Organic chemistry

 

COMPOSITION OF ORGANIC COMPOUNDS

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DETECTION OF ELEMENTS

The first step in the analysis of an organic compound is the detection of elements present in it. Most of these compounds contain 2 to 5 different elements. The principal elements present are carbon, hydrogen and oxygen. Often, in addition to these, they may contain nitrogen, sulphur and halogens. Phosphorus and metals are also present but only rarely. The order of abundance in which these elements are found in organic compounds is indicated below:

• Carbon- Always present
• Hydrogen- Nearly always present
• Oxygen- Generally present
• Nitrogen, Halogens & Sulphur- Less commonly present
• Phosphorus and Metals- Rarely present

DETECTION OF CARBON AND HYDROGEN

If the compound under investigation is known to be organic, there is no need to test for carbon. This test is performed only to establish whether a given compound is organic or not. With the exception of a few compounds for example, carbon tetrachloride. All organic compounds also contain hydrogen. The presence of both these elements is confirmed by the following common test.

Copper Oxide Test

The organic substance is mixed intimately with about three times its weight of dry copper oxide. The mixture is then placed in a hard glass test tube fitted with a bent delivery tube, the other end of which is dipping into lime water in another test tube. The mixture is heated strongly when the following reactions take place:

C + 2CuO → CO₂ + 2Cu

2H + CuO → H₂O + Cu

Dectection of carbon and hydrogen

Thus if carbon is present, it is oxidised to carbon dioxide which turns lime water milky. If hydrogen is also present, it will be oxidised to water which condenses in small droplets on the cooler wall of the test tube and inside the bulb. The formation of water is further confirmed by testing the condensed liquid with anhydrous copper sulphate (white) that is turned blue.

If the substance under investigation is a gas or a volatile liquid the above test is modified. The vapours of the substance are passed over heated copper oxide contained in a hard glass combustion tube.  The issuing gases are tested for carbon dioxide and water vapour as described before.

While testing for hydrogen, it is essential that the apparatus and copper oxide used are absolutely dry. Cupric oxide being hygroscopic in nature is heated strongly just before use.

DETECTION OF OXYGEN

There is no conclusive test for oxygen, though its presence in organic compounds is often inferred by indirect methods.  

1. The substance is heated alone in a dry test tube, preferably in an atmosphere of nitrogen. Formation of droplets of water on cooler parts of the tube obviously shows the presence of oxygen. A negative result, however, does not necessarily show the absence of oxygen.
2. The second method is to test for the presence of various oxygen containing groups such as hydroxyl (HO), carbonyl (COOH), nitro (NO₂) etc. If any of these is detected, the presence of oxygen is confirmed.
3. The sure test for oxygen depends on the determination of the percentage of all other elements present in the given compound. If the sum of these percentages fall short of hundred, the remainder gives the presence of oxygen and thus confirms its presence.

DETECTION OF NITROGEN

The presence of nitrogen in an organic compound is shown by the following tests:

1. Soda-lime Test

The given substance is mixed with double the amount of soda-lime (NaOH + CaO) and heated in a test tube. The vapours of ammonia evolved show the presence of nitrogen. A negative result, however, is not a proof of the absence of nitrogen. Many classes of nitrogenous compounds including nitro and diazo derivatives, do not respond to this test.

2. Sodium Test (Lassaigne’s Test)

This is a good test for the detection of nitrogen in all classes of nitrogenous compounds. It involves the following steps:

1. The substance is heated strongly with sodium metal.

Na + C + N → NaCN

       (from organic compound)

2. The water extract of the fused mass is boiled with ferrous sulphate solution.

FeSO4  + 2NaOH  → Fe(OH)2  + Na2SO4

             (from excess if sodium)

6NaCN+Fe(OH)2→Na[Fe(CN)6]+2NaOH    

                                (Sodium ferrocyanide)

3. To the cooled solution is then added a little ferric chloride solution and excess of concentrated hydrochloric acid.

3Na4[Fe(CN)6] + 4FeCl3 → Fe4[Fe(CN)6]3 + 12NaCl

   (Sodium ferrocyanide)         (Prussian blue)              

The formation of Prussian blue or green coloration confirms the presence of nitrogen.

Hydrochloric acid is added in this step to dissolve the greenish precipitate of ferrous hydroxide produced by the excess of sodium hydroxide on ferrous sulphate (in step b.), which would otherwise mark the Prussian blue precipitate.

In case sulphur is also present along with nitrogen in the given organic compound, a blood red coloration may appear while performing the above test. This is due to the formation of sodium sulphocyanide which again reacts with ferric chloride to produce blood red coloration.

