Chirality, Racemisation and Optical activity of Haloalkanes

Introduction

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.

Chirality

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.

Chirality, Racemisation and Optical activity of Haloalkanes

This chirality behavior 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 a 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.

Racemisation

Racemic is a sample of a chiral compound that consists of a 1:1 mixture of enantiomers is known as racemic. 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).

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.

The optical activity is of two types:

  1. Dextrorotatory or the d-form
    Dextrorotatory in Greek means right rotating, if the compound rotates the plane polarised 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.
  1. Laevorotatory or the l-form
    Laevorotatory in Greek means left rotating if the compound rotates the plane polarised light to the left, i.e., anticlockwise direction, it is called laevorotatory or the l-form and a negative (–) sign is 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.

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Practise This Question

Which of the following terms are used to describe a compound which rotates light in the clockwise direction?