Organic Chemistry: Some Basic Principles and Techniques • Topic 3 of 3

Purification & Analysis

Organic compounds prepared in the laboratory are rarely pure. The method of purification is chosen from the differences in physical properties — solubility, volatility, adsorption — between the compound and its impurities.

Methods of purification

  • Crystallisation: the most common method for solids. The compound is dissolved in a hot solvent in which it is sparingly soluble when cold; on cooling, pure crystals separate while soluble impurities stay in the mother liquor.
  • Sublimation: for solids that pass directly from solid to vapour on heating (e.g. naphthalene, camphor, benzoic acid), separating them from non-sublimable impurities.
  • Distillation: separates a volatile liquid from a non-volatile impurity, or two liquids with a large boiling-point gap. Fractional distillation separates liquids with close boiling points using a fractionating column. Distillation under reduced pressure purifies liquids that decompose at their normal boiling point (e.g. glycerol). Steam distillation purifies compounds that are steam-volatile and immiscible with water (e.g. aniline) below $100^\circ$C.
  • Differential extraction: shakes an aqueous solution with an immiscible organic solvent in which the compound is more soluble, so it transfers to the organic layer.
  • Chromatography: separates a mixture by differential adsorption/partition between a stationary and a moving phase — adsorption (column, TLC) and partition (paper) types.

Qualitative analysis — detecting elements

Carbon and hydrogen are detected by heating the compound with dry CuO: carbon gives $CO_2$ (turns lime-water milky) and hydrogen gives $H_2O$ (turns anhydrous $CuSO_4$ blue). N, S and halogens are covalently bound and are first converted to ionisable forms by Lassaigne's test: the compound is fused with sodium, forming $NaCN$, $Na_2S$, $NaX$ (and $NaSCN$). The fused extract is tested — nitrogen gives Prussian-blue $Fe_4[Fe(CN)_6]_3$, sulphur gives a violet colour with sodium nitroprusside, and halogens give silver-halide precipitates with $AgNO_3$.

Quantitative estimation

  • Carbon & hydrogen (Liebig): a known mass is burnt; $CO_2$ is absorbed in KOH and $H_2O$ in anhydrous $CaCl_2$. $\%C=\dfrac{12}{44}\times\dfrac{m_{CO_2}}{m}\times100$ and $\%H=\dfrac{2}{18}\times\dfrac{m_{H_2O}}{m}\times100$.
  • Nitrogen: Dumas oxidises N to $N_2$ gas, measured over KOH; Kjeldahl converts N to $(NH_4)_2SO_4$, liberates $NH_3$ with NaOH and titrates it. $\%N=\dfrac{1.4\times N_{acid}\times V_{acid}}{m}$.
  • Halogens (Carius): heated with fuming $HNO_3$ and $AgNO_3$ to give $AgX$, weighed.
  • Sulphur: oxidised to sulphate and precipitated as $BaSO_4$.
  • Phosphorus: oxidised to phosphoric acid and precipitated as ammonium phosphomolybdate or $Mg_2P_2O_7$.
Methods of purification and the property they exploit
MethodSuited toProperty used
Crystallisationsolids with soluble impuritiesdifference in solubility (hot vs cold)
Sublimationsublimable solids (camphor, naphthalene)direct solid → vapour transition
Simple distillationliquid + non-volatile impuritydifference in volatility/boiling point
Fractional distillationliquids with close boiling pointsrepeated vaporisation in a column
Distillation under reduced pressureliquids decomposing at their b.p. (glycerol)lowered boiling point at low pressure
Steam distillationsteam-volatile, water-immiscible (aniline)volatility with steam below $100^\circ$C
Differential extractioncompound in aqueous solutiongreater solubility in organic solvent
Chromatographycomplex mixtures, small amountsdifferential adsorption / partition
Solved Examples
1
Worked Example
Which purification method would you use for (i) a mixture of camphor and salt, and (ii) glycerol that decomposes at its boiling point?
Solution
  1. (i) Camphor sublimes but salt does not, so heating separates the camphor as vapour.
  2. That is sublimation.
  3. (ii) Glycerol decomposes at its normal boiling point, so lower the boiling point by reducing pressure.

Answer: (i) sublimation; (ii) distillation under reduced pressure.

