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Purification And Characterisation Of Organic Compounds

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Purification and Characterization of Organic Compounds PDF Notes, Important Questions and Synopsis

SYNOPSIS

  • Classification of organic compounds:
  • Purification of organic compounds:

  • Filtration: It is the technique used to separate an insoluble solid component of the mixture from a soluble component in a given solvent. 

  • Recrystallisation: This method is based on the differences in the solubility of the organic compound and its impurities in a suitable solvent. 

  • Simple distillation: This method is used for the purification of liquids which boils without decomposition and contain non-volatile impurities. 

  • Fractional distillation: Method is used to separate a mixture of two or more miscible liquids which have boiling points close to each other. Distillation is carried out by using fractionating columns. 
    Distillation under reduced pressure or vacuum distillation: Certain liquids have a tendency to decompose at a temperature below their boiling points. Under reduced pressure, the liquid will boil at a low temperature and the temperature of decomposition will not be reached. 

  • Steam distillation: This method is used for the separation and purification of a liquid which is appreciably volatile in steam from non-volatile components of a mixture.
    Differential extraction: This method is used to separate a given organic compound present in aqueous solutions by shaking with a suitable organic solvent in which the compound is more soluble than water. The basic requirement of the organic solvent is that it should be immiscible with water so that the organic and water layers can be easily separated. 

  • Chromatography: The technique of chromatography is based on the difference in the rates at which the components of a mixture move through a porous medium (called stationary phase) under the influence of some solvent or gas (called moving or mobile phase).

    Based on the principle involved, chromatography is classified as

    (a) Adsorption chromatography: It is based on the fact that different compounds are adsorbed on an adsorbent to different degrees. When a mobile phase is allowed to move over a stationary phase (adsorbent), the components of the mixture move by varying distances over the stationary phase because of the different adsorption tendencies.

    (i) Column chromatography: In this method, a suitable adsorbent such as alumina (Al2O3), silica (SiO2) or Fuller’s earth is packed as a column in a burette-like long tube and acts as a stationary phase. The mixture to be separated is dissolved in a suitable solvent, and the solution is poured on the top of the column of the adsorbent. The component which is adsorbed strongly gets adsorbed at the top.

    (ii) Thin layer chromatography: This method involves separation of a mixture of substances over a thin layer of an adsorbent coated on a glass plate. After drying the glass plate is placed in a closed jar containing eluant. As the eluant rises up the plate, the components of the mixture move up along with the eluant to different distances depending on their degree of adsorption and separation takes place.

    (b) Partition chromatography: This method is based on differences in tendencies of substances to distribute or partition between the stationary phase and mobile phase. Paper chromatography is a type of partition chromatography in which the mixture to be separated is applied in the form of a drop on the paper. This paper is then suspended in a suitable solvent or mixture of solvents. The solvent rises up by capillary action. Paper selectively retains different components according to their differing partition in two phases.

  • Qualitative Analysis of Organic Compounds: Analysis involving detection of all elements present in an organic compound.

  • Qualitative analysis (Detection of elements):

    1. Detection of Carbon and hydrogen by copper oxide test:
    2. Detection of Nitrogen, halogen and sulphur by Lassaigne’s test:



      • Detection of carbon and hydrogen: Carbon and hydrogen are detected by heating the compound with copper (II) oxide. Carbon present in the compound is oxidised to carbon dioxide and hydrogen to water. Carbon dioxide is tested with lime water. Carbon dioxide makes lime water milky and develops turbidity. Water is tested with anhydrous copper sulphate, which becomes blue on absorbing moisture.

        begin mathsize 12px style table attributes columnalign left end attributes row cell straight C text   end text plus text   end text 2 CuO text    end text rightwards arrow with straight capital delta on top text   end text 2 Cu text   end text plus text   end text CO subscript 2 end cell row cell 2 straight H text   end text plus text   end text CuO text   end text rightwards arrow with straight capital delta on top text   end text Cu text   end text plus text   end text straight H subscript 2 straight O end cell row cell CO subscript 2 text   end text plus text   end text Ca open parentheses OH close parentheses subscript 2 text   end text rightwards arrow with straight capital delta on top text   end text CaCO subscript 3 text   end text plus text   end text straight H subscript 2 straight O end cell row cell 5 straight H subscript 2 straight O text   end text plus text   end text stack CuSO subscript 4 with white below text   end text rightwards arrow with straight capital delta on top text   end text stack CuSO subscript 4.5 straight H subscript 2 straight O with Blue below end cell end table end style

