Question

3-Fluorophenol has a pKa of 8.40. 4-Nitrophenol has a pKa of 7.16. 3-Chlorobenzoic acid has a pKa of 3.82. You will need the following information: The pKa of HCl is –7; pKa of carbonic acid is 6.52 and the pKa of water is 15.

a) You have a mixture of 3-fluorophenol and 4-nitrophenol that you wish to separate by extraction. The mixture is dissolved in methylene chloride. Why would reacting the mixture with an aqueous solution of NaHCO3 NOT separate one substance from the other?

b) You have a mixture of 4-nitrophenol and 3-chlorobenzoic acid that you wish to separate by extraction. The mixture is dissolved in methylene chloride. Why would reacting the mixture with an aqueous solution of NaOH NOT separate one substance from the other?

c) Why would HCl not be a choice to use in order to extract any of these three components into the aqueous layer?

Answer 1

(a). Reacting the mixture with an aqueous solution of NaHCO3 does not separate one substance from the other because carbonic acid (pKa = 6.52) is the conjugate acid of NaHCO3 which is stronger than both 3-flurophenol and 3-nitrophenol. Hence, NaHCO3 will not deprotonate either either of the two weaker base 3-flurophenol and 3-nitrophenol.

(b). Reacting the mixture with an aqueous solution of NaOH does not separate one substance from the other because NaOH is a strong base and will deprotonate both the compounds 4-nitrophenol and 3-chlorobenzoic acid which are stronger than NaOH.

(c). It is because HCl cannot deprotonate any, stronger acid than all of them.