NEET General Organic Chemistry Purification And Characteristics Of Organic Compounds Notes

Purification And Characteristics Of Organic Compounds

Purification Of Organic Compounds: The methods which are employed for the purification of organic compounds or separation of individual components of a mixture containing two or more compounds are summarized below.

Read And Learn More: NEET General Organic Chemistry Notes, Question And Answers

Crystallisation Of Organic Compounds: The method is based on the difference in the solubilities of the compound and the impurities in a suitable solvent. Pure compound crystallises out from the solution and highly soluble impurities remain in the solution. Impure organic compounds like glucose, urea, cinnamic acid, etc are purified. Fractional crystallisation is used for the separation of a mixture of two compounds which are soluble in the same solvent but to a different extent. For example, Separation of sugar and salt.

Sublimation Of Organic Compounds: The process of direct conversion of a solid into the gaseous state on heating without passing through the intervening liquid state and vice versa on cooling is known as sublimation.

Only those substances whose vapour pressure becomes equal to the atmospheric pressure much before their respective melting points are capable of undergoing sublimation. There exists an equilibrium between the solid and its vapours. Camphor, naphthalene, anthracene, iodine, benzoic acid, salicylic acid, NH₂Cl, HgCl₂, etc. are purified by sublimation.

Distillation Of Organic Compounds: Distillation is a method used to separate constituents of a liquid mixture which differ in their boiling points.

Distillation is a process which involves two steps:

1. Vapourisation: Liquid is converted into vapours.
2. Condensation: Vapours are condensed again into liquid.

Depending upon the difference in the boiling points of the constituent liquids, different types of distillation methods are employed.

Simple distillation Of Organic Compounds: Simple distillation is applied only for volatile liquids which boil without decomposition at atmospheric pressure and contain non-volatile impurities. This method can also be used for separating liquids having sufficient differences in their boiling points. For example,

1. Benzene (boiling point 353 K) and aniline (boiling point 475 K)
2. Chloroform (boiling point 334 K) and aniline (boiling point 457 K)
3. Ether (boiling point 308 K) and toluene (boiling point 383 K)

Nitrobenzene prepared in the laboratory can also be purified by distillation.

Fractional Distillation of Organic Compounds: This method is used for the separation of two or more volatile liquids from a liquid mixture which has boiling points close to each other. Liquids forming a constant boiling mixture (azeotropic mixture) such as rectified spirit cannot be separated by this method. Fractional distillation is used these days in industries, especially, in the distillation of petroleum, coal tar and crude alcohol. A mixture of methanol (boiling point 338 K) and propanone (boiling point 330 K) or a mixture of benzene and toluene may be separated by fractional distillation.

Distillation under reduced pressure (Vacuum distillation) Of Organic Compounds: The compounds, which decompose at a temperature below their normal boiling points, cannot be purified by distillation under ordinary atmospheric pressure. Glycerine is one such compound which decomposes at its boiling point. The pressure is reduced by suction pump and the distillation is carried out at lower temperature as glycerine can be distilled at 453 K (normal boiling point 563 K) under a pressure of 10-12 mm. Cane juice can also be concentrated by this method. This technique can be used to separate glycerol from spent lye in soap industry.

Steam distillation Of Organic Compounds: This method is used to purify the organic compounds which

• Are volatile in steam but are immiscible with water.
• Possess high vapour pressure at the boiling point of water.
• Contain non-volatile impurities.

The compound to be purified is distilled with steam and impurities being non-volatile remains in mother liquor. For example, o-nitrophenol (volatile) and p-nitrophenol (non-volatile) are separated by this method.

Differential extraction (or solvent extraction) Of Organic Compounds: The process of separation of an organic compound (solid or liquid) from its aqueous solution by shaking with a suitable organic solvent is termed as solvent extraction. This method is employed for non-volatile compounds. For example, benzoic acid is extracted from its aqueous solution using benzene as solvent.

Chromatography

Chromatography Definition:

Chromatography is the technique of separating the components of a mixture in which the separation is achieved by the differential movement of individual components through a stationary phase under the influence of a mobile phase.

Depending upon the nature of the stationary phase (either a solid or a liquid tightly bound on a solid support) and the nature of the mobile phase (either a liquid or a gas), different types of chromatographic techniques are followed.

The various components on the developed TLC plate are identified through their retardation factor, i.e., R_f values.

⇒ \(\mathrm{R}_{\mathrm{f}}=\frac{\text { distance moved by the substance from base line }}{\text { distance moved by the solvent from base line }}\)

A component with highest value of R_f elute first. A component with greater tendency to adsorb on solid has lesser the R_f value.

