Qualitative Analysis

Qualitative Analysis

Qualitative analysis means to detect the various elements present in it. The elements which commonly occur in organic compounds are carbon, hydrogen, oxygen and nitrogen.

Detection of Carbon and Hydrogen

Principle: The presence of carbon and hydrogen, in an organic compound, is detected by heating the given compound with dry Copper(II) oxide or cupric oxide in a hard glass test tube when carbon present is oxidised to carbon dioxide and hydrogen is oxidised to water.

C +2 CuO —-> CO₂ + 2 Cu

2H + CuO ———-> H₂O + Cu
In general, if the organic compound containing only C and H has the molecular formula C,H, then the complete combustion equation may be written as

C_xH_y +(2 x + y/2) CuO——–> xCO₂ + y/2 H₂O+ (2x + y/2)Cu

Carbon dioxide turns lime water milky while water condenses on the cooler parts of the test tube and turns anhydrous copper sulphate blue.

Ca(OH)₂ + CO₂ ——-> CaCO₃ + H₂O

CuSO₄ + 5 H₂O ———> CuSO₄.5H₂O

Procedure for detection of Carbon and Nitrogen

A small quantity of the pure and dry compound is mixed with nearly five to six times its weight of dry and pure cupric oxide powder. The intimate mixture is strongly heated in a hard glass test tube fitted with a delivery tube having a bulb in the centre. The other end of the delivery tube is dipped in lime water taken in a test tube. The bulb in the delivery tube is packed with anhydrous copper sulphate supported over glass wool. On heating, the carbon is oxidised to CO₂ which turns lime water milky and hydrogen is oxidised to water which turns anhydrous copper sulphate blue.

 

Modifications in the estimation of Carbon and Hydrogen

(1) If the organic substance is a volatile liquid or a gas, the vapours of the compound are passed through heated cupric oxide taken in a hard glass test tube and gases evolved are tested for CO₂ and H₂O vapours.

(2) If the organic compound also contains sulphur then it is oxidised to sulphur dioxide which also turns lime water milky due to the formation of insoluble calcium sulphite.

4CuO + S ——-> 2 Cu₂O + SO₂

Ca(OH)₂ + SO₂ ———> CaSO₃ + H₂O

The out coming gases are first passed through an acidified solution of potassium dichromate which absorbs sulphur dioxide and turns it green, and then through lime water.

Detection of Nitrogen

The presence of nitrogen is detected by the following tests:

1. Dry heating test. If the organic compound containing nitrogen is heated strongly, it gives a smell of burning hair.

Limitation. This test is, however, not reliable since many compounds containing nitrogen do not give this test.

2. Soda lime test. A pinch of the organic compound is heated strongly in a dry test tube with soda-lime(CaO+ NaOH).A smell of ammonia indicates the presence of nitrogen.

NH₂CONH₂ + 2 NaOH ——–> 2NH₃ + Na₂CO₃

Limitation. This test is also not reliable since many organic compounds containing nitrogen such nitro , azo  groups, etc. do not give this test.

3. Lassaigne’s test.  In this test, the elements present in the organic compound are converted from covalent form into the ionic form by fusing the compound with sodium metal. This test is carried out as follows:

(1) Preparation of the Lassaigne’s extract. A small piece of freshly cut sodium is heated gently in a fusion tube till it forms a shiny globule.The tube is removed from the flame and a small amount of the organic compound (50-60 mg) is added and the tube heated strongly till it becomes red hot (2–3 minutes). The hot tube is then plunged into a china dish containing 10 – 15 ml. of distilled water. The contents of the china dish are boiled for a few minutes, cooled and then filtered. The filtrate is called Lassaigne’s extract or sodium fusion extract.

(a) Test for Nitrogen. The Lassaigne’s extract is alkaline since the excess of sodium react with water to produce sodium hydroxide. If not, it is made alkaline by adding a few drops of a dilute solution of sodium hydroxide. To a part of this alkaline solution is added a few drops of a freshly prepared solution of ferrous sulphate. The contents are warmed a little, cooled and then acidified with dil H₂SO₄ .

Appearance of a green or blue colouration indicates the presence of nitrogen. However, appearance of blood colour indicates the presence of both Nitrogen and sulphur.

Chemistry of the test. During fusion, carbon and nitrogen of the organic compound combine to form sodium cyanide.

