Notes on Qualitative Analysis of Organic Compounds | Grade 11 > Chemistry > Fundamental Principles of Organic Chemistry | KULLABS.COM

Qualitative Analysis of Organic Compounds

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Organic compounds are mostly composed up of the elements like carbon, hydrogen, oxygen, nitrogen, halogens, sulphur, and rarely phosphorous. Out of these elements, C and H are generally found in organic compounds. Elements other than H and C present in the organic compound is referred as foreign elements (or hetero elements). Examples are N, S, P, and halogens.

Detection of foreign elements

⇒Detection of carbon and hydrogen:

Principle: When the sample of an organic compound and a cupric oxide is heated in hard glass test-tube, carbon gets oxidized to carbon dioxide which turns lime water milky and hydrogen gets oxidized to water, which turns anhydrous copper sulphate into blue color. The following reaction takes place:

$$ 2CuO + C \xrightarrow {\Delta} 2Cu + CO_2$$

$$ CO_2 + Ca(OH)_2 \xrightarrow {} CaCO_3 + H_2$$

$$CuO +2H \xrightarrow {\Delta} Cu + H_2O$$

$$5H_2O + CuSO_4(white) \xrightarrow {} CuSO_4.5H_2O$$

Procedure: When the mixture of organic compound and a cupric oxide is heated in hard glass test tube, we get CO2 , which turns lime water milky. It shows the presence of CO2 gas and moisture turns anhydrous CuSO4 into blue color showing the presence of water,

⇒Lassaigne's test:

J.L.Lassaigne in 1843 developed a very simple and effective method for the detection of some foreign elements in the organic compounds.

  • PRINCIPLE OF LASSAIGNE'S TEST

Generally, organic compounds are covalent compounds and are insoluble in water. Hence, to turn them into ionic compounds (soluble in water), organic compounds are fused with highly reactive sodium metal and we get the following compounds like:

$$Na + C+ N \xrightarrow {\Delta} NaCN$$

$$2Na + S \xrightarrow {\Delta} Na_2S$$

$$Na+ C+N+S \xrightarrow {\Delta} NaCNS$$

$$Na + X \xrightarrow {\Delta} NaX (X = Cl, Br, I) $$

Preparation of sodium extract

A small piece of metal soaked inside the filter paper is melted in a fusion tube. When sodium metal fuses, cools in air, a pinch of the organic compound is added and the fusion tube is further heated strongly till brick-red-hot. The red hot tube is plunged into distilled water in a mortar. The content is grinded with a pestle and filtered. Thus, obtained filtrate is known as sodium or Lassaigne's extract. Sodium extract is usually alkaline in nature because sodium reacts with water to give sodium hydroxide.

$$2Na + 2H_2O \xrightarrow {} 2NaOH + H_2 $$

  • Detection of Nitrogen :

Few cc of sodium extract is taken and a little solution of sodium hydroxide is added. If the solution is not alkaline, freshly prepared ferrous sulphate solution is added. Then the solution is boiled, cooled in tap water, acidified with dilute or conc. HCl and at last few cc of ferric chloride solution is added. The formation of ferric ferrocyanide, which has a Prussian blue or green colouration confirms the presence of nitrogen in the organic compound.The following reaction takes place:

$$Na+C+N \xrightarrow {\Delta} NaCN$$

$$2Na + 2H_2O \xrightarrow {} 2NaOH + H_2$$

$$ FeSO_4 + 2NaOH \xrightarrow {} Fe(OH)_2\downarrow+ Na_2SO_4$$

$$ Fe(OH)_2 + 6NaCN \xrightarrow {} Na_4[Fe(CN)_6] + 2NaOH$$

$$3Na_4[Fe(CN)_6] + 4FeCl_3 \xrightarrow {} Fe_4[Fe(CN)_6]_3 + 12NaCl$$

  • Detection of sulfur :

When sodium extract is treated with sodium nitroprusside solution, we get violet or purple color due to the formation of tetrasodium thionitroprusside compound. This is also called Sodium Nitroprusside Test.The following reaction takes place:

$$ Na + S\xrightarrow {\Delta} Na_2S$$

$$Na_2S+Na_2[Fe(CN)_5NO] \xrightarrow {} Na_4[Fe(CN)_5NOS] (voilet\;or\; purple\; color) $$

When sodium extrct is acidified with acetic acid and is treated with lead acetate, we get black precipitate of lead sulphide, it shows the presence of sulphur. This is also called Lead acetate test.The following reaction takes place:

$$ Na + S\xrightarrow {\Delta} Na_2S$$

$$ Na_2S + (CH_3COO)_2Pb \xrightarrow {} 2CH_3COONa + PbS \downarrow (Black\;ppt.)$$

  • Detection of both Nitrogen and sulfur :

When sodium extract is treated with ferric chloride solution, we get blood-red color of ferric sulphocyanide. The following reaction takes place:

$$ Na+ C+ N+S \xrightarrow {\Delta} NaCNS(Thiocyanate)$$

$$ 3NaCNS + FeCl_3 \xrightarrow {} Fe(CNS)_3(Blood\;red) + 3NaCl$$

  • Detection of halogens :

When sodium extract is boiled with conc. HNO3 (to overcome CN-, S--), cooled and silver nitrate solution is added, we get different type of precipitaitons.The following reaction takes place:

$$Na+X \xrightarrow {\Delta} NaX (X= C, Br, I)$$

$$AgNO_3 + Cl \xrightarrow {} AgCl (white\;ppt. \downarrow) + NO_3$$

$$AgNO_3 + Br \xrightarrow {} AgBr (pale\;yellow\;ppt. \downarrow)+ NO_3$$

$$AgNO_3 + I\xrightarrow {} AgI (deep\;yellow\;ppt. \downarrow)+ NaNO_3$$

Sodium extract is treated with conc. HNO3 before the test of halides because sodium extract also contains CN-, S-- ions except halides. To overcome these radicals in the form of HCN and H2S gas, sodium extract is boiled with conc. nitric acid.

$$ AgCN^- +HNO_3 \xrightarrow {} HCN \uparrow + AgNO_3$$

$$Ag_2S + 2HNO_3 \xrightarrow {} H_2S \uparrow +2AgNO_3$$

Reference:

Adhikari, Rameshwar; Khanal, Santosh; Subba , Bimala; Adhikari, Santosh; Khatiwada, Shankar Pd. Universal Chemistry XI. First. Vol. 1st. Kathmandu: Oasis Publication, 2069.

Chaudhary, Ganga Ram; Karna, Shila Kant Lal; Sharma, Kanchan; Singh, Sanjay; Gupta, Dipak Kumar. A Textbook of Higher Secondary Chemistry XI. Ed. 2nd. Kathmandu: Vidyarthi Pustak Bhandar, 2069 (2012).



  • Organic compounds are mostly composed up of the elements like carbon, hydrogen, oxygen, nitrogen, halogens, sulphur, and rarely phosphorous. Out of these elements, C and H are generally found in organic compounds. Elements other than H and C present in the organic compound is referred as foreign elements (or hetero elements). Examples are N, S, P, and halogens.
  • Detection of carbon and hydrogen
  • J.L.Lassaigne in 1843 developed a very simple and effective method for the detection of some foreign elements in the organic compounds.
  • Generally, organic compounds are covalent compounds and are insoluble in water. Hence, to turn them into ionic compounds (soluble in water), organic compounds are fused with highly reactive sodium metal .
  • Preparation of sodium extract.
  • Detection of Nitrogen.
  • Detection of sulfur.
  • Detection of both Nitrogen and sulfur.
  • Detection of halogens.
.

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