Graduate
  1. Physical Chemistry  1.1. Thermodynamics  1.1.1. Laws of Thermodynamics  1.1.2. Enthalpy and heat capacity  1.1.3. Entropy and free energy  1.1.4. Phase Equilibrium  1.1.5. Statistical thermodynamics  1.1.6. Thermodynamic cycle  1.1.7. Fugacity and activity  1.2. Quantum Chemistry  1.2.1. Principles of quantum mechanics  1.2.2. Schrödinger Equation  1.2.3. Particle in a box  1.2.4. Operators and eigenvalues  1.2.5. Atomic orbitals  1.2.6. Molecular orbital theory  1.2.7. Perturbation theory  1.2.8. Valence bond theory  1.2.9. Hybridization and chemical bonding  1.3. Chemical kinetics  1.3.1. Rate laws and reaction mechanisms  1.3.2. Collision theory  1.3.3. Transition state theory  1.3.4. Enzyme Kinetics  1.3.5. Chain Reactions and Polymerization  1.3.6. Photochemical reactions  1.4. Statistical mechanics  1.4.1. Partitioning functions  1.4.2. Molecular distribution functions  1.4.3. Boltzmann Distribution  1.4.4. Bose–Einstein and Fermi–Dirac statistics  1.5. Spectroscopy  1.5.1. Rotational Spectroscopy  1.5.2. Vibrational Spectroscopy  1.5.3. Electronic Spectroscopy  1.5.4. Nuclear magnetic resonance spectroscopy  1.5.5. Mass Spectrometry  1.5.6. Raman Spectroscopy  1.5.7. Electron paramagnetic resonance spectroscopy  1.6. Surface and colloidal chemistry  1.6.1. Absorption isotherm  1.6.2. Surface tension and wetting  1.6.3. Colloidal Stability  1.6.4. Catalysis  1.6.5. Emulsions and micelles  1.7. Electrochemistry  1.7.1. Nernst equation  1.7.2. Electrochemical cells  1.7.3. Conductivity and mobility  1.7.4. War  1.7.5. Fuel cells  1.7.6. Electrolysis
  2. Organic chemistry  2.1. Reaction mechanism  2.1.1. Nucleophilic substitution reactions  2.1.2. Electrophilic addition reactions  2.1.3. Elimination reactions  2.1.4. Rearrangement reactions  2.1.5. Radical reactions  2.1.6. Pericyclic reactions  2.2. Spectroscopy and structural determination  2.2.1. UV-Vis Spectroscopy  2.2.2. IR Spectroscopy  2.2.3. NMR Spectroscopy  2.2.4. Mass Spectrometry  2.2.5. X-ray crystallography  2.3. Stereoscopic  2.3.1. Chirality and optical activity  2.3.2. Structural analysis  2.3.3. Geometrical isomerism  2.3.4. Dynamic Stereochemistry  2.4. Organometallic Chemistry  2.4.1. Organolithium and organomagnesium reagents  2.4.2. Palladium-catalyzed cross-coupling reactions  2.4.3. Transition Metal Complexes  2.4.4. Metal–carbon bond  2.5. Polymer chemistry  2.5.1. Polymerization mechanism  2.5.2. Polymer Characteristics  2.5.3. Biodegradable Polymer  2.5.4. Conducting polymer  2.5.5. Supramolecular polymers  2.6. Medicinal chemistry  2.6.1. Drug design and development  2.6.2. Pharmacokinetics and pharmacodynamics  2.6.3. Structure-activity relationships  2.6.4. Molecular docking and drug screening
  3. Inorganic chemistry  3.1. Coordination chemistry  3.1.1. Crystal field theory  3.1.2. Ligand field theory  3.1.3. Spectrochemical Series  3.1.4. Chelation and stability  3.2. Organometallic Chemistry  3.2.1. Metal Carbonyls  3.2.2. Catalysis by organometallic complexes  3.2.3. Metallocenes  3.3. Bio-inorganic chemistry  3.3.1. Metalloproteins and enzymes  3.3.2. The role of metals in biological systems  3.3.3. Metal ion transport and storage  3.4. Solid state chemistry  3.4.1. Crystal Structures  3.4.2. Band theory of solids  3.4.3. Superconductors  3.4.4. Defects in the crystal  3.5. Lanthanides and Actinides  3.5.1. Electronic configuration in lanthanides and actinides  3.5.2. Coordination chemistry of the lanthanides  3.5.3. Magnetic Properties of Lanthanides
  4. Analytical chemistry  4.1. Chromatography  4.1.1. Gas Chromatography  4.1.2. High Performance Liquid Chromatography  4.1.3. Thin Layer Chromatography  4.2. Spectroscopic Techniques  4.2.1. Atomic absorption spectroscopy  4.2.2. X-ray diffraction  4.2.3. Inductively coupled plasma spectroscopy  4.3. Electroanalytical methods  4.3.1. Potentiometry  4.3.2. Voltammetry  4.3.3. Colometry  4.4. Mass Spectrometry  4.4.1. Ionization Technique  4.4.2. Fragmentation Pattern  4.5. Chemometrics  4.5.1. Multidisciplinary analysis  4.5.2. Machine Learning in Chemistry
  5. Theoretical and Computational Chemistry  5.1. Molecular dynamics simulation  5.1.1. Force fields and energy minimization  5.1.2. Monte Carlo simulation in molecular dynamics simulations  5.2. Quantum chemical methods  5.2.1. Hartree–Fock theory  5.2.2. Density functional theory  5.2.3. Semi-empirical methods  5.3. Computational drug design  5.3.1. Molecular Docking  5.3.2. QSAR Modeling  5.3.3. Virtual Screening
  6. Biochemistry  6.1. Enzyme Kinetics  6.1.1. Michaelis-Menten kinetics  6.1.2. Inhibition mechanism  6.2. Metabolism and Bioenergetics  6.2.1. Glycolysis  6.2.2. Citric acid cycle  6.2.3. Electron transport chain  6.3. molecular Biology  6.3.1. DNA replication and repair  6.3.2. Protein synthesis  6.3.3. Gene Regulation  6.4. Structural Biochemistry  6.4.1. Protein Folding  6.4.2. Membrane Biophysics
  7. Environmental Chemistry  7.1. Atmospheric Chemistry  7.1.1. Greenhouse gases  7.1.2. Ozone depletion  7.1.3. Air pollutants and smoke  7.2. Water Chemistry  7.2.1. pH and water hardness  7.2.2. Wastewater treatment  7.2.3. Aquatic Chemistry  7.3. Soil Chemistry  7.3.1. Heavy metal contamination  7.3.2. Soil pH and Buffering  7.3.3. Nutrient cycling  7.4. Toxicology and Chemical Safety  7.4.1. Toxicokinetics  7.4.2. Risk Assessment in Toxicology and Chemical Safety in Environmental Chemistry  7.4.3. Effects of pollutants on the environment