2
Worked Example
In Lassaigne's test, what observation confirms the presence of nitrogen and why is sodium fusion needed?
Solution
  1. Covalently bound N must be converted to an ionic form; fusion with Na gives $NaCN$.
  2. The extract with $FeSO_4$ and then $Fe^{3+}$ forms sodium hexacyanoferrate, then ferric ferrocyanide.
  3. A Prussian-blue colour or precipitate $Fe_4[Fe(CN)_6]_3$ confirms nitrogen.

Answer: A Prussian-blue colour confirms N; sodium fusion converts covalent N into ionisable $CN^-$.

3
Worked Example
On combustion, $0.20\ \text{g}$ of an organic compound gave $0.44\ \text{g}$ of $CO_2$. Calculate the percentage of carbon.
Solution
  1. Use $\%C=\dfrac{12}{44}\times\dfrac{m_{CO_2}}{m}\times100$.
  2. $=\dfrac{12}{44}\times\dfrac{0.44}{0.20}\times100$.
  3. $=\dfrac{12}{44}\times 2.2\times100=0.2727\times220=60$.

Answer: $\%C=60\%$.

4
Worked Example
In a Kjeldahl estimation, $0.50\ \text{g}$ of a compound liberated $NH_3$ which neutralised $25\ \text{mL}$ of $0.5\ \text{N}$ acid. Find the percentage of nitrogen.
Solution
  1. Use $\%N=\dfrac{1.4\times N_{acid}\times V_{acid}}{m}$ with $V$ in mL.
  2. $=\dfrac{1.4\times 0.5\times 25}{0.50}$.
  3. $=\dfrac{17.5}{0.50}=35$.

Answer: $\%N=35\%$.

5
Worked Example
Name the method to estimate nitrogen that works for ALL nitrogen-containing compounds, and one limitation of the alternative.
Solution
  1. Dumas method oxidises nitrogen of any organic compound to $N_2$ gas, so it is general.
  2. Kjeldahl converts N to ammonium sulphate, but only if N can be reduced to $−3$.
  3. Kjeldahl fails for N in rings (pyridine), $−NO_2$ and azo ($−N=N−$) groups.

Answer: Dumas is general; Kjeldahl does not work for nitro, azo or ring nitrogen.

6
Worked Example
Which technique separates a small amount of a coloured plant pigment mixture, and on what principle?
Solution
  1. Small mixtures with components of differing affinities are best resolved by chromatography.
  2. Components move at different rates between a stationary phase and a moving solvent.
  3. This works by differential adsorption (or partition) of the components.

Answer: Chromatography, based on differential adsorption/partition between stationary and mobile phases.

Key Points

  • Purification methods exploit physical differences: crystallisation (solubility), sublimation (solid→vapour), distillation/fractional/reduced-pressure/steam (volatility), extraction (solvent partition), chromatography (adsorption/partition).
  • Carbon gives $CO_2$ (lime-water milky) and hydrogen gives $H_2O$ (blue with anhydrous $CuSO_4$) on heating with CuO.
  • Lassaigne's sodium fusion converts covalent N, S, X to ionic $CN^-$, $S^{2-}$, $X^-$: N gives Prussian blue, S a violet nitroprusside colour, X a silver-halide precipitate.
  • Carbon/hydrogen: $\%C=\dfrac{12}{44}\cdot\dfrac{m_{CO_2}}{m}\cdot100$, $\%H=\dfrac{2}{18}\cdot\dfrac{m_{H_2O}}{m}\cdot100$.
  • Nitrogen by Dumas (as $N_2$) or Kjeldahl ($\%N=\dfrac{1.4\,N_{acid}V_{acid}}{m}$); halogens by Carius (AgX), sulphur as $BaSO_4$, phosphorus as $Mg_2P_2O_7$.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.Glycerol, which decomposes at its normal boiling point, is best purified by:
Explanation: Lowering the pressure lowers the boiling point, so glycerol distils without decomposing.
Q2.In Lassaigne's test, a Prussian-blue colour confirms the element:
Explanation: Nitrogen (as $CN^-$ after fusion) forms ferric ferrocyanide, $Fe_4[Fe(CN)_6]_3$, which is Prussian blue.
Q3.On combustion $0.30\ \text{g}$ of a compound gave $0.22\ \text{g}$ of $CO_2$. The percentage of carbon is:
Explanation: $\%C=\dfrac{12}{44}\times\dfrac{0.22}{0.30}\times100=\dfrac{12}{44}\times73.3=20\%$.
Q4.The Kjeldahl method CANNOT be used to estimate nitrogen present as:
Explanation: Kjeldahl fails for nitro, azo and ring nitrogen because such N cannot be converted to ammonium sulphate.
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