                                                 

      • Detection of nitrogen: The sodium fusion extract (prepared by boiling fused mass of compound and sodium metal in distill water) is boiled with FeSO(iron (II) sulphate) and then acidified with concentrated sulphuric acid. The formation of prussian-blue colour confirms the presence of nitrogen.
        begin mathsize 12px style table attributes columnalign left end attributes row cell 6 CN to the power of minus text   end text plus text   end text Fe to the power of 2 plus end exponent text   end text rightwards arrow with blank on top text   end text open square brackets Fe left parenthesis CN right parenthesis subscript 6 close square brackets to the power of 4 minus end exponent end cell row cell 3 open square brackets Fe left parenthesis CN right parenthesis subscript 6 close square brackets to the power of 4 minus end exponent text   end text rightwards arrow with xH subscript 2 straight O on top text   end text stack Fe subscript 4 left square bracket Fe left parenthesis CN right parenthesis subscript 6 right square bracket subscript 3. xH subscript 2 straight O with Prussian text    end text blue below end cell end table end style

                                  

      • Detection of sulphur: Two tests can be performed for detection of sulphur.

      (a)   On adding lead acetate to acidified sodium fusion extract, the formation of black precipitate confirms the presence of sulphur.

                                                     begin mathsize 12px style straight S to the power of 2 minus straight thin space end exponent plus straight thin space Pb to the power of 2 plus end exponent straight thin space rightwards arrow with blank on top straight thin space PbS with Black below end style

      (b)  On adding sodium nitroprusside to acidified sodium fusion extract, appearance of violet colour indicates the presence of sulphur.

                                  begin mathsize 12px style straight S to the power of 2 minus straight thin space end exponent plus     left square bracket Fe left parenthesis CN right parenthesis subscript 5 NO right square bracket to the power of 2 minus end exponent straight thin space rightwards arrow with blank on top straight thin space stack left square bracket Fe left parenthesis CN right parenthesis subscript 5 NOS right square bracket to the power of 4 minus end exponent with Violet below end style

      • Detection of halogens: On adding a silver nitrate solution to sodium fusion extract acidified with nitric acid white precipitate, soluble in ammonium hydroxide shows the presence of chlorine, a yellowish precipitate, sparingly soluble in ammonium hydroxide shows the presence of bromine and a yellow precipitate, insoluble in ammonium hydroxide shows the presence of iodine.

      begin mathsize 12px style straight X to the power of minus text   end text plus text   end text Ag to the power of plus text   end text rightwards arrow with blank on top text   end text AgX text         end text left parenthesis straight X equals Cl comma text   end text Br comma text   end text straight I right parenthesis end style 

      • Detection of phosphorous: Compound containing phosphorous is heated with an oxidizing agent (sodium peroxide) which oxidises phosphorus to phosphate. The solution is boiled with nitric acid and then treated with ammonium molybdate. A yellow colouration or precipitate indicates the presence of phosphorus.

        begin mathsize 12px style table attributes columnalign left end attributes row cell text                                                           end text Na subscript 3 PO subscript 4 text   end text plus text   end text 3 HNO subscript 3 text   end text rightwards arrow with blank on top text   end text straight H subscript 3 PO subscript 4 text   end text plus text   end text 3 NaNO subscript 3 end cell row cell straight H subscript 3 PO subscript 4 text   end text plus stack text   end text 12 text   end text left parenthesis NH subscript 4 right parenthesis subscript 2 MoO subscript 4 with table attributes columnalign left end attributes row Ammonium row molybdate end table below text   end text plus text   end text 21 HNO subscript 3 text    end text rightwards arrow with blank on top text    end text stack left parenthesis NH subscript 4 right parenthesis subscript 3 PO subscript 4.12 MoO subscript 3 with table attributes columnalign left end attributes row cell text             end text Ammonium end cell row phosphomolybdate end table below text   end text plus text   end text 21 text   end text NH subscript 4 NO subscript 3 text   end text plus text   end text 12 straight H subscript 2 straight O end cell end table end style

  • Quantitative Analysis of Organic Compounds: It is the analysis that involves the determination of percentages of various elements present in a given compound.

