A student is working with a solution containing a mixture of two organic compounds: benzoic acid (pKa ≈ 4.2) and aniline (pKb ≈ 9.4). They want to separate these two compounds using an acid-base extraction procedure. Which of the following steps would be most effective in separating benzoic acid from aniline when starting with an organic solvent layer?
- AAdd an aqueous solution of NaOH to convert both compounds into their conjugate bases.
- BAdd an aqueous solution of NaHCO₃ (sodium bicarbonate) to selectively deprotonate benzoic acid.
- CAdd an aqueous solution of HCl to convert both compounds into their conjugate acids.
- DAdd an aqueous solution of a strong base like KOH to selectively deprotonate aniline.
Show answer & explanationAnswer & explanation
Correct answer: B. Add an aqueous solution of NaHCO₃ (sodium bicarbonate) to selectively deprotonate benzoic acid.
Acid-base extraction relies on selectively converting one component of a mixture into its charged (ionic) form, which is soluble in the aqueous layer, while the other component remains uncharged and soluble in the organic layer. Benzoic acid is a carboxylic acid (weak acid, pKa 4.2). Aniline is a weak base (pKb 9.4, so its conjugate acid has pKa 14-9.4 = 4.6). To separate them, we need to selectively ionize one. Sodium bicarbonate (NaHCO₃) is a weak base (conjugate acid H₂CO₃ has pKa1 6.4, so NaHCO₃ acts as a base with pKb ≈ 7.6). Its conjugate acid (H₂CO₃) has a pKa (6.4) that is greater than benzoic acid's pKa (4.2) but less than aniline's conjugate acid pKa (4.6). Therefore, NaHCO₃ is strong enough to deprotonate benzoic acid (making it water-soluble as benzoate) but not strong enough to protonate aniline significantly to make it water-soluble. This allows for selective separation. Options A and B would likely convert both or neither effectively for selective separation. Option D would not work as aniline is a base and would need an acid to be protonated.
Why the other options are wrong
- A. NaOH is a strong base and would deprotonate both benzoic acid and potentially deprotonate aniline's conjugate acid if aniline was already protonated, making the separation non-selective.
- C. HCl is a strong acid and would protonate both aniline (making it water-soluble) and keep benzoic acid uncharged, but this doesn't help separate them if both become aqueous or if the goal is to target benzoic acid specifically with a weaker base.
- D. KOH is a strong base. It would deprotonate benzoic acid, but it would not selectively deprotonate aniline (aniline is already a base, not an acid to be deprotonated by KOH).
Acid-Base Extraction Principles
Acid-base extraction is a technique used to separate organic compounds based on their differential solubility in aqueous solutions at different pH values. By converting a compound into its charged (ionic) form, it becomes water-soluble and can be extracted from an organic layer.
- Acids are extracted with bases (e.g., NaHCO₃, NaOH).
- Bases are extracted with acids (e.g., HCl).
- The choice of acid/base depends on the pKa/pKb values.
- Weak acids/bases allow for selective extraction.
Memory trick: Ionize One, Leave One Organic.