Qualitative Analysis (Detection Of Elements)

The qualitative analysis of an organic compound involves the detection of all the elements present in it. Carbon and hydrogen are generally present in all organic compounds. Other elements which may be present in organic compounds are oxygen, nitrogen, sulphur, halogens, phosphorus, etc. These elements are detected by the following tests.

Detection of Carbon and Hydrogen by Copper oxide test

Copper Oxide Test Principle Organic compounds undergo oxidation in the presence of a suitable oxidizing agent. In this process, carbon is oxidized to CO₂ and hydrogen is oxidized to water.

Copper Oxide Test Procedure The compound is intimately mixed with dry cupric oxide. The mixture is strongly heated in a hard glass test tube fitted with a cork and a delivery tube. The liberated gases are passed into lime water.

⇒ \(\underset{\text { organic compound }}{[\mathrm{C}]}+2 \mathrm{CuO} \stackrel{\text { heat }}{\longrightarrow} \mathrm{CO}_2+2 \mathrm{Cu}\)

⇒ \(\mathrm{CO}_2+\underset{\text { lime water }}{\mathrm{Ca}(\mathrm{OH})_2} \longrightarrow \underset{\text { (milky) }}{\mathrm{CaCO}_3}+\mathrm{H}_2 \mathrm{O}\)

⇒ \(\underset{\text { organic compound }}{[2 \mathrm{H}]}+\mathrm{CuO} \stackrel{\text { heat }}{\longrightarrow} \mathrm{H}_2 \mathrm{O}+\mathrm{Cu}\)

⇒ \(\underset{\text { (white) }}{\mathrm{CuSO}_4}+5 \mathrm{H}_2 \mathrm{O} \longrightarrow \underset{\text { (blue) }}{\mathrm{CuSO}_4 \cdot 5 \mathrm{H}_2 \mathrm{O}}\)

Detection of Nitrogen, Halogen and Sulphur by Lassaigne’s test

Detection of Nitrogen, Halogen and Sulphur by Lassaigne’s test Principle This is the most reliable test for the detection of nitrogen, sulphur, halogens etc. The given organic compound is fused with dry, metallic sodium. In this process, carbon and nitrogen present in the organic compounds get converted into cyanide ion (CN^–) sulphur (S) into sulphide ion S^2- and halogens (X^–) into halide ion (X“). The product is extracted into water. The solution contains sodium salts (NaCN, Na₂S and NaX).

Lassaigne’s Test For Nitrogen: A few crystals of ferrous sulphate are added to the first part of the filtrate. The mixture is boiled and cooled. It is acidified with hydrochloric acid and a few drops of ferric chloride solution are added. Sodium cyanide in the filtrate reacts with ferrous sulphate to give sodium ferrocyanide. It further reacts with ferric chloride to give a blue coloured solution of ferric ferrocyanide.

⇒ \([\mathrm{C}, \mathrm{N}]+\mathrm{Na} \stackrel{\text { fuse }}{\longrightarrow} \mathrm{NaCN}\)

⇒ \(\mathrm{FeSO}_4+2 \mathrm{NaCN} \longrightarrow \mathrm{Fe}(\mathrm{CN})_2+\mathrm{Na}_2 \mathrm{SO}_4\)

⇒ \(\mathrm{Fe}(\mathrm{CN})_2+4 \mathrm{NaCN} \longrightarrow \underset{\text { sodium ferrocyanide }}{\mathrm{Na}_4\left[\mathrm{Fe}(\mathrm{CN})_6\right]}\)

⇒ \(3 \mathrm{Na}_4\left[\mathrm{Fe}(\mathrm{CN})_6\right]+4 \mathrm{FeCl}_3 \longrightarrow \mathrm{Fe}_4\left[\mathrm{Fe}(\mathrm{CN})_6\right]_3+12 \mathrm{NaCl}\)

If an organic compound contains nitrogen and sulphur, sodium thiocyanate is formed in the sodium extract which gives blood red colouration with ferric chloride due to the formation of ferric thiocyanate.

⇒ \(\underset{\text { organic compound }}{[\mathrm{C}, \mathrm{N}, \mathrm{S}]}+\mathrm{Na} \stackrel{\text { fuse }}{\longrightarrow} \mathrm{NaCNS}\)

⇒ \(\mathrm{NaCNS}+\mathrm{FeCl}_3 \longrightarrow \mathrm{Fe}(\mathrm{CNS}) \mathrm{Cl}_2+\mathrm{NaCl}\)

Lassaigne’s Test For Sulphur

1. Lead acetate test: Second part of the filtrate is treated with excess of acetic acid and lead acetate solution. A black precipitate of lead sulphide is formed.

