Acid-Base Extraction Order: Why NaHCO3 Must Go First
A 2.0 g mixture contains benzoic acid (pKa about 4.2), 4-tert-butylphenol (pKa about 10.2), and biphenyl (no acidic proton). A student dissolves the mixture in 40 mL diethyl ether and plans liquid-liquid extractions with 20 mL portions of aqueous reagent, followed by drying over anhydrous sodium sulfate and rotary evaporation. Available reagents: 1.0 M NaHCO3 (aqueous pH about 8.3), 1.0 M NaOH (pH about 14), 1.0 M HCl, and saturated NaCl. The draft scheme is: (1) wash the ether with 1.0 M NaOH, acidify the aqueous layer, collect the solid; (2) then wash the ether with 1.0 M NaHCO3, acidify, collect the solid. A supervisor flags the order as wrong.
1. Using pKa and charge state, explain why the draft order fails to separate the two acidic components.
Read the worked answer
What 1.0 M NaOH does (pH ~14)
- Benzoic acid: A-/HA = 10^(14 - 4.2) = 10^9.8, so essentially complete deprotonation to benzoate.
- 4-tert-Butylphenol: A-/HA = 10^(14 - 10.2) = 10^3.8, so roughly 99.98% deprotonated to phenolate.
- Both anions are charged and partition into the aqueous layer, while biphenyl stays in ether.
Why that is a problem Acidifying that combined aqueous layer reprotonates both compounds, giving a mixed solid of benzoic acid plus the phenol. The draft scheme has therefore merged two components into one fraction, and the subsequent NaHCO3 wash has nothing acidic left to remove except residual traces.
Underlying rule An extraction reagent separates two acids only if it deprotonates one substantially and the other negligibly (roughly, if the aqueous pH sits between the two pKa values). pH 14 is far above both pKa values, so it is non-selective here.
2. Design the correct sequence and state exactly which compound ends up in which fraction.
Read the worked answer
Step 1 - 1.0 M NaHCO3 (pH ~8.3) on the ether layer
- Benzoic acid: ratio = 10^(8.3 - 4.2) = 10^4.1, so >99.99% as benzoate -> extracted into water.
- Phenol: ratio = 10^(8.3 - 10.2) = 10^-1.9, so only about 1.3% ionized -> stays in ether.
- Acidify the aqueous layer with 1.0 M HCl (vent CO2; keep pH below ~3) -> benzoic acid precipitates; collect by filtration.
Step 2 - 1.0 M NaOH on the remaining ether layer
- The phenol is now deprotonated quantitatively and moves into water; biphenyl remains in ether.
- Acidify the aqueous layer with HCl -> 4-tert-butylphenol separates (oil or solid depending on temperature); extract into fresh ether or isolate.
Step 3 - workup
- Dry the ether layer over Na2SO4, filter, evaporate -> biphenyl.
- A saturated NaCl wash helps break emulsions and removes residual water.
Judgment Order matters: go from the weakest base to the strongest so each reagent removes only the most acidic remaining component. The phenolate should not be exposed to strong base until the carboxylic acid is gone.
3. Suppose the student runs the flawed NaOH-first scheme anyway. What would the isolated 'first fraction' be, and what recovery strategy could salvage pure compounds from it?
Read the worked answer
What you get A mixed solid of benzoic acid plus 4-tert-butylphenol - not a pure compound. Melting range would be broad and depressed, and TLC in ether/hexane would show two spots.
Salvage route Re-dissolve the mixed solid in ether and perform the selective extraction that was skipped:
- Wash with 1.0 M NaHCO3 -> benzoate into water; acidify -> benzoic acid.
- Wash the ether with 1.0 M NaOH -> phenolate into water; acidify -> phenol. This re-partitions the mixture according to the same pKa logic and recovers both components, though with yield loss from extra transfers and possible emulsion formation.
Alternatives and caveats
- Recrystallization or column chromatography can also separate them, but they depend on solvent choice and are less predictable than the acid-base route.
- Phenols are prone to oxidation and darkening under strong base and air; prolonged exposure to 1.0 M NaOH lowers yield. Benzoic acid may also carry phenol as an inclusion in the crude solid, so purity must be checked (melting range, NMR) rather than assumed from appearance.