Purification of Organic Compounds
A reaction flask rarely hands you a pure product. Alongside the compound you actually want sit unreacted starting material, side products, solvent traces and coloured tars — and before you can measure a melting point, record an infrared spectrum, or trust any analysis, that mixture has to be pulled apart. This is the job of purification techniques, and Class 11 organic chemistry asks you to know five of them at a principle level: crystallisation, sublimation, distillation, differential extraction, and chromatography. The trick that ties all five together is simple: every method exploits one specific physical property in which the desired compound and its impurities differ — solubility, volatility, boiling point, or how strongly each substance clings to a surface. Once you spot which property differs and by how much, the correct technique nearly picks itself. This article works through each method, the one quantitative tool of the chapter — the value in chromatography — and how to choose between distillation's variants.
Each purification method separates a compound from its impurities by exploiting one specific physical-property difference between them — solubility for crystallisation, volatility for sublimation, boiling point for distillation, relative solubility for extraction, and adsorption/partition rate for chromatography.
The core idea
1. Crystallisation — differences in solubility. The impure solid is dissolved in the minimum amount of hot solvent to make a nearly saturated solution; impurities present in small amounts are removed by hot filtration (or stay behind in the mother liquor). On cooling, the desired compound's solubility drops sharply and it crystallises out first, while the impurity — never reaching its own, higher, solubility threshold — remains dissolved. Works only for solid organic compounds, and only when the compound and impurity have sufficiently different solubility curves in the chosen solvent.
2. Sublimation — difference in volatility. Some solids pass directly from solid to vapour without ever melting into a liquid. Gentle heating vaporises the sublimable compound, which is then collected as a pure solid on a cold surface, while non-volatile impurities are left behind in the heated vessel. This works only when the impurity itself does not sublime.
3. Distillation — difference in boiling point. Heating a liquid mixture vaporises the more volatile component first; the vapour is condensed and collected separately, leaving less volatile material behind. Simple distillation suits a large boiling-point gap. Three variants handle harder cases: fractional distillation uses a fractionating column to repeat the vaporisation–condensation cycle many times, so it can resolve liquids whose boiling points are close together; steam distillation co-distils a water-immiscible compound with steam, letting it come over at a temperature below its own normal boiling point; and vacuum distillation lowers the pressure (and hence the boiling point) to distil substances that would decompose if heated to their normal boiling point.
4. Differential extraction — difference in relative solubility. A compound dissolved in one solvent (often water) is shaken with a second, immiscible solvent in which it is more soluble. The compound partitions preferentially into the second solvent, which is then separated off — the basis of a separating-funnel extraction.
5. Chromatography — difference in adsorption/partition rate. A mixture is carried by a mobile phase (a solvent) past a stationary phase (an adsorbent). Components that cling more weakly to the stationary phase move faster and separate from those that cling more strongly. Column chromatography packs the adsorbent into a column and collects fractions as the eluent flows through. Thin-layer chromatography (TLC) spreads a thin adsorbent layer on a plate; the solvent rises by capillary action, carrying each component a characteristic distance, quantified by
where: both distances are measured from the same starting (spotting) point, along the same straight line up the plate; is dimensionless and always lies between (component never moves — sticks to the adsorbent) and (component moves exactly as far as the solvent — no affinity for the adsorbent). A given compound, on a given adsorbent, with a given solvent, has a reproducible , which is why it is used to identify or compare spots on a TLC plate.
The goal of this derivation is to compute a component's from raw plate measurements and to read off, from the resulting number, what it tells you about that component's affinity for the stationary phase — a routine skill any TLC question in this chapter can demand, and the only quantitative tool this chapter expects you to apply by hand.
Step 1. Identify the two distances needed. On a developed TLC plate, mark the original spotting point (the baseline), the centre of the separated spot, and the solvent front — the highest point the solvent itself reached. Measure, along the same line, the distance from the baseline to the spot's centre, and the distance from the baseline to the solvent front.
Step 2. Take the ratio. . Because both distances are measured from the same starting line, this ratio is a pure number with no units, and by construction it can never exceed (the component cannot outrun the solvent that carries it) nor fall below (a spot that never leaves the baseline).
