Class 11 Chemistry Organic Chemistry Methods of purification of organic compounds

Methods of purification of organic compounds:

The various methods involved in the purification are:

  • Sublimation
  • Crystallisation
  • Distillation
  • Differential extraction
  • Chromatography

Sublimation: It is the process in which solid directly changes to gaseous form on heating and on cooling the gaseous state convert in to solid state.

The idea behind using this technique is to separate substance,that show sublimation from substance that do not sublime.

Crystallisation: This method is based on the difference in the solubility of compound and the impurities in the suitable solvent.

  • The impure compound in dissolved in a solvent in which it is sparingly soluble at room temperature, but appreciably soluble at higher temperature.
  • The solution is concentrated to get saturated solution.
  • On cooling the solution pure compound crystallizes out and is removed by filtration.
  • The filtrate contains impurities and small quantity of compound .

Distillation : This method is used to separate volatile liquids from non volatile impurities and also the liquids that have difference in their boiling points .

  • The liquids with different boiling point vaporise at different temperatures .Then these vapours are cooled and the liquids formed are collected separately.

Like chloroform and aniline can be easily separated by this technique as both have different boiling points.

For this technique:

(i) Take the mixture in a distillation flask. Fit the flask with a thermometer.

(ii) Arrange the apparatus as shown in the figure.

(iii) Heat the mixture slowly keeping a close watch at the thermometer and observe what happens.The vapours of one component with lower boiling point can be seen rising up in the distillation flask with the increase in temperature.

 (ii) These vapours get condensed in the condenser and can be collected (as pure liquid distillate) from the condenser outlet.

Fractional distillation: This method is used for those liquids which have nearly same boiling points that is the difference in their boiling point is not much .So, as they have same boiling points. Therefore ,liquids from both liquids condense and formed at same time.

  • In this technique ,vapours of liquid mixture are passed through a fractionating column before condensation .This fractionating column is fitted over round bottom flask.
  • Vapours of liquid with higher boiling point condense before the vapours of liquid with lower boiling point.
  • On reaching the top, the vapours become pure in low boiling component and passed through the condenser. The pure liquid is collected in receiver .Each successive condensation and vaporisation unit in the fractionating column is called theoretical plate. Example: separation of different components of crude oil

Distillation under reduced pressure: This method is used to purify liquids having very high boiling points and those which decompose at or below their boiling points.

  • Such liquids are made to boil at a temperature lower than their boiling points, by reducing the pressure on their surface. A liquid boils at a temperature, at which its vapour pressure is equal to external pressure .The pressure is reduced with the help of water pump. Example glycerol can be separated from spent-lye by this method

Steam distillation: This technique is applied to separate substances which are steam volatile and are immiscible in water.

  • In steam distillation steam from a steam generator is passed through a heated flask containing the liquid to be distilled.
  • The mixture of steam and the volatile organic compound is condensed and collected. Then compound is later separated from water using separating funnel.
  • In it the liquid boils ,when sum of vapour pressures due to organic liquid and due to water becomes equal to the atmospheric pressure.

 i.e. =P1 +p2

 

  • Since p1 is lower than p, the organic liquid vaporises at lower temperature than its boiling point.

Example: Aniline and water can be separated by this method

Differential extraction: When the organic compound is present in an aqueous medium, then it is separated by shaking it with an organic solvent in which it is more soluble than in water .

  • The organic solvent and the aqueous solution should be immiscible with each other .So, that they form two distinct layers which can be separated with separating funnel.
  • The organic solvent is later removed by distillation or by evaporation.

Chromatography: It is a technique used to separate mixtures in to their components, purify. Compounds which can be solid or liquid and also test the purity of compounds.

  • In this technique the mixture of substances are applied on to a stationary phase .Then a pure solvent or a mixture of solvents is allowed to move slowly over stationary phase .The components of mixture gets gradually separated from one another .The moving phase is mobile phase .

 The chromatography is classified into two categories:

  • Adsorption chromatography
  • Partition chromatography

Adsorption chromatography: It is based on the facts that different compounds are adsorbed on adsorbent at different degrees.

The commonly used adsorbents are silica gel and alumina.

  • When a mobile phase is allowed to move over a stationary phase the components of mixture move by varying distances over a stationary phase.