⇒ \([\mathrm{S}]+2 \mathrm{Na} \stackrel{\text { fuse }}{\longrightarrow} \mathrm{Na}_2 \mathrm{~S}\)

⇒ \(\mathrm{Na}_2 \mathrm{~S}+\mathrm{Pb}\left(\mathrm{CH}_3 \mathrm{COO}\right)_2 \longrightarrow 2 \mathrm{CH}_3 \mathrm{COONa}+\underset{\text { black precipitate }}{\mathrm{PbS} \downarrow}\)

2. Sodium nitroprusside test: To the sodium fusion extract, a few drops of sodium nitroprusside are added. The appearance of violet colour indicates the presence of sulphur.

⇒  \(\mathrm{Na}_2 \mathrm{~S}+\mathrm{Na}_2\left[\mathrm{Fe}(\mathrm{CN})_5 \mathrm{NO}\right] \longrightarrow \underset{\text { sodium sulpho nitroprusside (violet colour) }}{\mathrm{Na}_4\left[\mathrm{Fe}(\mathrm{CN})_5 \mathrm{NOS}\right]}\)

Lassaigne’s Test For Halogens

1. Silver nitrate test A portion of the stock solution is boiled with dil. HN03, cooled and silver nitrate is added. A white precipitate soluble in ammonium hydroxide shows the presence of chlorine.A pale yellow precipitate slightly soluble in ammonium hydroxide shows the presence of bromine.A yellow precipitate insoluble in ammonium hydroxide shows the presence of iodine.

⇒ \(\mathrm{NaCl}+\mathrm{AgNO}_3 \longrightarrow \underset{\text { white }}{\mathrm{AgCl} \downarrow}+\mathrm{NaNO}_3\)

⇒ \(\mathrm{AgCl}+\underset{\text { ammonium hydroxide }}{2 \mathrm{NH}_4 \mathrm{OH}} \longrightarrow \underset{\text { diamine silver (I) chloride (soluble) }}{\mathrm{Ag}\left(\mathrm{NH}_3\right)_2 \mathrm{Cl}}+2 \mathrm{H}_2 \mathrm{O}\)

⇒  \(\mathrm{NaBr}+\mathrm{AgNO}_3 \longrightarrow \underset{\text { pale yellow }}{\mathrm{AgBr} \downarrow}+\mathrm{NaNO}_3\)

⇒ \(\mathrm{NaI}+\mathrm{AgNO}_3 \longrightarrow \underset{\text { yellow }}{\mathrm{AgI} \downarrow}+\mathrm{NaNO}_3 \\\)

If nitrogen sulphur or both are found to be present in the organic compound, the sodium extract must be boiled with dilute nitric acid to expel hydrogen cyanide or hydrogen sulphide. If cyanide and sulphide ions are not removed, they interfere with the test forming precipitate of AgCN (white) and Ag₂S (black).

2. Beilstein’s test: Beilstein’s test is used to detect the halogen in an organic compound. A copper wire is heated in a Bunsen flame till no colour is imparted to the flame. The copper wire is dipped in the given organic compound and exposed to the non-luminous zone of the Bunsen flame. A bluish-green-coloured flame indicates the presence of halogen.

Lassaigne’s Test For Phosphorus: Organic compound containing phosphrous is fused with sodium peroxide. The phosphorus of the compound is oxidised to phosphate. The fused mass is extracted with water and filtered. The filtrate containing sodium phosphate is boiled with nitric acid and then treated with ammonium molybdate. A yellow solution or precipitate indicates the presence of phosphorus.

⇒ \(\mathrm{Na}_3 \mathrm{PO}_4+3 \mathrm{HNO}_3 \rightarrow \mathrm{H}_3 \mathrm{PO}_4+3 \mathrm{NaNO}_3\)

⇒ \(\mathrm{H}_3 \mathrm{PO}_4+12\left(\mathrm{NH}_4\right)_2 \mathrm{MoO}_4+2 \mathrm{HNO}_3 \rightarrow\left(\mathrm{NH}_4\right)_3 \mathrm{PO}_4 12 \mathrm{MoO}_3+21 \mathrm{NH}_4 \mathrm{NO}_3+12 \mathrm{H}_2 \mathrm{O}\)