Key ideas
Organic lab work is about making a compound and then purifying and checking it. Chromatography, recrystallization, distillation, extraction and melting-point measurement are the standard tools, each with a simple calculation.
- In thin-layer chromatography, Rf = distance moved by the spot ÷ distance moved by the solvent front. It is always between 0 and 1 and depends on how polar the compound is.
- Recrystallization purifies a solid. Percent recovery = mass recovered ÷ mass of crude solid × 100, and it is never 100 %.
- Distillation separates liquids by boiling point, simple when they differ by more than about 25 °C and fractional when they are closer.
- A pure solid melts sharply at a fixed temperature, and impurities lower and broaden the melting range. Several small extractions remove more than one large one.
Key equations
Rf = distance of spot ÷ distance of solvent frontpercent recovery = recovered ÷ crude × 100fraction left = (1 ÷ (1 + K·V₂/V₁))ⁿpercent yield = actual ÷ theoretical × 100
Worked example
A spot travels 3.2 cm while the solvent front travels 8.0 cm, so Rf = 0.40. Recrystallizing 2.00 g of crude solid gives 1.60 g of pure crystals, a recovery of 80 %. If a compound has K = 3 between ether and water, one extraction of 100 mL of water with 100 mL of ether removes 75 %. Two extractions of 50 mL each leave (1 ÷ (1 + 3 × 0.5))² = 0.16, so 84 % is removed.
Common mistakes
- Measuring Rf from the origin line to the wrong point. Use the center of the spot and the position of the front.
- Using a lot of hot solvent in recrystallization. It keeps the product dissolved and lowers the recovery.
- Thinking a wide, low melting range shows a purer sample. It shows impurities.
Practice
Test yourself with practice questions on this topic: Laboratory techniques