Chirality, Racemisation and Optical Activity of Haloalkanes (For CBSE, ICSE, IAS, NET, NRA 2022)

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Title: Chirality, Racemisation and Optical Activity of Haloalkanes

  • Racemisation refers to the process of converting an optically active (d- or /-) compound into the racemic modification. This article focuses on stereochemical aspects of nucleophilic substitution reaction. Here, the terms will be explained first and then their relationship with haloalkanes will be explained.
  • Racemisation is defined as “a process by which an optically active substance either dextro or levorotatory is directly converted into a racemate” . Resolution is the method when a racemic modification is divided into its constituent enantiomers. Racemisation is a thermodynamically favourable method and it proceeds spontaneously if a suitable pathway is accessible for the interconversion of the enantiomers.

Racemisation

  • Racemic is a sample of a chiral compound that consists of a 1: 1 mixture of enantiomers is known as racemic. The racemization is to become racemic. A sample whose enantiomeric excess decreases in the course of a reaction (S) , or a stereocenter whose configurational purity decreases in the course of a reaction (A) .
  • The process of racemization involves the conversion of half of its dextro form into levo form so that the mixture is optically inactive due to the presence of equal amounts of the two enantiomers. Racemization can be brought about by
    • Action of heat – Often optically active enantiomer changes into a racemic mixture by the action of heat.
    • The action of chemical reagents – It can also be brought by the presence of foreign substances.
    • Antiracialization – Also takes place by merely keeping the substance at room temperature for some time.

Chirality

  • Chirality is defined as “an object which is asymmetric and cannot be superimposed over its mirror image is known as chiral or stereocenter” . This property is known as chirality.
    • For example- our Hand, legs etc.
  • The object which is symmetric in nature and can be superimposed over its mirror image is known as achiral.
    • For example- cube, cone etc.
Example for Chirality

Example for Chirality

  • This chirality behaviour is observed in lots of organic compounds also. The chirality is due to the three dimensional or spatial arrangements of molecules. Dutch scientist, J. Van՚t Hoff and a French scientist, C. Le Bel in the year 1874, showed that the spatial arrangement of four groups (valences) around a central carbon is tetrahedral and if all the substituents attached to that carbon are different; such carbon is called asymmetric carbon or stereocentre.
  • The resulting molecule would lack symmetry and is referred to as asymmetric molecule. The asymmetry of the molecule or chirality is responsible for the optical activity in such organic compounds.

Optical Activity

  • Optical activity of an organic compound refers to the property of an organic compound by the virtue of which, it rotates the plane polarised light (produced by passing ordinary light through Nicol prism) when it is passed through their solutions and the compounds are known as optically active compounds.
  • Optical activity of an organic compound refers to the property of an organic compound by the virtue of which, it rotates the plane polarized light (produced by passing ordinary light through Nicol prism) when it is passed through their solutions and the compounds are known as optically active compounds.
  • He also observed that a mixture of both, a racemic mixture (or a racemic modification) , did not rotate light because the optical activity of one molecule cancelled out the effects of the other molecule. Pasteur was the first to explain the presence of chiral molecules.

The Optical Activity is of Two Types

  • Dextrorotatory or the d-form Dextrorotatory in Greek means right rotating, if the compound rotates the plane polarized light to the right, i.e.. , clockwise direction, it is called dextrorotatory or the d-form and is indicated by placing a positive (+) sign before the degree of rotation.
  • Laevorotatory or the l-form Laevorotatory in Greek means left rotating if the compound rotates the plane polarized light to the left, i.e.. , anticlockwise direction, it is called Laevorotatory or the l-form and a negative (–) sign are placed before the degree of rotation.
  • The angle by which the plane polarized light is rotated is measured by an instrument called polarimeter.
  • Such (+) and (–) isomers of a compound are called optical isomers and the phenomenon is termed as optical isomerism.

Measuring Optical Activity

  • Optical activity is measured by a polarimeter and depends on several factors,
    • concentration of the sample
    • temperature
    • length of the sample tube or cell and
    • the wavelength of the light passing through the sample.
  • Rotation is given in degrees, based on whether the sample contains (positive) or l- (negative) enantiomers.
  • The normal rotation calculation for a particular chemical compound is called a particular rotation, defined as an angle measurement with a path length of 1 decimeter and a concentration of 1 g/ml. The specific rotation of a pure substance is an inherent property of the substance.

Question

What is Racemization and Racemic Mixture?

Answer:

Racemization is a process in which optically active compounds (consisting of a single enantiomer) are converted into an equal mixture of enantiomers with zero optical activity (a racemic mix) . The rate of racemization depends on the molecule and conditions such as pH and temperature.

What is the Difference between Optically Active and Inactive?

Answer:

If the light moves to the right, it is called to show optical activity of the dextrorotatory type and if it moves to the left, it is said to show optical activity of the Laevorotatory type. And if the light band goes across the lens of Nicol, unnoticed. Optically it is assumed to be inactive.

Is a Racemic Mixture Optically Active?

Answer:

A solution in which both enantiomers of a compound are present in equal amounts is called a racemic mixture, or racemate. Since enantiomers have equal and opposite specific rotations, a racemic mixture exhibits no optical activity.

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