A mixture of p-cresol (4-methylphenol), pK_a = 10.2, and 2,4-dinitrophenol, pKa = 4.11, is dissolved in ether. The ether solution is then vigorously shaken with one of the following aqueous solutions. Which solution effects the best separation of the two phenols by dissolving one in the water layer and leaving the other in the ether solution?

Explain.

(1) A 0.1 M aqueous HCl solution

(2) A solution that contains a large excess of pH = 4 buffer

(3) A solution that contains a large excess of pH = 7 buffer

(4) 0.1 M NaOH solution