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Chirality, Racemisation, and Optical Activity
1.0What is Chirality?
A molecule is said to be chiral if it cannot be superimposed on its mirror image. Chirality comes from the Greek word cheir, meaning “hand,” because just like left and right hands, chiral molecules exist as non-superimposable mirror images.
Chiral Center and Stereogenic Atom
- The most common cause of chirality in organic molecules is the presence of a chiral center (asymmetric carbon atom).
- A chiral center is a carbon atom attached to four different groups.
- Example: In 2-butanol, the second carbon atom is attached to –H, –OH, –CH₃, and –CH₂CH₃, making it a chiral center.
Examples of Chiral Molecules
- Lactic acid
- 2-butanol
- Amino acids (except glycine)
- Sugars like glucose and fructose
2.0Optical Isomerism and Chirality
Optical isomerism is a type of stereoisomerism that arises due to chirality. Optical isomers are also called enantiomers, and they are non-superimposable mirror images of each other.
Key Points:
- Enantiomers have identical physical properties (boiling point, melting point, density).
- They differ in the direction in which they rotate plane-polarised light.
- One enantiomer is dextrorotatory (d- or + form) and the other is levorotatory (l- or – form).
3.0Optical Activity
Definition of Optical Activity
Optical activity is the ability of a chiral compound to rotate the plane of polarised light when it passes through the substance.
Plane Polarised Light
- Light waves usually oscillate in all directions perpendicular to the direction of travel.
- When passed through a polariser, light oscillates in only one plane — this is called plane-polarised light.
Dextrorotatory and Levorotatory Compounds
- Dextrorotatory (d- or +): Rotates plane-polarised light to the right (clockwise).
- Levorotatory (l- or –): Rotates plane polarised light to the left (anticlockwise).
- The direction of rotation cannot be predicted from the structure; it is determined experimentally.
Measurement using a Polarimeter
- A polarimeter is used to measure the angle of rotation (α).
- The specific rotation [α] is given by:
where,
- = observed angle of rotation
- l = path length of sample tube (dm)
- c = concentration of solution (g/mL)
4.0Racemisation
Definition of Racemisation
Racemisation is the process by which an optically active compound (chiral compound) is converted into a racemic mixture, losing its optical activity.
A racemic mixture (racemate) contains equal amounts of the two enantiomers (d- and l- forms). Since the two rotations cancel each other out, the mixture is optically inactive.
Causes of Racemisation
- Heat or Light: Exposure to high temperature or UV light can cause racemisation.
- Chemical Reagents: Acids, bases, or other reagents may facilitate conversion between enantiomers.
- Reaction Mechanism: Certain reaction intermediates (like planar carbocations) lead to racemisation.
Mechanism of Racemisation
- A chiral molecule undergoes a reaction, forming a planar intermediate (e.g., carbocation).
- This intermediate has no chirality.
- Attack by nucleophiles can occur from either side with equal probability.
- The result is an equal mixture of both enantiomers.
Examples of Racemic Mixtures
- Racemic lactic acid
- Racemic mandelic acid
- Many pharmaceutical drugs are synthesized as racemic mixtures (though only one enantiomer may be active).
5.0Relationship between Chirality, Optical Activity, and Racemisation
- Chirality gives rise to optical activity because chiral molecules rotate plane-polarised light.
- Optical activity is a measurable property used to distinguish enantiomers.
- Racemisation destroys optical activity by converting a pure enantiomer into a racemic mixture.
Thus, chirality → optical activity → racemisation (loss of activity).
6.0Applications of Chirality and Optical Activity in Chemistry
- Pharmaceuticals: The biological activity of drugs depends on chirality. Example: One enantiomer of thalidomide was therapeutic, while the other caused birth defects.
- Biochemistry: Amino acids (proteins) are chiral, and so are sugars (DNA backbone).
- Food Chemistry: Chirality affects taste and smell. Example: Limonene enantiomers smell like oranges (d-form) or lemons (l-form).
- Industrial Chemistry: Used in asymmetric synthesis to selectively produce one enantiomer.
- Forensics & Toxicology: Chiral analysis helps in detecting drugs and metabolites.