Question

Organic Chemistry Question

This question is about an extraction experiment with the compounds biphenyl and 2-chlorobenzoic acid.

A student, when conducting this experiment overlooked the instruction "Repeat steps 2 and 3 above two more times with fresh 2 mL portions of sodium bicarbonate solution." Upon conducting TLC analysis, this student found that the final isolated biphenyl was contaminated with some 2-chlorobenzoic acid. Explain, on a molecular level, how and why this student's error led to poor separation. Be specific.

Answer 1

Biphenyl is an aromatic compound which is insoluble in water, dilute HCl or dilute NaOH or dilute sodium bicarbonate solution.

2-Chlorobenzoic acid is acidic thus is souble in aqueous sodium bicarbonate solution.

From the mixture of the two the separation is based on dissolving the acidic component in aqueous medium and extracting the biphenyl into organic solvent medium.

However , repititive washing with sodium bicarbonate is required to completely remove all the acid, failing which the biphenyl has acid contamination.