Question

What pH would you make the water layer in order to cause the acid to dissolve in the water layer and the amine to dissolve in the ether layer? The pKa for CH₃COOH is 4.8 and the pKa for CH₃NH₃ is 10.7. Explain how you get your answer

Answer 1

To force acetic acid alone to dissolve in the water layer, leaving pure methylamine into the ether layer, a strong base, with pKb much greater than that of the base and the pKa of the acid can be used. This will cause all of the acid to form a salt with the base, making it to dissolve in water, as a salt leaving the base in its unprotonated form due to common ion effect tipping the equilibrium to the reverse direction owing to the presence of a much stronger base in the medium. Thus, the acetic acid and methylamine will cease to form a salt among themselves as the strong base would have formed a salt with the acid already and it would have also stripped methylammmonium cation of its extra proton and put it back into its neutral state, thus relieving it of its charge, thereby making it to dissolve in the ether layer alone. For this, any strong base like sodium hydroxide, potassium hydroxide or lithium hydroxide can be used. But due to the high pKa of methylamine, even slightly weaker bases like potassium carbonate can be used. Any pH that goes well above the pH of the acid and the base will effect the separation. Hence, any solution of pH above 9 will be suitable for the job. For strong bases, pOH = -log[OH] and pH = 14-pOH. In case of a weak base, taking its pKb into account, pOH is calculated as pOH = -log[OH] where [OH] = sq.root of K_bxC_b where C_b is the concentration of the base and then from this, pH can be found as mentioned before.