Question

You wish to isolate the acetal from acetophenone and methanol explain why one of the following organic acids is the best choice to catalyze the reaction: a) p-toluenesulfonic b) trifluoroacetic acid c) hydrochloric acid?

Answer 1

Solution:

Acetal obtained from acetophenone and methanol is acetophenone dimethyl acetal.

The formation of acetals is acid catalyzed and reversible. The reaction is driven to the right either by the use of excess alcohol as the solvent or by removal of the water by-product, or both or by increasing the concentration of acetophenone.

Dimethyl acetals can be prepared from carbonyl compounds with excess methanol catalyzed by a Brønsted (i.e. protic) acid. Additon of H+ to carbonyl makes it more reactive with the weak nucleophile CH₃OH. p-toluenesulfonic, trifluoroacetic acid , hydrochloric acid acts as H+ donor and hence are used as acid catalysts.

Note: p-Toluenesulfonic acid ( pTsOH) or tosylic acid (TsOH) is an organic acid which is a white solid and soluble in water, alcohol and other polar organic solvents and hence is widely used in organic sysnthesis.

Trifluoroacetic acid is an organofluorine compound with the chemical formula CF₃COOH. It is a structural analogue of acetic acid with the three hydrogen atoms replaced by three fluorine atoms and is a colorless liquid. It is a stronger acid than acetic acid and is widely used in organic synthesis.