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 racemisation 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. Racemisation 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.
  • Autoracemisation Also takes place by merely keeping the substance at room temperature for some time.

Racemisation via Carbocation Formation

In this process, a carbocation, which is stabilized by resonance, is formed by the elimination of an electron withdrawing group. The carbocation has a planar structure which undergoes recombination with anion. This recombination can take place from either side equally resulting in a 1:1 mixture enantiomers which means racemisation. Allylic, benzylic and tertiary carbocations are good examples for this sort of racemization.

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.

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.

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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 of the substance is a measure of the ability of the substance to rotate the polarization plane if the solution of the substance is placed in the path of plane-polarized light. For this work, the polarimeter of Perkin-Elmer Model 141 with a Sodium lamp of 584 nm wavelength (D-line) and fitted with a digital counter was used to calculate the amount of degrees of rotation of the plane-polarized light at room temperature by 8.3 percent concentration of each L-, D-and DL-alanine solution.

Initially, the French scientist Jean-Baptiste Biot detected optical phenomena. He concluded that the change in direction of the plane-polarized light, when moving through such objects, was simply a rotation of light and had a molecular origin. His dissertation was influenced by an observation by Louis Pasteur. Pasteur observed the presence of two crystals which were mirror images of tartaric acid, an acid present in wine. Through meticulous experimentation, he discovered that one group of molecules rotated the clockwise polarized light while the other group rotated the clockwise light to the same degree.

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 canceled 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:

  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 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
  • wavelength of the light passing through the sample.

Rotation is given in + /- degrees, based on whether the sample contains d-(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 1g / ml. The specific rotation of a pure substance is an inherent property of the substance.

Frequently Asked Questions – FAQs

What is Racemisation and racemic mixture?

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 rachemisation depends on the molecule and conditions such as pH and temperature.

How do you separate a racemic mixture?

By reacting them with an optically active reagent, you can separate the constituents of a racemic mixture. The product is a diastereomer compound. Diastereomers possess various physical properties. Use ordinary isolation methods like fractional crystallization you can isolate them.

Is Achiral optically active?

Rotation direction and magnitude depend on the nature of the electron cloud, so it is reasonable that two identical molecules with identical electron clouds rotate light exactly the same way. Accordingly, achiral molecules do not show optical activity.

What is the difference between optically active and inactive?

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.

How is a racemate formed?

Racemic mixtures are often formed by converting achiral substances into chiral ones. This is because chirality can only be discerned in a chiral environment. In an achiral environment an achiral substance has no preference for forming one enantiomer over another.

What is optical isomerism with example?

Isomers are molecules of the same chemical composition but with a different configuration of atoms in vacuum. Unless the structure in space makes the two isomers non-superimposable mirror images of each other, we call them optical isomers or enantiomers. An example of this is the amino acid alanine.

What molecules have optical activity?

Once techniques have been developed to determine the three-dimensional structure of a molecule, the source of the optical activity of the substance has been identified: compounds that are optically active contain molecules that are chiral. Chirality is the property of a molecule that is the result of its structure.

What is the difference between optically active and inactive?

If the light moves to the right, it is called to show the dextrorotatory type of optical activity, and if it moves to the left, it is said to show the laevorotatory type of optical activity. And as the band of light goes through the lens of Nicol, it is unchanged. It’s claimed to be optically inert.

How does optical activity depend on state of substance?

Many compounds are present under every aggregation state; this means that their optical behavior is determined by the properties of the individual molecules. The specific rotation depends not only on the type of substance, but also on factors such as the state of aggregation, temperature , pressure and type of solvent.

What is optical activity of a medium?

Optical activity, the ability of a substance to rotate the plane of polarization of a light beam that passes through it. Since the specific rotation depends on the temperature and the wavelength of the light, these quantities must also be specified.

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