Purification and Characterisation of Organic Compounds
Getting a compound clean, then asking what it is made of — the physical methods that separate and the analytical tests that reveal the elements and their amounts
- How to choose a purification method — sublimation, crystallisation, distillation, extraction or chromatography — from a compound's properties.
- The four kinds of distillation and when each one applies.
- Chromatography by adsorption and partition, and the meaning of the Rf value.
- How to detect C, H, N, S and the halogens, including Lassaigne's test.
- How to estimate C and H (Liebig) and nitrogen (Dumas, Kjeldahl).
- The Carius method for halogens and sulphur, and the percentage formulae for every element.
Why Purify and Characterise?
An organic compound straight from a reaction is rarely pure. Before its identity can be settled, it must be purified — freed from unreacted starting material, by-products and solvent — and then characterised: its elements detected (qualitative analysis) and their amounts measured (quantitative analysis). Purification rests on physical differences such as volatility and solubility; characterisation rests on chemistry that converts hidden covalent elements into things we can weigh or titrate.
Sublimation & Crystallisation
Sublimation purifies solids that pass directly from solid to vapour on heating, leaving non-volatile impurities behind. It suits compounds such as camphor, naphthalene and anthracene.
Crystallisation is the workhorse for solids and exploits differences in solubility. The impure solid is dissolved in the minimum of hot solvent (one that dissolves the compound well when hot, poorly when cold), the hot solution is filtered to remove insoluble impurities, and on cooling the pure compound crystallises while soluble impurities stay in the mother liquor. Coloured impurities are removed with a little activated charcoal.
Distillation & Its Variants
Distillation separates liquids by boiling point. Four variants cover the cases you meet.
| Method | Used when | Example |
|---|---|---|
| Simple distillation | liquid + non-volatile impurity, or boiling points far apart | chloroform from aniline; water from salt |
| Fractional distillation | boiling points close together | petroleum refining; acetone–water |
| Distillation under reduced pressure | liquid decomposes at its normal boiling point | glycerol; sugar-cane juice concentration |
| Steam distillation | compound steam-volatile and immiscible with water | aniline; bromobenzene; essential oils |
Because the two immiscible liquids contribute their vapour pressures independently, the mixture boils below 100 °C — so a compound that would otherwise decompose at its own boiling point distils over gently with the steam. It must be steam-volatile and immiscible with, and unreactive toward, water.
Extraction & Chromatography
Differential (solvent) extraction recovers a compound from an aqueous solution by shaking it in a separating funnel with an immiscible organic solvent in which the compound is far more soluble. The compound moves into the organic layer, which is run off and evaporated. Several small extractions remove more than one large one.
Chromatography separates a mixture by the differing affinities of its components for a stationary and a mobile phase. Two mechanisms appear in the syllabus.
| Type | Separates by | Stationary / mobile phase |
|---|---|---|
| Adsorption (column, TLC) | differential adsorption on a solid | silica gel / alumina · a liquid eluent |
| Partition (paper) | differential partition between two liquids | water held on paper · a moving solvent |
Detecting C and H (Qualitative)
Carbon and hydrogen are detected together by heating the compound with dry copper(II) oxide. Carbon is oxidised to carbon dioxide and hydrogen to water.
The carbon dioxide turns lime water milky; the water turns anhydrous white copper(II) sulphate blue (or pink cobalt chloride paper). Both observations together confirm carbon and hydrogen.
Lassaigne's Test: N, S, Halogens
Nitrogen, sulphur and halogens are held in covalent form and give no ionic tests directly. Lassaigne's test first fuses the compound with metallic sodium, converting these elements into ionic sodium salts that dissolve in water as the sodium fusion extract.
| Element | Reagent on the extract | Positive result |
|---|---|---|
| Nitrogen | \(\ce{FeSO4}\), then \(\ce{Fe^3+}\)/acid | Prussian-blue colour (ferric ferrocyanide) |
| Sulphur | sodium nitroprusside | violet colour (also \(\ce{Pb(OAc)2}\) → black \(\ce{PbS}\)) |
| Halogen | boil with \(\ce{HNO3}\), add \(\ce{AgNO3}\) | white \(\ce{AgCl}\) / pale-yellow \(\ce{AgBr}\) / yellow \(\ce{AgI}\) |
Estimating C and H (Liebig)
In Liebig's method a known mass of compound is burnt in excess oxygen over hot copper oxide. The water formed is absorbed in anhydrous calcium chloride and the carbon dioxide in concentrated potassium hydroxide; both absorbers are weighed before and after.
$$ \%\,\ce{C} = \frac{12}{44}\times\frac{m_{\ce{CO2}}}{m_{\text{compound}}}\times 100 $$
$$ \%\,\ce{H} = \frac{2}{18}\times\frac{m_{\ce{H2O}}}{m_{\text{compound}}}\times 100 $$
Estimating Nitrogen
Two methods estimate nitrogen, and the choice depends on how the nitrogen is bound.