  • Quantitative analysis:

  • Relation between Empirical and molecular formula:

  • Determination of Empirical and molecular formula:

Principle involved in estimation of various elements

  •  Carbon and hydrogen: A known mass of the given dry organic compound is heated strongly with dry cupric oxide in an atmosphere of air or oxygen free from CO2. The carbon and hydrogen of the organic compounds are oxidized to CO2 and water vapour. Carbon dioxide produced is collected in potash bulbs (containing KOH solution) while vapour are absorbed in anhydrous calcium chloride tubes. From the amounts of carbon dioxide and water vapour produced, the percentage of carbon and hydrogen can be calculated.

                                 begin mathsize 12px style straight C subscript straight x straight H subscript straight y text   end text plus text   end text open parentheses text   end text straight x plus straight y divided by 4 close parentheses straight O subscript 2 text    end text rightwards arrow with blank on top text    end text xCO subscript 2 text   end text plus text    end text open parentheses straight y divided by 2 close parentheses straight H subscript 2 straight O end style

  • Nitrogen

Duma’s method: A known mass of an organic compound is heated with dry cupric oxide in an atmosphere of carbon dioxide. If any oxide of nitrogen is produced during this process, then it is reduced to nitrogen by passing over heated copper gauze. The gaseous mixture is collected over an aqueous solution of KOH, when all the gases except nitrogen are absorbed. The volume of nitrogen produced is measured at room temperature and atmospheric pressure.

 begin mathsize 12px style table attributes columnalign left end attributes row cell text                       end text straight C subscript straight x straight H subscript straight y straight N subscript straight z text   end text plus text   end text open parentheses 2 straight x plus straight y divided by 2 close parentheses text   end text CuO text   end text end cell row cell text                                                          end text downwards arrow end cell row cell xCO subscript 2 text    end text plus text     end text straight y divided by 2 straight H subscript 2 straight O text    end text plus text    end text straight z divided by 2 straight N subscript 2 text    end text plus text    end text open parentheses 2 straight x plus straight y divided by 2 close parentheses Cu end cell end table end style

Kjeldahl’s method: A known mass of the organic compound is heated with concentrated H2SO4 so that nitrogen is quantitatively converted into ammonium sulphate. The resulting solution is then heated with excess of sodium hydroxide. The ammonia gas evolved is passed into a known but excess volume of standard acid (HCl or H2SO4). The acid left unused is estimated by titrating the solution with standard alkali. From the amount of acid left unused, the amount of acid used for neutralization of ammonia can be calculated.

 begin mathsize 12px style table attributes columnalign left end attributes row cell Organic text   end text compound text   end text plus text   end text straight H subscript 2 SO subscript 4 text   end text rightwards arrow with blank on top open parentheses text   end text NH subscript 4 close parentheses subscript 2 SO subscript 4 end cell row cell text                    end text rightwards arrow with 2 NaOH on top text   end text Na subscript 2 SO subscript 4 text   end text plus text   end text 2 NH subscript 3 text   end text plus text   end text 2 straight H subscript 2 straight O end cell row cell 2 NH subscript 3 text   end text plus text   end text straight H subscript 2 SO subscript 4 text   end text rightwards arrow with blank on top text   end text open parentheses NH subscript 4 close parentheses subscript 2 SO subscript 4 end cell end table end style

  • Halogens (Carius method): A known mass of the organic substance is heated with fuming HNO3 in a Carius tube. The silver halide obtained is separated, washed, dried and weighed. From the weight of silver halide formed, the percentage of halogen can be calculated.

                                                         begin mathsize 12px style thin space straight X        +     AgNO subscript straight 3 superscript blank        rightwards arrow with blank on top      AgX end style

  • Sulphur (Carius method): A known mass of the organic compound is heated with sodium peroxide or fuming HNO3 in a Carius tube when sulphur is quantitatively converted into sulphuric acid. It is then precipitated with barium chloride as barium sulphate.

                                                           begin mathsize 12px style table attributes columnalign left end attributes row cell text         S+H end text subscript text 2 end text end subscript text O+3O end text rightwards arrow with text HNO end text subscript text 3 end text end subscript on top text H end text subscript text 2 end text end subscript text SO end text subscript text 4 end text end subscript end cell row blank row cell text H end text subscript text 2 end text end subscript text SO end text subscript text 4 end text end subscript text   end text plus text  BaCl end text subscript text 2 end text end subscript text   end text downwards arrow rightwards arrow with blank on top text   BaSO end text subscript text 4 end text end subscript text    end text plus text   end text 2 text HCl end text end cell end table end style

