt of benzoic acid. Addition of HCl to the aqueous layer precipitates the insoluble benzoic acid.

A more realistic separation is given in a modern practical book after a Cannizzaro reaction. You will meet this reaction in Chapters 26 and 39 but all you need to know now is that there are two products, formed in roughly equal quantities. Separation of these from starting mate rial and solvent, as well as from each other, makes this a useful reaction.

The products under the basic reaction conditions are the salt of the acid (soluble in water) and the alcohol (not soluble in water). Extraction with dichloromethane removes the alcohol and leaves the salt in the aqueous layer along with solvent methanol and residual KOH. Rotary evaporation of the CH₂Cl₂ layer gives crystalline alcohol and acidification of the aqueous layer precipitates the neutral acid.

In the same way, any basic compounds dissolved in an organic layer can be extracted by washing the layer with dilute aqueous acid and recovered by raising the pH, which will precipitate out the less soluble neutral compound. A general way to make amines is by ‘reductive amination.’ Ignore the details of this reaction for now (we come back to them in Chapter 11) but consider how the amine might be separated from starting material, by products, and solvent.

As the reaction mixture is weakly acidic, the amine will be protonated and will be soluble in water. The starting material and intermediate (of which very little is present anyway) are soluble in organic solvents. Extracting the aqueous layer and neutralizing with NaOH gives the amine. Whenever you do any extractions or washes in practical experiments, just stop and ask yourself: ‘What is happening here? In which layer is my compound and why?’ You will then be less likely to throw away the wrong layer (and your precious compound)!

مواضيع ذات صلة

The definition of pKa

The ionization of water

Water can behave as an acid or as a base

Every acid has a conjugate base

The pH scale and pKa

An isolated proton is extremely reactive—formation of H3O+ in water

Acids, bases, and pKa

Organic compounds are more soluble in water as ions

Heterocyclic aromatic compounds

Hückel’s rule tells us if compounds are aromatic

Molecular orbitals and aromaticity

The π molecular orbitals of other conjugated cyclic hydrocarbons