Qualitative analysis is an analytical technique which helps identifying elements/ compounds present in a sample, while quantitative analysis helps in determining the proportions of the elements or compounds in that sample.

The organic compounds are analyzed qualitatively and quantitatively to ascertain the presence and proportion of certain elements like C(Carbon) & H(Hydrogen) along with some other elements like N(nitrogen), S(Sulphur), P(Phosphorous), X (Halogen) etc.

Qualitative analysis

The elements present in organic compounds are carbon and hydrogen. In addition to these, they may also contain nitrogen, sulphur, halogens and phosphorus, and oxygen.

Detection of C and H

Carbon and hydrogen are detected by heating the compound with copper(II) oxide. Carbon present in the compound is oxidized to carbon dioxide (lime-water test develops turbidity due to CaCO₃ formation) and hydrogen to water (tested with anhydrous copper sulphate, which turns blue due to formation of CuSO₄.5H₂O (blue vitriol)).

(i). 2CuO + C → CO₂ + 2Cu (i)-a. CO₂ + Ca(OH)₂ → CaCO₃ + H₂O [Test for C]

(ii) CuO + 2H → H₂O+ Cu (ii)-a. 5H₂O+ Cu(SO)₄→ Cu(SO)₄. 5H₂O [Test for H]

Detection of Other Elements

After detecting Carbon and Hydrogen, it is necessary to check if the compound has other elements like Nitrogen, sulphur, halogens, phosphorus, and oxygen. Nitrogen, sulphur, halogens present in an organic compound are detected by “Lassaigne’s test”.

In this test, the elements convert into ionic form from covalent form when sodium is fused with the elements present in the compound.

Briefly, sodium fusion extract (SFE) was prepared by heating a small amount of sodium along with the organic compound in a glass tube (Lassaigne tube) until it is become red hot.

The glass tube containing the mixture is put into distilled water, broken into pieces with a mortar pestle. This solution thus formed is called sodium fusion extract or Lassaigne’s extract. It is filtered and the filtrate if containing N, S, and X (halogen) which converted to cyanide, sulphide, and halide are tested for their presence in the organic compund.

1. Na + C + N → NaCN (Sodium cyanide)
2. 2Na +S → Na2S (Sodium sulphide)
3. Na + X → NaX (Sodium halide)

Detection of Nitrogen

The SFE thus formed is alkaline in nature and contains NaOH. A small amount of SFE is boiled with iron(II) sulphate (FeSO4) which forms Fe(ii) hydroxide (Fe(OH)2) and sodium sulphate.

The (Fe(OH)2) thus formed react with sodium cyanide to sodium ferro cyanide [Na₄(Fe(CN)6] which when treated with Fe(iii) chloride (FeCl3) and acidified with HCL turns prussian blue due to ferri ferro cyanide formation confirms N in the organic compound.

1. FeSO₄ + 2NaOH → Na2SO4 + Fe(OH)₂
2. Fe(OH)₂+ 6NaCN → Na₄(Fe(CN)₆+ 2NaOH
3. 4FeCl₃+ 3Na₄(Fe(CN)₆ → Fe⁽ⁱⁱⁱ⁾₄[(Fe⁽ⁱⁱ⁾(CN)₆]₃ (prussian blue)+ 12NaCl

Detection of Sulphur

Lead acetate test: A black precipitate of lead sulphide indicates the presence of sulphur when the sodium fusion extract is acidified with acetic acid and lead acetate is added to it.

Na₂S + Pb(CH₃COO)₂ → PbS [black] + 2NaCOOCH₃

Nitroprusside test: On treating sodium fusion extract with sodium nitroprusside, appearance of a violet colour (due to sodium thio nitroprusside formation) further indicates the presence of sulphur.

Na₂S + Na₂(Fe(CN)₅NO) → Na₄(Fe(CN)₅NOS) [violet]

If both N, & S present in organic compound

In case, nitrogen and sulphur both are present in an organic compound, sodium thiocyanate is formed. It gives blood red color and no Prussian blue since there are no free cyanide ions

Na + C + N + S → NaSCN

If sodium fusion is carried out with excess of sodium, the thiocyanate decomposes to yield cyanide and sulphide. These ions give their usual tests.

NaSCN + 2Na → NaCN + Na₂S

Detection of Halogen

The SFE is acidified with HNO₃ and then treated with AgNO₃ (silver nitrate).

NaX+ AgNO₃ → NaNO₃ + AgX

1. A white precipitate (AgCl), soluble in ammonium hydroxide shows the presence of chlorine. Here X=Cl; AgCl + NH4OH → Ag(NH)₃Cl + 2H₂O
2. A yellowish precipitate (AgBr), sparingly soluble in ammonium hydroxide shows the presence of bromine. Here X=Br; AgBr + NH4OH → Ag(NH)₃Br + 2H₂O
3. A yellow precipitate (AgI), insoluble in ammonium hydroxide shows the presence of iodine

If nitrogen or sulphur is also present in the compound, the SFE is first boiled with concentrated nitric acid to decompose cyanide or sulphide of sodium formed during Lassaigne’s test. These ions would otherwise interfere with silver nitrate test for halogens.

Detection of Phosphorous

The compound is heated with an oxidizing agent (sodium peroxide). The phosphorus present in the compound is oxidized to phosphate. The solution is boiled with nitric acid and then treated with ammonium molybdate. A yellow coloration or precipitate indicates the presence of phosphorus.

1. 2P + 5Na₂O₂ → 2Na₃PO₄ + 2Na₂O
2. Na₃PO₄ + 3 HNO₃ → H₃PO₄ + 3NaNO₃
3. H₃PO₄ + 12(NH₄)₂MoO₄ + 3 HNO3 → (NH₄)₃PO₄ .12MoO₃ + 21NH₄NO₃ + 12H₂O