Dumas method. The compound is burnt with copper oxide; all the nitrogen comes off as \(\ce{N2}\) gas, whose volume is measured over potassium hydroxide (which absorbs the \(\ce{CO2}\)). It works for all nitrogen compounds.
Kjeldahl method. The compound is heated with concentrated sulphuric acid, converting its nitrogen to ammonium sulphate. Adding alkali liberates ammonia, which is distilled into a known excess of standard acid; back-titration gives the acid that reacted with the ammonia. It is quick and accurate but fails for nitro, azo and ring nitrogen.
Halogens, Sulphur, Phosphorus & Oxygen
The Carius method handles halogens and sulphur: the compound is heated with fuming nitric acid in a sealed Carius tube, which oxidises and frees the element to be precipitated and weighed. Phosphorus is treated similarly and oxygen is usually found by difference.
| Element | Weighed as | Percentage |
|---|---|---|
| Halogen (X) | silver halide \(\ce{AgX}\) | \( \dfrac{\text{at. mass } X}{M_{\ce{AgX}}}\times\dfrac{m_{\ce{AgX}}}{m}\times 100 \) |
| Sulphur | barium sulphate \(\ce{BaSO4}\) | \( \dfrac{32}{233}\times\dfrac{m_{\ce{BaSO4}}}{m}\times 100 \) |
| Phosphorus | magnesium pyrophosphate \(\ce{Mg2P2O7}\) | \( \dfrac{62}{222}\times\dfrac{m_{\ce{Mg2P2O7}}}{m}\times 100 \) |
| Oxygen | — | by difference: \(100 - \sum(\text{others})\) |
Putting It to Work
Problem. Aniline (b.p. 184 °C, steam-volatile, immiscible with water) is to be purified. Which method, and why?
Solution. Steam-volatile and water-immiscible points to one method:
Problem. Burning \(0.20\,\text{g}\) of a compound gives \(0.44\,\text{g}\) \(\ce{CO2}\) and \(0.18\,\text{g}\) \(\ce{H2O}\). Find % C and % H.
Solution. Apply the Liebig formulae:
Problem. \(0.50\,\text{g}\) of a compound needs \(20\,\text{mL}\) of \(0.5\,\text{N}\) acid to neutralise the ammonia. Find % N.
Solution. Use the Kjeldahl formula:
Problem. \(0.30\,\text{g}\) of a chloro-compound gives \(0.287\,\text{g}\) \(\ce{AgCl}\) (\(M=143.5\)). Find % Cl. (\(\ce{Cl}=35.5\))
Solution. Fraction of Cl in AgCl times the mass ratio:
Problem. A Lassaigne extract from a compound containing both N and S gives a blood-red colour with \(\ce{Fe^3+}\), not Prussian blue. Why?
Solution. N and S together form thiocyanate:
Problem. On a TLC plate a spot moves \(3.0\,\text{cm}\) while the solvent front moves \(5.0\,\text{cm}\). Find Rf.
Solution. Divide the two distances:
Chapter Summary
Sublimation (volatile solids) and crystallisation (solubility difference) purify solids.
Simple, fractional, reduced-pressure and steam — chosen by boiling point and stability.
Adsorption and partition separate mixtures; Rf identifies each component.
CuO for C and H; Lassaigne's sodium fusion for N, S and halogens.
Liebig (C, H); Dumas and Kjeldahl (N) — each with its percentage formula.
Carius weighs AgX or BaSO₄; P as Mg₂P₂O₇; oxygen by difference.
Problems
For each item, first decide whether it is purification, detection or estimation, then apply the matching principle. Difficulty rises down the list.
- State the principle of crystallisation and the properties of an ideal solvent.
- Name the four types of distillation and give one application of each.
- Explain why steam distillation lets a compound distil below 100 °C.
- Define the Rf value and state its range. How is it useful?
- How are carbon and hydrogen detected in an organic compound?
- Describe Lassaigne's test and why sodium fusion is necessary.
- Why must the extract be boiled with nitric acid before testing for halogens?
- A compound contains both N and S. What colour does the nitrogen test give, and why?
- Burning \(0.246\,\text{g}\) of a compound gives \(0.198\,\text{g}\) \(\ce{CO2}\) and \(0.1014\,\text{g}\) \(\ce{H2O}\). Find % C and % H.
- State the Dumas and Kjeldahl percentage formulae and the limitation of Kjeldahl.
- \(0.35\,\text{g}\) of a compound gives \(0.466\,\text{g}\) \(\ce{BaSO4}\). Find % S.
- Explain how oxygen is usually estimated and why a direct method is harder.