  • Phosphorus: The phosphorus in the compound is oxidized to phosphoric acid on heating with fuming nitric acid. It is treated with magnesia mixture. A precipitate of magnesium ammonium phosphate MgNH4PO4 is formed. This is filtered, washed, dried and ignited to given magnesium pyrophosphate (Mg2P2O7)

    begin mathsize 12px style table attributes columnalign left end attributes row cell stack text 2P end text with table attributes columnalign left end attributes row cell text In  organic end text end cell row cell text compound end text end cell end table below text   + 5 end text open square brackets text O end text close square brackets text    end text rightwards arrow with text heat end text on top text  P end text subscript text 2 end text end subscript text O end text subscript text 5 end text end subscript end cell row cell text P end text subscript text 2 end text end subscript text O end text subscript text 5 end text end subscript text  + 3H end text subscript text 2 end text end subscript text O  end text rightwards arrow with text heat end text on top text    end text stack text H end text subscript text 3 end text end subscript text PO end text subscript text 4 end text end subscript with table attributes columnalign left end attributes row cell text Phosphoric end text end cell row cell text acid end text end cell end table below end cell row cell text H end text subscript text 3 end text end subscript text PO end text subscript text 4 end text end subscript text  +  end text stack open parentheses text NH end text subscript text 4 end text end subscript text Cl + MgCl end text subscript text 2 end text end subscript close parentheses with text Magnesia  mixture end text below rightwards arrow with blank on top text   end text stack text MgNH end text subscript text 4 end text end subscript text PO end text subscript text 4 end text end subscript with text Whiteppt. end text below end cell row cell text MgNH end text subscript text 4 end text end subscript text PO end text subscript text 4 end text end subscript text   end text rightwards arrow with text Heat end text on top text   end text stack text Mg end text subscript text 2 end text end subscript text P end text subscript text 2 end text end subscript text O end text subscript text 7 end text end subscript text   end text with text Whiteppt end text below text + 2NH end text subscript text 3 end text end subscript end cell end table end style
 Phosphorus:A known mass of an organic compound is heated with fuming nitric acid. Due to this phosphorus present in the compound is oxidised to phosphoric acid. It is precipitated as ammonium phosphomolybdate, (NH4)3PO4.12MoO3, by adding ammonia and ammonium molybdate. This is filtered, washed, dried and weighed.


begin mathsize 12px style table attributes columnalign left end attributes row cell text H end text subscript text 3 end text end subscript text PO end text subscript text 4 end text end subscript text  +  end text stack text 12 end text open parentheses text NH end text subscript text 4 end text end subscript close parentheses subscript text 2 end text end subscript text MoO end text subscript text 4 end text end subscript with text Ammonium   Molybdate end text below text  + 21HNO end text subscript text 3 end text end subscript end cell row cell text                                                     end text downwards arrow text Heat end text end cell row cell stack open parentheses text NH end text subscript text 4 end text end subscript close parentheses subscript text 2 end text end subscript text PO end text subscript text 4 end text end subscript text .12MoO end text subscript text 3 end text end subscript with table attributes columnalign left end attributes row cell text Ammonium  phosphate  Molybdate end text end cell row cell text                                    Yellow  ppt. end text end cell end table below text  + 21NH end text subscript text 4 end text end subscript text NO end text subscript text 3 end text end subscript text  + 12 H end text subscript text 2 end text end subscript text O end text end cell end table end style

Oxygen: A definite amount of an organic compound is decomposed by heating in a stream of nitrogen gas. The mixture of gaseous products containing oxygen is passed over red hot coke when oxygen gets converted to carbon monoxide. The mixture is then passed through warm iodine pentoxide (I2O5) when carbon monoxide is oxidised to carbon dioxide producing
iodine. The percentage of oxygen can be calculated from the amount of carbon dioxide or iodine produced. 
begin mathsize 12px style table attributes columnalign left end attributes row cell text Compound  end text rightwards arrow with text Heat end text on top text  O end text subscript text 2 end text end subscript text  +  other   gaseous   products end text end cell row cell text 2C + O end text subscript text 2 end text end subscript text    end text rightwards arrow with text 1373K end text on top text  2CO end text end cell row cell text I end text subscript text 2 end text end subscript text O end text subscript text 5 end text end subscript text  5CO  end text rightwards arrow with blank on top text  I end text subscript text 2 end text end subscript text  +  5CO end text subscript text 2 end text end subscript end cell end table end style

Oxygen is generally estimated by subtracting the sum of the percentage of all other elements in the compound from 100.

 

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