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Chemical Reactions Of Alkyl Halides: Nucleophilic Substitution-Stereochemical Aspect - I

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Chemical Reactions Of Alkyl Halides: Nucleophilic Substitution-Stereochemical Aspect - I - Lesson Summary

Elementary concepts of stereo chemical principles and notations:

  • plane polarised light
  • asymmetry
  • chirality
  • racemisation
  • Inversion of configuration and retention of configuration.


Plane polarised light and optical activity:

Ordinary light is light vibrating in all directions through the line of propagation.

When ordinary light is passed through a lens made of Polaroid, it produces a light that vibrates in only one plane called plane polarised light.

When plane polarised light is passed through a solution of certain compounds, they rotate the plane polarised light in a specific direction. Such compounds are called optically active compounds.

If a substance rotates plane polarised light to the right or clockwise, then it is called dextrorotatory or d-form. It is indicated by placing a + sign before the degree of rotation.

If a substance rotates plane polarised light to the left angle or counter-clockwise direction then the compound is said to be laevorotatory or l-form. It is indicted by placing a - sign before the degree of rotation.

EX: lactic acid rotates plane polarised light to the right, and hence, is known as dextrorotatory lactic acid or (+)-lactic acid.

2-methyl -1-butanol rotates plane polarised light to the left, and it is known as laevorotatory 2-methyl 1-butanol or (-) 2-methyl 1-butanol.

The plus or minus isomers of a compound are called optical isomers, and the phenomenon is known as optical isomerism.

Asymmetry:

If all the four substituents or groups attached to the central carbon atom are different, then such a carbon is said to be an asymmetric carbon or stereo centre. The resulting molecule is said to be an asymmetric molecule. This asymmetry of the molecule is responsible for optical activity.

EX: A sphere & a coneanda cube are identical to their mirror images and are super-imposable.

Many objects are non-super-imposable on their mirror images.

EX: The left hand and the right hand are mirror images of each other, but they do not super-impose, and hence, are said to be non-super-imposable.

Objects that are non-super-imposable on their mirror images are said to be chiral, and this property is known as chirality. Objects that are super-imposable on their mirror images are called achiral.

Chiral molecules may contain one are more asymmetric carbons or stereocentres. An asymmetric carbon is represented with a star on it.

Consider two simple molecules, propan-2-ol and butan-2-ol and their mirror images

Propan-2-oldoesnot contain an asymmetric centre, the four groups attached to the tetrahedral carbon are not all different. Thus, it is an achiral molecule.

Butan-2-ol contains four different groups attached to the tetrahedral carbon. It is an asymmetric molecule, and is chiral and optically active.

Enantiomers:

Non-super-imposable mirror image isomers are known as enantiomers. Enantiomers possess identical physical properties like melting points, boiling points, solubility and refractive index.

If a mixture contains equimolar proportions of enantiomers, then it shows zero optical rotation. Such a mixture is known as racemic mixture.

It is represented with the symbol dl or (±) before the name of the compound.

The process of the formation of an equimolar proportion of an enantiomericmixture from an optically pure compound is known as raceimisation.

Inversion of configuration and retention of configuration:

Retention of configuration is the preservation of integrity of the spatial arrangement of bonds to an asymmetric centre during a chemical transformation.

Inversion of configuration in a reaction yields a product whose configuration is opposite to that of the reactant or the substrate.

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