Acid-Base Extraction of an Amine, Acid, and Naphthalene
A 2.00-g mixture is known to contain equal masses of benzoic acid (pKa 4.2), 4-methylaniline (pKa of its conjugate acid ≈5.1), and naphthalene. A student dissolves the mixture in 60 mL diethyl ether and extracts once with 30 mL of 1.0 M HCl. She separates the aqueous layer, then extracts the ether once with 30 mL of 1.0 M NaOH. She basifies the HCl layer to pH 12 and extracts with ether, recovering 0.70 g of a white solid. She acidifies the NaOH layer to pH 2 and recovers 0.55 g of a white solid. The ether layer is dried with anhydrous Na2SO4 and evaporated to give 0.60 g of naphthalene. The instructor notes that the amine mass exceeds its nominal share and asks the student to account for each layer and evaluate whether one 30-mL extraction was sufficient.
1. Assign benzoic acid, 4-methylaniline, and naphthalene to the HCl aqueous layer, NaOH aqueous layer, or final ether layer. Support each assignment with an ionization ratio at the stated pH.
Read the worked answer
At pH 1, 4-methylaniline (conjugate acid pKa ≈5.1) is protonated: [RNH3+]/[RNH2] = 10^(5.1−1) = 10^4.1 ≈ 1.3×10^4, so >99.99% is the water-soluble ammonium salt. Benzoic acid at pH 1 has [A−]/[HA] = 10^(1−4.2) = 10^−3.2 ≈ 6×10^−4, so it remains mostly neutral HA in ether. Naphthalene is neutral throughout and stays in ether. The HCl layer therefore contains the 4-methylanilinium salt; basifying to pH 12 gives [RNH3+]/[RNH2] = 10^(5.1−12) ≈ 1×10^−7, so the free amine is recovered. The NaOH layer contains benzoate: at pH 12, [A−]/[HA] = 10^(12−4.2) = 10^7.8, so benzoic acid is deprotonated; acidifying to pH 2 gives HA, which precipitates. Naphthalene remains in the final ether.
2. The nominal mass of each component is 0.667 g. Calculate percent recovery for each and for the total, and identify the most likely source of the amine result above 100%.
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Amine: 0.70/0.667 × 100 = 105%. Benzoic acid: 0.55/0.667 × 100 = 82.5%. Naphthalene: 0.60/0.667 × 100 = 90.0%. Total: 1.85/2.00 × 100 = 92.5%. The amine value above 100% cannot be a true recovery; it suggests residual water, NaCl, or solvent in the amine solid, or incomplete drying. The acid recovery is the lowest, consistent with incomplete extraction, loss during pH adjustment, or solubility of benzoic acid in the aqueous phase. The naphthalene loss may be from evaporation or transfer. The total below 100% means material was lost, not created; the apparent amine excess means the other recoveries are even lower on a mass-balance basis.
3. Would replacing the single 30-mL HCl extraction with two 15-mL extractions improve recovery of the amine? Explain using partition-coefficient reasoning and state a practical caveat.
Read the worked answer
For extraction of a solute with partition coefficient K between ether and water, the fraction left in ether after one extraction with volume V is 1/(1 + K V/V_ether). For two extractions each with V/2, the residual fraction is [1/(1 + K(V/2)/V_ether)]^2. For any positive K, this is smaller than 1/(1 + K V/V_ether), so two smaller portions recover more than one larger portion of the same total volume. The effect is largest when K is modest; if K is very large, both methods are nearly complete. Practical caveat: the amine salt has a high affinity for water at pH 1, so one extraction may already be efficient; the low recovery is more likely from emulsion, incomplete phase separation, or transfer losses than from equilibrium. Multiple extractions can also increase handling losses and emulsion formation.