Step 3. Interpret the value. A small (close to ) means the component is strongly adsorbed onto the stationary phase and moves little; a large (close to ) means the component has little affinity for the stationary phase and travels almost with the solvent front. Two different compounds developed under identical conditions (same adsorbent, same solvent, same plate) that give different values are, by that fact, resolved — proof the mixture actually contained two components.
Step 4. Use for identification. Because is reproducible for a given compound under fixed conditions, it can be compared against a known reference run on the same plate to confirm or rule out a compound's identity — the basis of TLC as an analytical (not just preparative) tool.
Worked example
A TLC plate is spotted with a mixture and developed. The solvent front travels from the baseline. Spot A is found from the baseline and spot B is found from the baseline. Find the of each spot and state which component is more strongly adsorbed on the stationary phase.
Step 1. Write down the formula and the measured distances. , with solvent front distance , spot A distance , spot B distance .
Step 2. Compute for spot A. .
Step 3. Compute for spot B. .
Step 4. Compare and interpret. Spot A has the smaller ( versus ), meaning it moved less relative to the solvent front — it is more strongly adsorbed onto the stationary phase. Spot B, with the larger , has weaker affinity for the adsorbent and travels closer to the solvent front. Answer: , ; spot A is more strongly adsorbed.
Competitive stretch
If a third spot, C, is later found to have on an identical plate run under the same conditions (same adsorbent, same solvent, same temperature), it is consistent with being the same substance as spot A — but this is supporting evidence only, since two genuinely different compounds can coincidentally share an under one particular solvent system. A stronger identification runs the comparison again with a second, different solvent and checks that the values still match.
Common traps
- Treating sublimation as identical to evaporation or ordinary distillation — sublimation has no liquid phase at all; the solid passes directly to vapour and back to solid.
- Assuming simple distillation works for any two liquids: it only cleanly separates liquids with a large enough boiling-point gap; close boiling points require fractional distillation with its repeated vaporisation–condensation cycles.
- Forgetting that steam distillation needs the compound to be immiscible/insoluble in water and volatile in steam — it fails for water-miscible liquids, however volatile they are.
- Measuring distances from the wrong reference point, or on plates run with different solvents/adsorbents, and then comparing the resulting numbers as if they were directly comparable — is only meaningful for spots developed under identical conditions.
Ask the tutor: not sure which purification technique fits a described mixture, or want practice computing from a TLC plate diagram? Ask your AI tutor for more worked scenarios.
How is fractional distillation actually different from simple distillation if both just boil the liquid?
Both rely on the same underlying idea — the more volatile component vaporises preferentially — but simple distillation gives only one vaporisation–condensation event before the vapour is collected, which is not enough to cleanly split liquids with close boiling points. A fractionating column forces the rising vapour to repeatedly condense and re-vaporise as it climbs, and each cycle enriches the vapour a little further in the more volatile component. By the time the vapour reaches the top, many cycles have compounded into a much sharper separation, which is why fractional distillation is the standard choice whenever boiling points are close.
Why does cooling a saturated solution make only the desired compound crystallise, not the impurity too?
Solubility depends on both the substance and its concentration relative to its own solubility limit at that temperature. The hot solvent is prepared to be nearly saturated in the major compound, so a small drop in temperature pushes its concentration above its (now lower) solubility limit, and the excess crystallises out. The impurity, present in far smaller quantity, typically never reaches its own solubility limit at the same temperature drop, so it stays dissolved in the mother liquor. This only works cleanly when the impurity's amount is genuinely small and its solubility behaviour differs enough from the main compound's — a poor solvent choice or too large an impurity load can spoil the separation.
Can Rf values from two different chromatography experiments ever be compared directly?
Only if every experimental condition matches: the same adsorbent (stationary phase), the same solvent or solvent mixture (mobile phase), the same plate thickness, and roughly the same temperature and humidity. Rf is a ratio of distances, not an absolute property of the molecule alone — it depends on how strongly the compound interacts with that specific stationary phase versus that specific solvent. Change the solvent and the same compound can show a very different Rf. So Rf is best used to compare spots run side by side on the same plate, or under carefully reproduced identical conditions, never across unrelated setups.