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Separations and Purifications

Chapter 12: Separations and Purifications

Updated Apr 17, 2026
Read the entire chapter on one page Every section in order, with the sidebar tracking where you are as you scroll.
🎯 Diagnostic: Test Your Starting Level 24 questions (2 per section). No prior reading required, see what you already know.

Aim to answer every question before checking. Missed questions point you to the sections you need most.

1. (12.1) Separation techniques exploit:
B. The choice of technique follows from which property differs most between the components.
2. (12.1) Purification refers to:
A. Purification is central to any organic workflow; the final product is only as good as the last cleanup step.
3. (12.2) Liquid-liquid extraction separates by:
D. Multiple smaller extractions are more efficient than one large one (multi-fold extraction theorem).
4. (12.2) The partition coefficient K is:
C. log KowK_{\text{ow}} (octanol-water partition coefficient) is a common measure of drug lipophilicity.
5. (12.3) Acid-base extraction:
B. This is how MCAT-style workup problems separate a mixture of an acid, a base, and a neutral molecule.
6. (12.3) A carboxylic acid in an organic layer can be pulled into aqueous by:
A. Subsequent acidification of the aqueous layer reprotonates and releases the original acid, cleanly isolated.
7. (12.4) Simple distillation works well when:
D. Smaller BP differences require fractional distillation with theoretical plates.
8. (12.4) To separate two liquids whose BP difference is small (<25 °C):
C. Each theoretical plate effectively repeats a simple distillation, magnifying a small BP gap.
9. (12.5) Vacuum distillation is used for:
B. Remember: boiling occurs when vapor pressure equals atmospheric (or ambient) pressure. Lower ambient pressure = lower BP.
10. (12.5) Boiling point under reduced pressure:
A. Critical for thermally sensitive compounds that decompose near atmospheric BP.
11. (12.6) Recrystallization purifies a solid by:
D. Sharp, well-defined melting point of the crystalline solid is a classic confirmation of purity.
12. (12.6) A good recrystallization solvent:
C. Choosing the right solvent (or solvent pair) is often the most important decision in recrystallization.
13. (12.7) Thin-layer chromatography (TLC) separates compounds by:
B. A quick TLC spot test is used routinely to monitor reactions in the lab.
14. (12.7) The retention factor Rf in TLC is:
A. A compound's Rf in a given solvent system is a fingerprint under standardized conditions.
15. (12.8) Column chromatography:
D. The most common laboratory-scale purification method in organic chemistry.
16. (12.8) In reverse-phase HPLC:
C. Reverse-phase is the most common HPLC mode for biological and pharmaceutical samples.
17. (12.9) Gas chromatography separates:
B. GC is excellent for volatile organic compounds but requires thermal stability.
18. (12.9) Common GC detectors include:
A. GC-MS combines separation with mass identification, often used in forensics and environmental analysis.
19. (12.10) HPLC is:
D. Essential in pharmaceutical analysis and quality control.
20. (12.10) A common HPLC application is:
C. Drug purity, pharmacokinetics, and biomarker discovery all rely heavily on HPLC.
21. (12.11) Gel electrophoresis works by:
B. Agarose gels are used for DNA/RNA; polyacrylamide for proteins.
22. (12.11) In SDS-PAGE for proteins:
A. Native gels (no SDS) separate proteins by charge and shape as well as size.
23. (12.12) The right separation technique depends on:
D. Think about what differs most between target and impurities, then pick a technique that amplifies that difference.
24. (12.12) To separate two liquids whose BPs are very close:
C. Lots of theoretical plates let a column resolve BP differences of a few degrees.

This final chapter covers how organic chemists actually isolate and purify compounds. Every reaction you have studied produces a mixture; separation techniques pull the desired product out of that mixture. Each technique exploits a different physical or chemical property:

  • Extraction: polarity / solubility differences.
  • Distillation: boiling point differences.
  • Recrystallization: differential solubility at different temperatures.
  • Chromatography: differential affinity for stationary vs. mobile phases.
  • Gel electrophoresis: size and/or charge.

On the MCAT, separation chapters are usually conceptual - they test whether you know which technique to apply to a given mixture, not how to set up the apparatus.

The Central Analogy

In This Chapter

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