MCAT® ExamChemical and Physical Foundations of Biological SystemsMedium
A student wants to separate a mixture of two organic compounds: compound X (a carboxylic acid) and compound Y (an aldehyde). Both compounds are soluble in diethyl ether. Which of the following extraction techniques would be most effective for separating these two compounds?
- AColumn chromatography
- BDistillation
- CAcid-base extraction
- DRecrystallization
Show answer & explanationAnswer & explanation
Correct answer: C. Acid-base extraction
Carboxylic acids are acidic and can be deprotonated by a base (like NaHCO₃ or NaOH) to form a water-soluble carboxylate salt. Aldehydes are generally neutral and not deprotonated by weak bases. This difference in acid-base properties allows for separation using acid-base extraction, where the carboxylic acid moves into the aqueous layer as a salt, while the aldehyde remains in the organic layer.
Why the other options are wrong
- A. Column chromatography separates compounds based on differential adsorption to a stationary phase and elution by a mobile phase. While it could work, acid-base extraction is a much more direct and efficient method for compounds with distinct acid-base properties.
- B. Distillation separates compounds based on boiling point differences. While compounds X and Y might have different boiling points, this method doesn't exploit their distinct chemical properties for a more selective separation.
- D. Recrystallization purifies solid compounds based on differences in solubility in a hot solvent, not typically used for separating two compounds from a liquid mixture or exploiting acid-base properties.
Acid-Base Extraction
A technique used to separate organic compounds based on their differential solubility in aqueous solutions at varying pH, by converting them into ionic salts.
- Acids (e.g., carboxylic acids, phenols) can be deprotonated by bases to form water-soluble salts.
- Bases (e.g., amines) can be protonated by acids to form water-soluble salts.
- Neutral compounds remain in the organic solvent.
- The pH is adjusted to control the protonation state, allowing compounds to move between immiscible organic and aqueous layers.
Memory trick: Acids go aqueous with base, neutrals stay in their place!