Depending upon differential adsorption the two types of techniques are:

  • Column chromatography
  • Thin layer chromatography

Column chromatography: It involves separation of mixture over a column of adsorbent packed in a glass tube.

  • The column is fitted with a stop cock at its lower end .The mixture adsorbed on adsorbent is placed on the top of the adsorbent column packed in a glass tube.
  • An appropriate eluant which is liquid or a mixture of liquids is allowed to flow down the column slowly.
  • Depending upon the degree to which the compounds are adsorbed complete separation takes place.
  • The most readily adsorbed substance is retained near the top and the other comes down to various distances in the column.

Thin layer chromatography: This is another type of adsorption which involves separation of substances of mixture over a thin layer of adsorbent coated on glass tube.

  • A thin layer of adsorbent is spread over a glass plate of suitable size. The plate is known as thin layer chromatography plate.
  • The solution of mixture to be separated is applied as a small spot about 2 cm above one end of the TLC plate.
  • Then glass plate is then placed in a closed jar containing the eluant.
  • As the eluant rises up ,the plate the different components of mixture move along with eluant to different distances depending on their degree of adsorption .Hence, separation takes place .
  • The relative adsorption of each component of mixture is expressed in terms of retardation factor (Rf).
  • The spots of the compounds are visible on TLC plate due to their original color. The spots which are not visible to eye but fluoresce in ultraviolet light can be detected by putting the plate under ultra violet light.

Position chromatography: It is based on the continuous differential partitioning of components of a mixture between stationary and mobile phases.

  • Paper chromatography: Is a type of partition chromatography .In this a special paper called chromatography paper is used .It contains water trapped in it which acts as a stationary phase.

 (i)In this we take a thin strip of filter paper (25cm x 5cm approx).

(ii) Draw a line on it using a pencil, approximately 3cm above the lower edge.

(iii) Put a small drop of solution of mixture .Lets say ink from a sketch pen or fountain pen) at the centre of the line. Let the ink dry.

(iv) Attach the paper strip on the thread with the help of cello tape.

(v) Lower the filter paper strip into a large size gas jar in such a way that the drop of ink on the paper is just above the water level as shown in the figure [2.8 (b)]. Adjust the thread and fix it on the sides of gas jar with the help of cello tape.

(vi)Cover the gas jar with a lid and leave it undisturbed.

(vii) Watch carefully as the water rises up on the filter paper.

(viii) Remove the filter paper strip dry it and observe.

The paper retains different components according to their differing partition in the two phases .the paper strip is called chromatogram. The spots of separated coloured compounds are visible at different heights from the position of initial spot.

Detection of carbon and hydrogen:

They are detected by heating the compound wit copper oxide. Carbon present in the compound is oxidised to Carbon-dioxide (tested with Lime water) and Hydrogen to Water(tested with anhydrous Copper sulphate ).

Class_11_Concepts_Of_Chemistry_Detecting_Of_Carbon_&_Hydrogen

Detection of other elements : Nitrogen ,Sulphur, Halogen ,Phosphorous presence in organic compound are detected by Lasagne’s test .The elements present in the compound are converted into covalent form in to an ionic form by fusing the compound with Sodium metal .

Class_11_Concepts_Of_Chemistry_Detecting_Of_Elements

Test for Nitrogen : The Sodium fusion extract is boiled with Ferrous Sulphate and then acidified with concentrated Sulphuric acid . The formation of Prussian Blue colour confirms the presence of Nitrogen .

Class_11_Concepts_Of_Chemistry_Test_Of_Nitrogen

Test for Sulphur : The Sodium fusion extract is acidified with Acetic acid with Lead acetate is added to it .A Black precipitate of Lead Sulphide indicates the presence of Sulphur.

S2-    +   Pb2+ --> PbS

Sulphide Lead    Lead Sulphide

On treating Sodium fusion extract with Sodium Nitro Prusside, the Violet colour appears that confirms Sulphur .

Class_11_Concepts_Of_Chemistry_Test_Of_Sulphur

Test for halogens: The Sodium fusion extract is acidified with Nitric acid and then treated with Silver Nitrate.

