Why Is Chirality Important in Chemical Reactions?
Racemisation is a process of converting an optically active compound such as dextro of levorotatory into a direct racemic modification. It is a thermodynamically favourable process that acts spontaneously if it gets a proper pathway that is accessible for enantiomers' interconversion.
What is Racemisation?
A mixture of equal quantity of d-isomer as well as l-isomer, one can get a 'racemic mixture' and the said procedure is known by the name of racemisation. For example, on mixing equal amounts of l-tartaric acid as well as d-tartaric acid, you get racemic tartaric acid. This resulting mixture is also known to be an optically inactive combination. Also, the racemisation process also revolves around converting half the amount of dextro into the levo in order to get an optically inactive mixture because of the presence of two enantiomers and that too equal quantity.
Racemisation can be attained through:
Action of Chemical Reagents: This can be brought about by foreign substances.
Heat Action: Heat can easily convert an optically active enantiomer into the said racemic mixture.
Process of Auto Racemisation: This process takes place by getting the substance at room temperature.
Formation of Racemic Modification
The racemic modification can be formed through these different methods:
Mixing
Intimate mixing is the perfect and obvious process of getting a racemic modification. Equal amounts of laevorotatory (-) and dextrorotatory (+) isomers leads to this result. This process of mixing relates to the process of blending as a racemic modification is a representation of a random state of affairs rather than unrelated enantiomers.
Racemisation
Racemisation is a procedure of racemic modification production initiated with pure enantiomers. As these two enantiomers possess the same free energy quotient, the equilibrium mixture will lead to a 50-50 mixture i.e. racemic modification. Different racemisation equilibrium processes include thermal racemisation, anion formation, cation formation, Walden inversion, and reversible formation of inactive intermediaries.
Synthesis
Synthesis of dissymmetric molecules, initiating from the racemic modification's symmetric molecule while making use of optically active reagent as well as no asymmetric physical influence always results in the racemic modification of the two enantiomeric types of product molecules.
Carbocation Formation via Racemisation
During this procedure, the carbocation which gets stabilized through resonance gets formed by eliminating an electron-withdrawing group. The carbocation that possesses a planar structure then undergoes reunion with anion. This reunion takes place resulting in 1:1 mixture enantiomers which are known as racemisation. Some good examples of this type of racemisation are benzylic, tertiary carbocations, and allylic.
Optical Activity
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The optical activity of the said organic compound implies the organic compound's property which rotates a plane-polarized light when it gets passed through different solutions. These compounds are known to be optically active compounds. It implies that the substance's optical activity is basically the measurement of the substances' ability to rotate the polarization plane when the said substance gets in the plane-polarised light's path.
The optical phenomenon was invented by French scientist Jean-Baptiste Biot. He inferred that the change in the plane-polarised light's direction when it is moving through different objects was a result of light's rotation having a molecular origin. He was influenced by Louis Pasteur's observation. Pasteur witnessed two crystals that were known to be the tartaric acid's mirror images. Tartaric acid is basically an acid that is present in the wine. He then inferred that a molecule group led to the rotation of clockwise polarized light while another group resulted in the rotation of clockwise light.
The Optical Activity is of Two Different Types:
Laevorotatory or the l-Form: When a particular compound leads to the rotation of a plane polarised light in an anticlockwise direction, it is termed as the laevorotatory or the l-form.
Dextrorotatory or the D-Form: When a particular compound leads the rotation of a plane polarized light in a clockwise direction, it is termed as the dextrorotatory or the d-form.
FAQs on Chirality, Racemisation, and Optical Activity in Chemistry
1. What are optically active compounds as per the CBSE Class 12 syllabus?
Optically active compounds are substances that have the ability to rotate the plane of plane-polarised light when it is passed through them. This property arises from molecular chirality, meaning the molecule is non-superimposable on its mirror image. For example, 2-butanol exists as two enantiomers; one rotates light clockwise (dextrorotatory) and the other rotates it anti-clockwise (laevorotatory).
2. What is the fundamental relationship between chirality and optical activity?
Chirality is the necessary condition for a compound to exhibit optical activity. A chiral molecule lacks an internal plane of symmetry and is asymmetric. It is this three-dimensional asymmetry that interacts with plane-polarised light, causing the plane to rotate. In contrast, an achiral molecule is superimposable on its mirror image and will not rotate plane-polarised light, making it optically inactive.
3. What is a racemic mixture and why is it optically inactive?
A racemic mixture is an equimolar (50:50) mixture of two enantiomers, the dextrorotatory (+) and laevorotatory (-) forms of a chiral compound. It is optically inactive due to a phenomenon called external compensation. The clockwise rotation of plane-polarised light caused by one enantiomer is perfectly cancelled out by the equal and opposite anti-clockwise rotation caused by the other enantiomer, resulting in no net optical rotation.
4. How does the process of racemisation typically occur in a chemical reaction?
Racemisation is the conversion of an optically active compound into a racemic mixture. This often occurs during chemical reactions that proceed through a planar, achiral intermediate, such as a carbocation. Since this intermediate is flat, an incoming group or nucleophile can attack it from either face (top or bottom) with equal probability. This leads to the formation of both enantiomers in equal amounts, resulting in a product that is optically inactive.
5. Why do SN1 reactions often lead to racemisation, whereas SN2 reactions cause inversion?
This difference is due to their reaction mechanisms.
- SN1 reactions proceed through a flat, trigonal planar carbocation intermediate. The nucleophile can attack this planar intermediate from either side with equal ease, producing a 50:50 mixture of enantiomers, which is a racemic mixture.
- SN2 reactions involve a backside attack by the nucleophile, forcing the leaving group to exit from the opposite side in a single, concerted step. This leads to an inversion of configuration at the chiral centre, a process known as Walden inversion.
6. What is the key difference between a racemic mixture and a meso compound?
While both are optically inactive, the reasons are different.
- A racemic mixture is optically inactive due to external compensation, where the rotational effects of two different types of molecules (the d- and l-enantiomers) cancel each other out.
- A meso compound is optically inactive due to internal compensation. A meso compound is a single achiral molecule that contains chiral centres and a plane of symmetry. The rotational effects from different parts within the *same molecule* cancel each other out.
7. How can the individual enantiomers in a racemic mixture be separated?
The separation of enantiomers from a racemic mixture is a process called resolution. Since enantiomers have identical physical properties, they cannot be separated by simple methods like distillation. A common chemical method involves reacting the mixture with a pure chiral reagent (a resolving agent). This converts the enantiomers into a pair of diastereomers, which have different physical properties (like solubility) and can then be separated by techniques like fractional crystallisation.
8. What instrument is used to measure optical activity, and what factors influence the reading?
A polarimeter is the instrument used to measure the angle of rotation of plane-polarised light. The observed optical rotation is influenced by several factors:
- Concentration of the sample in the solution.
- Length of the polarimeter tube that holds the sample.
- Temperature of the solution.
- Wavelength of the light used for the measurement (commonly the D-line of a sodium lamp).
