JEE Chemistry Organic Chemistry Some Basic Principles and Techniques Notes Set 05

Purification of Organic Compounds

1. Simple distillation: used when the liquid has non-volatile impurities or the boiling points of the components differ by about 80 K or more. Examples: chloroform (334 K) and aniline (457 K); ether (308 K) and toluene (384 K).
2. Fractional distillation: used when the boiling points differ by only about 10 K or less; a fractionating column is used. Examples: crude oil fractions, acetone (329 K) and methanol (338 K).
3. Distillation under reduced pressure (vacuum distillation): used for liquids that decompose below their boiling point, for example glycerol, and for concentrating sugar juice.
4. Steam distillation: used for substances that are immiscible with water and steam volatile, for example aniline, nitrobenzene, bromobenzene, turpentine oil and o-nitrophenol.

Qualitative Analysis: Detection of Elements

1. Carbon and hydrogen: heat the compound with dry copper(II) oxide. Carbon gives CO2, which turns lime water milky; hydrogen gives water, which turns anhydrous \( CuSO_4 \) blue.
2. Lassaigne's test: the compound is fused with sodium metal, plunged into water and filtered. The covalent elements become ionic: \( Na + C + N \to NaCN \), \( 2Na + S \to Na_2S \), \( Na + X \to NaX \). The filtrate is the sodium fusion extract.
3. Nitrogen: boil the extract with \( FeSO_4 \), then add \( FeCl_3 \) and acidify; \( Na_4[Fe(CN)_6] \) gives Prussian blue, \( Fe_4[Fe(CN)_6]_3 \).
4. Sulphur: with lead acetate (acidified with acetic acid) a black precipitate of PbS forms; with sodium nitroprusside a violet colour forms.
5. If both N and S are present, sodium thiocyanate NaSCN forms and gives a blood red colour with \( FeCl_3 \) (no Prussian blue); an excess of sodium in the fusion decomposes it to NaCN and \( Na_2S \).
6. Halogens: boil the extract with conc. \( HNO_3 \) to expel \( H_2S \) and HCN, then add \( AgNO_3 \): AgCl white and soluble in ammonia; AgBr pale yellow and sparingly soluble; AgI yellow and insoluble.
7. Phosphorus: oxidise with \( Na_2O_2 \) to phosphate, then warm with conc. \( HNO_3 \) and ammonium molybdate to get a canary yellow precipitate of ammonium phosphomolybdate.

Quantitative Analysis: Formulae

Let W be the mass of the organic compound in grams.
1. Carbon and hydrogen (Liebig's method): the compound burns in excess \( CuO \); water is absorbed by anhydrous \( CaCl_2 \) and \( CO_2 \) by KOH solution. \[ \%C = \frac{12}{44}\times\frac{m_{CO_2}}{W}\times100,\qquad \%H = \frac{2}{18}\times\frac{m_{H_2O}}{W}\times100 \] 2. Nitrogen by Dumas method: nitrogen is collected over water; correct its volume to STP, using pressure minus aqueous tension. 22400 mL of N2 at STP weigh 28 g. \[ \%N = \frac{28}{22400}\times\frac{V_{STP}\,(mL)}{W}\times100 \] 3. Nitrogen by Kjeldahl's method: the nitrogen is converted to ammonium sulphate, ammonia is liberated with alkali and absorbed in a known volume of standard acid. \[ \%N = \frac{1.4\times N\times V}{W} \] where N is the normality of the acid and V is the volume of acid (mL) neutralised by ammonia. The method does not work for nitro, azo and azoxy compounds or for nitrogen in a ring (pyridine).
4. Halogens by Carius method: the compound is heated with fuming nitric acid and \( AgNO_3 \); the silver halide formed is weighed. \( \%X = \dfrac{\text{atomic mass of X}}{\text{molar mass of AgX}}\times\dfrac{m_{AgX}}{W}\times100 \).
5. Sulphur by Carius method: it is oxidised to sulphuric acid and precipitated as \( BaSO_4 \). \( \%S = \dfrac{32}{233}\times\dfrac{m_{BaSO_4}}{W}\times100 \).
6. Phosphorus is precipitated as magnesium pyrophosphate \( Mg_2P_2O_7 \): \( \%P = \dfrac{62}{222}\times\dfrac{m}{W}\times100 \). Oxygen is usually found by difference.