Chemistry

Purification And Characterisation of Organic Compounds - 1

1. Introduction — necessity of purification.2. Methods of purification of organic

compound.(a) Crystallisation(b) Fractional crystallisation(c) Sublimation(d) Distillation(e) Fractional distillation(f) Steam distillation(g)Vacuum distillation(h)Differential extraction(i) Chromatography

3. Detection of elements (Qualitative analysis)

(a) Detection of carbon(b) Detection of hydrogen(c) Detection of nitrogen(d) Detection of sulphur(e) Detection of halogen(f) Detection of oxygen

• Filtration• Crystallization• Sublimation• Distillation• Differential extraction• Chromatography

Only one of the compoundsis soluble in the givensolventeg.(i) Urea and Naphthalene(ii) Benzoic acid and anthracene

Most common method for purification of solid organic compounds.

Impurities and organic compound have different solubilities in the given solvent.

The organic compound should be insoluble or sparingly soluble in the solvent at room temperature but readily soluble in hot conditions.

The impurities should be insoluble in the solvent even in hot conditions.

The organic compound should not react with the solvent.

The mixture of benzoic acid (water soluble) and naphthalene (water insoluble) purified by.

How will you separate mixture of benzoic acid and napthalene ?

benzoic acid (water soluble ) and napthalene (water insoluble)

Difference in solubilities of organic compounds and impurities are very less.

eg. urea and copper sulphate.

Used to separate volatile organic compounds from non volatile impurities e.g. Naphthalene, benzoic acid, anthracene, camphor.

A mixture of benzene and anilinecan be separated by(a) alcohol(b) NaOH (c) HCl(d) Hot water

Amines are basic in nature. Therefore, aniline reacts with HCl to form a salt which dissolves in water.

Hence, aniline can be separated from benzene by using HCl.

Hence, answer is (c).

(i) Simple distillation- difference in boiling points of compounds is more than 40ºC. e.g.– chloroform(b. p. 334K) and aniline(b. p. 457K).

(ii) Fractional distillation- difference in boiling points of compounds is less than 40ºC. e.g. – acetone (b. p. 329K) and methyl alcohol(b. p. 338K).

(iii) Vacuum distillation- used for organic compounds which decompose at or below their boiling points.e.g. Glycerol.

(iv) Steam distillation- used for organic compounds which are immiscible with water and are steam volatile. e.g. Aniline.

The most suitable method for theseparation of mixture of ortho andpara nitrophenol in the ratio of 1:1is(a) distillation(b) crystallisation (c) vapourisation(d) colour spectrum

The boiling point of o-nitrophenol is 210oC and that of p-nitrophenol is 245oC.

Therefore, vapours of o-nitrophenol will be formed first (lower boiling point) and collected seperately and vapours of p-nitrophenol will be formed latter.

Hence, answer is (a).

Used to extract pure organic compounds from their aqueous solution by shaking with organic solvent in which they arehighly soluble.

Eg. Benzoic acid from its aqueous solution using benzene.

• Used to purify small samples.• Based on selective adsorption or partition between stationary and mobile phase.• Column Chromatography- based on adsorption, used for bulk quantities.• Thin layer Chromatography- based on adsorption, used for quantitative analysis.

• Paper Chromatography- based on partition and used for quantitative and qualitative analysis.

• Gas Chromatography- used to separate and analyse volatile organic liquids.

• High performance liquids Chromatography (HPLC)- used for qualitative and quantitative analysis of organic compounds.

1. Boiling point.

2. Melting point.

3. Mixed melting point.

Detection of C, H, N, halogens P, S, and oxygen.

Lassaigne’s extract is prepared toconvert covalency of organic compound into electrovalency by fusing with Na.

Fe(CN)6 Na4

Fe 4 Fe(C N ) 6

3

Organic compound Lassaignes’s extract

Fused with sodium (Na)

(has NaCN)sodium cyanide

Few drops of NaOH

and freshly prepared FeSO 4 Sodium ferrocyanide

Ferric ferrocyanideprusslain blue

colouration confirms presence of nitrogenin organic compounds

Excess of HCland FeCl 3(ferric chloride)

Organic compound Heated over

Copper wireGreen flame confirms the presenceof halogen/s in the organic compound

Beilstein's test

Organic compound Lassaignes extract

Fused with sodium (has covalent bondswith carbon and halogens)

(has NaXsodium halide)

dil. HNO3

and AgNO3

White ppt ofsilver chloride(AgCl) soluble inammonia confirmsthe presence ofchlorine inorganic compound

Pale yellow pptofsilver bromide(AgBr) sparinglysoluble inammoniaconfirms thepersence ofbromine in organiccompound

Yellow ppt ofsilver iodide(Agl)insolublein ammoniaconfirms thepresence ofiodine in organiccompound

boiled with nitric acid(HNO3) and addedammonium molybdate[(NH4)2MoO4]

Organic compound Phosphorus present in organic compoundchange to phosphate (Na3PO4)

With Na2O(sodium peroxide)

Yellow ppt ofammoniumphosphomolybdate[(NH4)3PO4.12MoO3]confirms the presenceof phosphorus inorganic compound

Organic compound fused with mixture of

Potassium nitrite (KNO3)and sodium carbonate

Sulphur present in organic compound changes into sulphate

Extracted withwater, filteredand added dil. HClBaCl2

white ppt ofbarium sulphate(BaSO4) confirmsthe presence ofsulphur in organic compound

When acetic acid and lead acetatesolutions are added to sodiumextract, black percipitate is formed.It shows that organic compound has_____ element.(a) chlorine(b) oxygen(c) phosphorus(d) sulphur

Hence, answer is (d).

Sodium extract + CH3COOH + (CH3COO)2Pb

Black ppt(PbS)

It shows the presence of sulphur in organic compound.

Organic compound Lassaignes's extract

Fused with sodium (has Na2Ssodium shulphide)

To one portion add acetic acid (CH3COOH) and lead acetate[Pb(CH3COO)2]

Divide the extract in two parts

Black ppt. of lead sulphide(PbS) confirms the presenceof sulphur in organic compound

To other portion add freshlyprepared sodiumnitroprussideNa2[Fe(CN)5(NO)]

Violet colourationof Na4[Fe(CN)5NOS]confirms the presenceof sulphur in organiccompound

Presence of oxygen in organic compound is detected by testing for functional group containing oxygen eg- alcohol (–OH), aldehyde (–CHO), ketone (RCOR), carboxylic acid (–COOH), ester (–COOR) and nitro (–NO2).

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