Qualitative Analysis of Organic Compounds

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What is Qualitative Analysis?

Some of the most commonly occurring elements in organic compounds are carbon, hydrogen, sulphur, nitrogen and halogen elements. Detection of these elements can be done through qualitative analysis, as there is no direct method for the detection of oxygen.

Qualitative analysis is an analysis method used to measure changes in melting point, colour, odour, radioactivity, reactivity, bubble production, boiling point and precipitation. On the other hand, quantitative analysis is a method of analysis to determine the number of elements or molecules produced during a chemical reaction. We can therefore highlight that qualitative analysis is a method to analyse the species present in a compound and is more focused on finding the elements and ions present in the given compound whereas quantitative analysis is based on finding the quantity (how much) of elements present in the compound.

Importance of Qualitative Analysis of Organic Compounds

The identification and analysis of unknown organic compounds make a very important aspect of experimental organic chemistry. A systematic approach based on the scheme helps in fetching good results as there is no definite set of procedures which can be applied for all.

General Qualitative Analysis Scheme

  1. Preliminary Tests: In these tests, one can note all the physical characteristics of the compound that includes solid, liquid, colour or odour. Also, performing an ignition test by heating a small amount of metal spatula helps in determining whether the compound is aromatic or aliphatic as aliphatic compounds produce luminous flame and aromatic compounds produce a sooty flame.

  2. Physical Properties Detection can be made by determining the boiling point or melting point. In the case of liquids, it’s better to go for distillation that serves the dual purpose for knowing the boiling point and also the purification of liquid that helps in performing subsequent tests.

  3. Analysis of Elements present in the compound can be performed with the help of various methods.

  4. Solubility Tests: The test is performed using various reagents such as dil. HCl, dil. NaOH, sodium bicarbonate and hot or cold water. Useful information can be obtained by observing the compound’s solubility.

  5. Group Classification Tests: Various tests can help in deducing the functional groups in the unknown compound.

Different Tests for Detection of Elements

  1. Detection of Carbon and Hydrogen

Carbon and Hydrogen can be detected by heating the compound with copper oxide in a dry test tube. These are then oxidised to carbon dioxide and water respectively where the former turns lime water milky and water turns anhydrous copper sulphate blue in colour. Thus, it confirms the presence of carbon and hydrogen. 

  1. Detection of Phosphorus

For determining the phosphorus, the organic compound is heated with an oxidising agent that helps in oxidising the phosphorus present in it to phosphate. Thereafter, the solution is boiled with concentrated HNO3 and is then treated with ammonium molybdate. Presence of phosphorus is then confirmed with a yellow precipitate.

The reactions involved in the process of estimation of phosphorus methods are as follows:

Na3PO4 + 3HNO3 → H3PO4 + 3NaNO3

H3PO4 + 12(NH4)2MoO4 + 21HNO3 → (NH4)3PO4.12MoO3 + 21NH4NO3 + 12H2O

  1. Estimation of Nitrogen

  • Dumas Method

The estimation of Nitrogen is done with the help of the Dumas method where a known mass or weight of the compound is heated with copper oxide (CuO) in the carbon dioxide atmosphere. This yields free Nitrogen along with carbon dioxide and water.

The reaction is as follows:

CxHyNz + (2x+ 0.5y) CuO → xCO2 + 0.5y H2O + 0.5z (N2) + (2x+ 0.5y)Cu

When the gases are passed over a hot copper gauge, it converts trace amounts of nitrogen oxides to dinitrogen. The gaseous mixture is collected over KOH solution which absorbs carbon dioxide. Nitrogen is then collected in the upper part of the graduated tube.

  • Kjeldahl Method

In this method, a known mass of an organic compound, for example, 0.5 g is mixed with 10 g of potassium sulphate and 1 g of copper sulphate with 25 ml of sulphuric acid and is heated in a Kjeldah’s flask. Here, copper sulphate acts as a catalyst and potassium sulphate raises the boiling point of concentrated sulphuric acid. Further, Nitrogen in the compound quantitatively converts to Ammonium sulphate. The resultant mixture reacts with an excess of sodium hydroxide and Ammonia evolves. It is then passed through an excess volume of standard acid.

The acid which doesn’t react is left out which is estimated by titration with some standard alkali. This way, the percentage of Nitrogen can be calculated. 

Various reactions involved in the estimation of Nitrogen by Kjeldahl method are as follows:

C + H + S → CO2 + H2O + SO2

N → (NH4)2SO4

(NH4)2SO4 + 2NaOH → Na2SO4 + 2NH3 + 2H2O

2NH3 + H2SO4 → (NH4)2SO4

FAQs (Frequently Asked Questions)

Q1. What is Qualitative Analysis in Chemistry?

Ans: Qualitative analysis of organic compounds is done to detect the presence of various elements such as oxygen, nitrogen, sulphur and halogen elements. Carbon and hydrogen are generally detected by heating the given compound with cupric oxide. During this process, carbon gets oxidised to carbon dioxide and hydrogen gets converted to water. Carbon dioxide can further be tested by lime water test and water through anhydrous copper sulphate test.

Q2. How is the Test of Halogens Done for its Detection in an Organic Compound?

Ans: Just like the test of phosphorus and other elements present in organic compounds, a test for halogens can be done. It is performed by acidifying a portion of the fusion solution with dilute nitric acid. Further, excess of silver nitrate solution is added where a precipitate formed indicates the halogen presence. This is the test of halogen applicable if nitrogen and sulphur are not present in the compound. There is a different test applicable to the test of halogens when nitrogen or/and sulphur are present. During this, cyanide and sulphide ions both interfere with this kind of reaction by the formation of silver cyanide and silver sulphide precipitates. For the removal of cyanide and sulfide ions, we need to acidify the fusion solution with dilute nitric acid.