Qualitative Analysis of Organic Compounds

Detection of carbon and hydrogen. The compound is heated with dry copper(II) oxide, $\text{CuO}$, which acts as an oxidising agent. Carbon in the sample is oxidised to carbon dioxide, which turns lime water milky; hydrogen is oxidised to water, which turns anhydrous copper sulphate blue or condenses as droplets on the cooler part of the test tube. Copper oxide itself is reduced to metallic copper in the process.

where: the carbon and hydrogen atoms belong to the organic sample, $\text{CuO}$ is the solid oxidant packed around the sample, $\text{CO}_2$ is confirmed by lime water going milky, and $\text{H}_2\text{O}$ is confirmed by anhydrous $\text{CuSO}_4$ turning blue (it is white when anhydrous). This pair of reactions is specific to carbon and hydrogen only — it says nothing about nitrogen, sulphur, halogens, or phosphorus.

Sodium fusion (Lassaigne's test). The remaining elements — nitrogen, sulphur, and halogens — are covalently locked into the molecule and will not give an ionic test as they are. The compound is fused with a small piece of sodium metal, which is reactive and reducing enough to break these covalent bonds and rebuild the elements as sodium salts: sodium cyanide ($\text{NaCN}$) if nitrogen is present, sodium sulphide ($\text{Na}_2\text{S}$) if sulphur is present (and, when both N and S are present together, some sodium thiocyanate $\text{NaSCN}$ also forms), and sodium halides ($\text{NaX}$) if a halogen is present. The fused mass is extracted with distilled water to give the Lassaigne's extract, an aqueous solution now carrying these species as free ions, ready for standard inorganic spot tests.

The three confirmatory tests on the extract: Nitrogen — the extract is boiled with freshly prepared ferrous sulphate solution and then acidified with dilute sulphuric acid. A Prussian blue colouration or precipitate confirms nitrogen, formed via sodium ferrocyanide reacting with ferric ions present from partial air-oxidation of the ferrous salt. Sulphur — either the extract treated with sodium nitroprusside solution gives a violet or purple colouration, or the extract acidified with acetic acid and treated with lead acetate gives a black precipitate of lead sulphide. Halogens — the extract is acidified with dilute nitric acid and treated with silver nitrate. A white precipitate that dissolves in ammonium hydroxide signals chlorine; a pale yellow precipitate that is only partly soluble signals bromine; a yellow, insoluble precipitate signals iodine. If nitrogen or sulphur is also present, the extract must first be boiled with concentrated nitric acid to destroy residual $\text{CN}^-$ and $\text{S}^{2-}$, which would otherwise form their own misleading precipitates with silver nitrate.

Detection of phosphorus. The compound is oxidised — for example by fusion with sodium peroxide, or by heating with an oxidising mixture — converting phosphorus into phosphate, $\text{PO}_4^{3-}$. The resulting solution is boiled with concentrated nitric acid and treated with ammonium molybdate solution; a yellow precipitate or colouration of ammonium phosphomolybdate confirms phosphorus.

There is no single formula behind qualitative organic analysis — it is a chain of reasoning about reactivity, so the logic itself is the thing to master.

Why copper oxide works for carbon and hydrogen but nothing else. $\text{CuO}$ is a mild, general oxidising agent strong enough to oxidise carbon to $\text{CO}_2$ and hydrogen to $\text{H}_2\text{O}$ at the heating temperatures used, but it is not aggressive enough to break apart nitrogen, sulphur, or halogen bonding in a way that produces a distinctive, easily trapped product. So this single test is deliberately narrow: two elements, two gaseous products, two classic spot checks (lime water, anhydrous copper sulphate).