  • A White precipitate soluble in Ammonium hydroxide shows the presence of Chlorine.
  • A Yellow precipitate soluble in Ammonium hydroxide shows the presence of Bromine.
  • A Yellow precipitate not soluble in Ammonium hydroxide shows the presence of Iodine.
  • X-  +     Ag+ --> AgX
  • Halide Silver ion   SilverHalide

 Test for phosphorous: The compound is heated with Sodium Peroxide. The Phosphorus gets oxidised to Phosphate .Then the solution is boiled with Nitric acid and then treated with Ammonium mol bate .A Yellow precipitate indicates phosphorous.

Quantitative analysis

  • Carbon and hydrogen: Both of them are estimated in one experiment. A known mass of Organic compound is burnt in presence of excess of Oxygen and Copper (ii)oxide . Carbon and Hydrogen both are oxidised to form Carbon dioxide and Water.

 Class_11_Concepts_Of_Chemistry_Quatitative_Analysis

The mass of Water produced is determined by passing a mixture through U-tube containing anhydrous Calcium chloride. Carbon dioxide is absorbed in another U-tube containing solution of Potassium hydroxide. The increase in mass of calcium chloride and Potassium hydroxide gives the amount of Carbon dioxide and water .From it the percentage of C and H can be calculated.

  Let the mass of organic compound = mg

     Mass of water = m1

     Mass of carbodioxide =m2

    % of C = (12 x m2 x 100)/(44x)m

    % of H = (2 x m1 x 100)/(18x)m

      Nitrogen : There are two methods for estimation of Nitrogen :

  • Dumas method
  • Kjeldahl’s method

Dumas method: In this Nitrogen containing compound is heated with Copper oxide in an atmosphere of Carbon dioxide,yields free Nitrogen in addition to Carbon dioxide and water .

Class_11_Concepts_Of_Chemistry_Dumas_Method

The Nitrogen oxides formed are reduced to Nitrogen by passing the gaseous mixture over a heated Copper gauze. The mixture of gases so produced is collected over aqueous Potassium hydroxide which absorbs Carbon dioxide. Nitrogen is collected in the upper part of graduated tube.

 Volume of N at STP = (P1V1 x 273)/(760 x T1)

Kjeldahl’s method: The compound containing Nitrogen is heated with concentrated Sulphuric acid . The Nitrogen in the compound gets converted to Ammonium sulphate. The resulting acid mixture is then heated with excess Sodium hydroxide .The liberated Ammonia gas is absorbed in excess of standard solution of Sulphuric acid. The amount of Ammonia produced is determined by estimating the amount of Sulphuric acid consumed in the reaction.

Class_11_Concepts_Of_Chemistry_Kjeldahl's_Method

  • Halogens : For them we have Carious method. A known mass of organic compound is heated with fuming Nitric acid in the presence of Silver nitrate contained in a hard glass tube known as Carious tube in a furnace.

The Carbon and Hydrogen present in compound are oxidised to Carbon dioxide and water. The Halogens present forms the Silver halide .It is filtered, washed, dried and weighed.

          Let the mass of organic compound = mg

         Mass of AgX = m1 g

         1 mole of AgX contains 1 mole of X

          % of Halogen = (Atomic mass of X x m1 x 100)/(molecular mass of AgX) x m

  • Sulphur: A known mass of organic compound is heated in Carious tube with Sodium peroxide or fuming Nitric acid. Sulphur present is oxidised to Sulphuric acid .It is precipitated with Barium sulphate by adding excess of Barium chloride solution in water .The precipitate is filtered ,washed and dried and weighed .

         Let the mass of organic compound = mg

          Let the mass of ammonium phosphor molybate = m1 g

          % of Sulphur = (32 x m1 x 100)/(233) x m 

  •  Phosphorous: A known mass of organic compound is heated with fuming Nitric acid. As a result Phosphorous gets oxidised to Phosphoric acid. It is then precipitated with Ammonium molybdate .

          Let the mass of organic compound = mg

          Let the mass of ammonium phosphor molybate = m1 g

          % of Phosphorous = (62 x m1 x 100)/(222) x m 

  • Oxygen: A definite mass of organic compound is decomposed in stream of Nitrogen gas .The mixture of gaseous products containing oxygen is passed over red hot coke when all the oxygen present is converted into Carbon monoxide .Then this mixture is passed over Iodine pent oxide, when CO is oxidised to Carbon dioxide producing Iodine.

 

 

 

 

 

 

 

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