Nomenclature and General Principles: Qualitative Analysis of Organic Compounds (For CBSE, ICSE, IAS, NET, NRA 2022)

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Stereoisomerism is exhibited by the compounds which have the same structural formula i.e.. their atoms are connected in the same order, but they differ from each other in the way these atoms (or groups) are arranged in space.

Geometrical Isomerism

Consider example of 2-butene as shown below:

Geometrical Isomerism
  • A cis- isomer is the one having identical groups on same side of double bond.
  • A trans-isomer has identical groups on opposite side.
  • The above two structures, cis-2- butene (two – groups on the same side) and trans-2-butene (two – groups on different sides) are two geometrical isomers (stereoisomers) .
  • They differ in the geometries of the groups around the double bond.

Optical Isomerism

  • The optical isomers have identical physical properties except optical activity.
  • They rotate the plane of plain polarized light in opposite directions.
  • The plane polarized light is defined as the light that vibrates in one plane only.
  • The rotation of plane of polarized light is called optical activity.
  • These substance that can rotate the plane of polarized light are said to be optically active.
  • Compound that rotate the plane of plane polarized light to the right (clockwise) are said to be dextrorotatory and denoted by ‘d’ or (+) before the name of the compound.
  • Compounds that rotate the plane to the left (anticlockwise) are called levorotatory and denoted by placing ‘l’ or (-) before the name of the compound.
  • A mixture containing equal amounts of d- and l- isomers is called a racemic mixture and is optically inactive.
  • The optical isomerism is shown by the compounds having at least one carbon atom joined to four different atoms or groups.
  • For example, 1,2-dihydroxypropane
Optical Isomerism

Qualitative Analysis of Organic Compounds

Detection of C and H

  • and are detected by heating the compound with (copper oxide) in a dry test tube.
  • They are oxidized to and , respectively.
  • turn lime water milky and turns anhydrous to hydrated CuSO4 which is blue in colour.

Detection of C and H

Detection of Other Elements

N, S, halogens, and phosphorus present in an organic compound are detected by Lassaigne՚s test, by fusing the compound with sodium metal, which convert the elements present in the compound from covalent to ionic form.

C, N, S, and X come from organic compound.

Test for Nitrogen

The sodium fusion extract or L. E. is boiled with and then acidified with conc. .

The appearance of Prussian blue colour confirms the presence of N.

Ferriferous Cyanide (Prussian Blue)

This test is not given by compounds which do not contain C atoms but contain N atoms.

This test is also not given by diazonium salts (e. g. ) , although they contain both C and N elements.

If S is present along with N, the appearance of blood red colour confirms the presence of both.

Ferric thiocyanate (Blood-red Colour)

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