Why sodium fusion is necessary at all. Nitrogen, sulphur, and halogens in an organic molecule are held by covalent bonds — nitrogen inside an amine, sulphur inside a thiol, chlorine inside a C–Cl bond. None gives the instant precipitate or colour test expected of a free ion like $\text{Cl}^-$ or $\text{S}^{2-}$, because the atom is locked inside a covalent framework, not free in solution. Molten sodium metal is violently reactive and strongly reducing: fusing the compound with it rips these bonds apart and forces the liberated atoms to recombine with sodium as new, water-soluble ionic salts. This one step is the pivot of the whole method — it turns a detection problem in organic chemistry into an ordinary inorganic-ion test.

Why the specific reagents give specific signals. Each downstream test exploits a known, unambiguous inorganic reaction: $\text{CN}^-$ ions form ferric ferrocyanide, an intense blue complex, only under the ferrous sulphate/acid sequence, so Prussian blue is essentially unambiguous for nitrogen. Lead and sulphide ions combine to form insoluble black lead sulphide, a colour hard to confuse with anything else. Silver ions form halide precipitates whose colour and solubility in ammonia vary systematically down the group — chlorine most soluble, iodine least — because the silver–halide lattice becomes harder to break up by ammonia complexation as the halide ion grows larger and more polarisable. This trend is why colour and solubility together, not colour alone, distinguish the three halogens.

An organic compound is fused with sodium metal and its aqueous extract is tested. Test 1: the extract is boiled with $\text{FeSO}_4$ solution, then acidified with dilute sulphuric acid — a Prussian blue colour appears. Test 2: a separate portion of the extract, after first being boiled with concentrated nitric acid, is acidified with dilute nitric acid and treated with silver nitrate — a pale yellow precipitate forms that dissolves only partially in excess ammonium hydroxide. Identify every element confirmed present and explain each conclusion.

Step 1. Interpret the first test. During sodium fusion, any nitrogen in the compound is converted to sodium cyanide, $\text{NaCN}$, in the extract. Boiling this with ferrous sulphate forms sodium ferrocyanide; acidifying with dilute sulphuric acid, in the presence of a little ferric ion from partial air-oxidation, produces ferric ferrocyanide — the intense Prussian blue colour. This colour is the specific confirmatory signal for nitrogen, so nitrogen is confirmed present.

Step 2. Note why the second portion was pre-boiled with concentrated nitric acid. If sulphur (as $\text{Na}_2\text{S}$) or residual cyanide were present in the extract, they would react with silver nitrate to give their own precipitates, contaminating the halogen result. Boiling with concentrated nitric acid destroys $\text{CN}^-$ and $\text{S}^{2-}$ before the silver nitrate test is applied, so any precipitate that forms afterward can be attributed to halide alone.

Step 3. Interpret the precipitate. A pale yellow precipitate, partially soluble in excess ammonium hydroxide, is the characteristic signature of silver bromide, $\text{AgBr}$ — distinct from the fully soluble white $\text{AgCl}$ and the insoluble yellow $\text{AgI}$. This confirms bromine is present in the original compound.

Step 4. State the conclusion. Nitrogen and bromine are both confirmed present. Sulphur cannot be confirmed or ruled out from the tests given — no lead acetate or sodium nitroprusside test was performed — so it remains undetermined from this data alone.

• Forgetting why sodium fusion is needed in the first place — organic compounds hold N, S, and halogens as covalent atoms, which give none of the usual ionic tests until fusion converts them into ionic sodium salts.
• Skipping the concentrated nitric acid pre-boil before testing for halogens when nitrogen or sulphur may also be present — without it, leftover $\text{CN}^-$ or $\text{S}^{2-}$ gives false precipitates with silver nitrate.
• Mixing up the three silver halide signatures — white and fully soluble means chlorine, pale yellow and partly soluble means bromine, yellow and insoluble means iodine — these must be read together, not guessed from colour alone.
• Assuming the copper oxide test detects anything beyond carbon and hydrogen — it does not touch nitrogen, sulphur, halogens, or phosphorus, which all require the separate sodium-fusion-based tests.

Ask the tutor: want to work through the full Lassaigne's test sequence on a sample compound step by step, or practise reading a set of test results together?