Na + C + N + S → NaCNS

                            (Sodium sulphocyanide)

3NaCNS  + FeCl3 → Fe(CNS)3 + 3NaCl

                                 (Ferric sulphocyanide)

                                 (blood red)

Procedure of Sodium Test

Fix a fusion tube in an iron stand clamping it just near the upper end. Take a freshly cut piece of sodium of the size of a pea and dry it from below. When it melts to a shining globule, put a pinch of the organic compound on it. Heat the tube with the tip of the flame till all the reaction ceases and it becomes red hot. Now plunge it in about 50ml of distilled water taken in a china dish break the bottom of the tube by striking against the dish. Boil the contents of the dish for about 10 minutes and filter. Label the filtrate as sodium extract and proceed with it as follows:

Take a portion of the sodium extract in a test tube note if it is alkaline. If it is not, make it so by adding sodium hydroxide. Then add to it freshly prepared ferrous sulphate solution and heat it to boiling. Put 2-3 drops of ferric chloride solution, cool and acidify with conc. Hydrochloric acid. A Prussian blue or green precipitate, or even coloration, confirms the presence of nitrogen.

DETECTION OF SULPHUR

The presence of sulphur in organic substances is shown as described below.

1. Sodium Test

Sulphur, if present, in the given organic compound, upon fusion with sodium reacts to form sodium sulphide.

2Na + S → Na2S

                  (Sodium sulphide)

Thus, the sodium extract obtained from the fused mass may be tested as:

1. To a portion, add freshly prepared sodium nitroprusside solution. A deep violet coloration indicates sulphur.
2. Acidify a second portion of the extract with acetic acid and then add lead acetate solution. A black precipitate of lead sulphide confirms the presence of sulphur.

Pb(CH3COO)2  + Na2S  → PbS + 2CH3COONa

(lead acetate)                   (lead sulphide)

                                           (black ppt.)

2. Oxidation Test

The organic substance is fused with a mixture of potassium nitrate and sodium carbonate. The sulphur, if present, is oxidised to sulphate.

Na2CO3 + S + 3O → Na2SO4 + CO2

The fused mass is extracted with water, acidified with hydrochloric acid and then barium chloride solution is added to it. A precipitate indicates the presence of sulphur.

BaCl2  + Na2SO4  → BaSO4 + 2 NaCl

                                (barium sulphate)

                                (white ppt.)

DETECTION OF HALOGENS

1. Sodium Test

Upon fusion with sodium, the halogens in the organic compound are converted to the corresponding sodium halides. Thus,

Cl + Na → NaCl

Br + Na → NaBr

I + Na → NaI

Acidify a portion of sodium extract with dilute nitric acid and add to it silver nitrate solution.

White precipitate soluble in ammonia indicates Chlorine.

Yellow precipitate sparingly soluble in ammonia indicates Bromine.

Yellow precipitate insoluble in ammonia indicates Iodine.

When nitrogen or sulphur is also present in the compound, the sodium extract before testing the halogens is boiled with strong nitric acid to decompose  the cyanide and the sulphide formed during the sodium fusion. If not removed, these radicals will forma white and black precipitate respectively on the addition of silver nitrate.

NaCN + HNO3 → NaNO3 + HCN

Na2S + 2HNO2 → 2NaNO2 + H2S

2. Copper Wire Test (Beilstein’s Test)

The copper wire flattened at one end is heated in an oxidising Bunsen flame till it ceases to impart any green colour to the flame. A small quantity of substance under investigation is now taken on the flattened end of the wire which is reinserted in the in the Bunsen flame. Upon heating for a while, the halogen present in the substance is converted into a volatile copper halide which imparts a blue or green color to the flame. This test though very sensitive, is not always reliable. A substance like urea contains no halogen, also colors the flame green.

DETECTION OF PHOSPHORUS

The solid substance is heated strongly with an oxidising agent such as conc. Nitric acid or a mixture of sodium carbonate and potassium nitrate. The phosphorus present in the substance is thus oxidised to phosphate. The residue is extracted with water, boiled with some nitric acid, and then a hot solution of ammonium molybdate is added to it in excess. A yellow coloration or precipitation indicates the presence of phosphorus.

DETECTION OF METALS

The substance is strongly heated in a crucible, preferably of platinum, till all reaction ceases. An incombustible residue indicates the presence of a metal in the substance. The residue is extracted with dilute acid and the solution tested to the presence of metallic radical by the usual scheme employed for inorganic salts.