Na + C +  N ———-> NaCN

On heating with ferrous sulphate solution, sodium ferrocyanide. i.e.,sodium hexacyanoferrate(II) is formed and at the same time some ferrous (Fe²⁺) ions are oxidised to ferric (Fe³⁺)ions. These Fe³⁺ ions then react then react with sodium hexacyanoferrate  to produce iron (III) hexacyanoferrate which is prussion blue in colour.

2NaCN + FeSO₄ ———> Na₂SO₄ + Fe(CN)₂

Fe(CN)₂ + 4 NaCN ——-> Na₄[Fe(CN)₆]

3Na₄[Fe(CN)₆] +4 Fe³⁺ ———-> Fe₄[Fe(CN)₆]₃ + 12 Na⁺

If nitrogen and sulphur both are present in the organic compound, they may combine during fusion to form sodium thiocyanate (sulphocyanide) due to insufficient sodium. This when heated with ferrous sulphate a blood red colouration due to ferric thiocyanate (or sulphocyanide) by reaction with ferric ions is formed by oxidation of ferrous ions.

Na + C + S + N —–> NaSCN

Fe³⁺ + 3 NaSCN———-> Fe(SCN)₃ + 3Na⁺

But the absence of blood red colouration does not necessarily mean that sulphur is absent. This is due to the reason that in presence of excess of sodium metal, sodium thiocyanate decomposes to form sodium cyanide and sodium sulphide

2Na + NaSCN ——-> Na₂S+ NaCN

 

Detection of Halogens

The presence of halogens in an organic compound is detected by the following tests:

(1) Beilstein test:  In this test, a clean and stout copper wire is heated in the non-luminous flame of the Bunsen burner until it ceases to impart any green or bluish green colour to the flame. The heated end is then dipped in the organic compound and again introduced into the Bunsen flame. The appearance of a green or bluish flame due to the formation of volatile cupric halides indicates the presence of halogens in the organic compounds.

Limitations

(1) Organic compounds like urea, thiourea, etc. which do not contain halogens also give this due to the formation of volatile cupric cyanide.

(2) It  not tell as to which halogen (chlorine, bromine, or iodine) is actually present in the organic compound.

(2) Lassaigne’s test. It is a very reliable test for the detection of halogens in an organic compound.

Preparation of the Lassaigne’s extract

(1) Test for halogens. A part of the Lassaigne’s extract is boiled with dil. HNO₃ and cooled. A few drops of silver nitrate solution are then added.
(a) A white precipitate soluble in ammonia and insoluble in dil. HNO, indicates the presence of chlorine.
NaCl +AgNO₃ ———> AgCl+  NaNO₃

(b) A pale yellow precipitate partially soluble in ammonia indicates the presence of bromine.
NaNO₃ + AgBr ———> AgBr + NaNO₃

c) A yellow precipitate insoluble in ammonia indicates the presence of iodine.
NaI + AgNO₃ ——–> AgI + NaNO₃

Function of Nitric Acid

If the organic compound also contains nitrogen or sulphur, the Lassaigne’s extract on boiling with dil. HNO₃  decomposes sodium cyanide or sodium sulphide formed during fusion.
If cyanide and sulphide ions are not decomposed, they will react with silver nitrate and hence will interfere with the test.

NaCN+AgNO₃ ——-> AgCN+  NaNO₃

Na₂S + 2 AgNO₃ ——> Ag₂S + 2 NaNO₃

(3) Carbon disulphide test for Bromine and Iodine

A small portion of the Lassaigne’s extract is boiled with dil H₂SO₄ , to decompose sodium cyanide and sodium sulphide. The solution is then cooled and a few mL of freshly prepared chlorine water and carbon disulphide or carbon tetrachloride are added. The solution is vigorously shaken and allowed to stand when:

(1)  An orange colour in CS₂ or CCl₄ layer indicates the presence of bromine.
(2) A violet colour in CS₂ or CCl₄, layer indicates the presence of iodine.

Theory of the test. Chlorine displaces bromine and iodine from their corresponding halides. The halogen thus liberated dissolves in CS₂ or CCl₄ , to produce the specific colour.

2 NaBr +Cl₂——–>2NaCl +Br₂ (Dissolves in CS2 or CCl4 to give orange colour)

2NaI + Cl₂——-> 2NaCl+ I₂ (Dissolves in CS2, or CCl4, to give violet colour)

 

Detection of Sulphur

Detection of Phosphorus