Extraction order for an amine, an acid, and a phenol
An organic chemistry student has 100 mL of diethyl ether containing 0.10 mol aniline (conjugate acid pKa 4.6), 0.10 mol benzoic acid (pKa 4.2), and 0.10 mol p-cresol (4-methylphenol, pKa 10.3). The student wants to isolate all three solids. Available aqueous solutions are 1 M HCl, 1 M NaOH, saturated NaHCO3 (pH ≈ 8.3), and saturated NaCl. The student plans a sequence of extractions in a separatory funnel, keeping the ether layer after each extraction and saving each aqueous layer for back-extraction. A teaching assistant warns that using 1 M NaOH as the first extractant will still remove an acidic component but will make later separation of the acidic compounds difficult. The student must choose an order that avoids cross-contamination and explain which species enters which layer at each step, using pKa and charge arguments. The student also wants to minimize the number of back-extractions.
1. Propose a three-step extraction sequence using the available solutions that separates aniline, benzoic acid, and p-cresol. For each step, state the layer and the charged species.
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Step 1: saturated NaHCO3 (pH ≈ 8.3). Benzoic acid (pKa 4.2) is deprotonated to benzoate and enters the aqueous layer; at pH 8.3 the benzoate/benzoic acid ratio is about 10^(8.3−4.2), so deprotonation is essentially complete. p-Cresol (pKa 10.3) remains mostly neutral (phenolate/phenol ratio about 10^(8.3−10.3) = 10^-2), and aniline (conjugate acid pKa 4.6) remains neutral. Keep the ether layer.
Step 2: 1 M NaOH. p-Cresol is deprotonated to phenolate (pH 14 is far above pKa 10.3) and enters the aqueous layer. Aniline remains neutral in ether.
Step 3: 1 M HCl. Aniline is protonated to anilinium (pH 0 is far below pKa 4.6) and enters the aqueous layer. The ether layer now contains no target compound; it may contain neutral impurities.
2. Explain why 1 M NaOH as the first extractant is a poor choice for separating benzoic acid and p-cresol. Use pKa values.
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At pH near 14, both benzoic acid (pKa 4.2) and p-cresol (pKa 10.3) are deprotonated. Benzoate and phenolate enter the same aqueous NaOH layer, so the first extraction is not selective for either acidic compound. Later separation would require careful pH adjustment: acidifying the combined aqueous layer to about pH 7 would protonate p-cresol (pH below its pKa 10.3) while leaving benzoate mostly deprotonated (pH above its pKa 4.2), then further acidification to pH below 4.2 would recover benzoic acid. That is more steps and more risk of contamination.
NaHCO3 at pH 8.3 is selective because it is above benzoic acid's pKa by about 4 units but below p-cresol's pKa by about 2 units. Benzoic acid is removed while p-cresol stays in ether.
3. Describe how to recover each solid from its aqueous layer after the sequence in Task 1. Include the approximate pH target.
Read the worked answer
Benzoate layer: acidify with 6 M HCl to pH below 4.2, ideally about pH 2, to protonate benzoic acid; cool and filter. p-Cresolate layer: acidify to pH below 10.3, ideally about pH 7–8, to protonate p-cresol; cool and filter or extract into ether. Anilinium layer: basify with 6 M NaOH to pH above 4.6, ideally about pH 9–10, to free aniline; extract with ether, dry, and evaporate. Use a brine wash and drying agent, and avoid overshooting pH because strong acid or base can cause hydrolysis or emulsions.