Graduate → Organic chemistry → Stereoscopic ↓


Chirality and optical activity


Stereochemistry is a subdiscipline of chemistry that involves the study of the spatial arrangements of atoms in molecules and their effects on the physical and chemical properties of these substances. Two primary concepts fundamental to stereochemistry are chirality and optical activity. These concepts are important in understanding the behavior of various molecules in biological systems and other chemical environments.

Understanding chirality

The word chirality comes from the Greek word "cheir", meaning hand. Just as our hands are mirror images but cannot be superimposed, chiral molecules are those that cannot be superimposed on their mirror image. Molecules are chiral if they do not have an internal plane of symmetry.

A B

Take the example of a molecule in which a central carbon atom is bonded to four different substituents. Such a carbon atom is called a chiral center or stereocenter. The simplest example of a chiral molecule is a carbon atom bonded to four different groups such as hydrogen, chlorine, bromine, and iodine.

* *  / C —— /  * *
* *  / C —— /  * *

A notable example of this is lactic acid, which has the following structure:

CH₃ | HC-OH | COOH
CH₃ | HC-OH | COOH

Lactic acid exists in two enantiomeric forms, (R)-lactic acid and (S)-lactic acid. These enantiomers are non-superimposable mirror images. The presence of a chiral center (a carbon atom attached to four different groups) makes it possible for lactic acid to exist in these two different forms.

Optical activity

Optical activity is a property of chiral substances where they rotate the polarization plane of plane-polarized light. This is a key feature that distinguishes chiral compounds from their achiral counterparts. Optical activity arises because the spatial arrangement of atoms affects how they interact with polarized light.

Polarized light optical rotation

When plane-polarized light passes through a solution containing a chiral molecule, the angle of rotation of the plane can be measured using a polarimeter, an instrument designed for this purpose. The direction and degree to which the light is rotated helps determine the nature of the chiral compound:

  • A dextrarotatory or (+) compound rotates light clockwise.
  • A levorotatory or (-) compound rotates light in a counterclockwise direction.

The degree of rotation depends on several factors, including the length of the light path through the solution, the concentration of the chiral compound, and the specific rotation, which is an inherent property of the particular chiral compound.

Mathematics of optical activity: specific rotation

The specific rotation [α] of a chiral compound is calculated using the formula:

[α] = (α_obs) / (l × c)
[α] = (α_obs) / (l × c)
  • [α] = specific rotation
  • α_obs = observed rotation in degrees
  • l = length of the cell in decimeters
  • c = concentration of the sample in grams per millilitre

Enantiomers and diastereomers

Enantiomers are chiral molecules that are mirror images of one another. Each enantiomer of a pair will rotate plane-polarized light by the same amount but in opposite directions. Thus, if the (+) form of an enantiomer rotates light +5°, the (-) form will rotate it -5°.

Another category of stereoisomers that is important in understanding chirality are diastereomers. Unlike enantiomers, diastereomers are not mirror images of each other and have different physical properties. Molecules with multiple chiral centers can have multiple stereoisomers, because these centers can adopt different possible configurations.

For example, tartaric acid has two chiral centers and can exist in the following stereoisomers:

HOOC-CHOH-CHOH-COOH (2R,3R) HOOC-CHOH-CHOH-COOH (2S,3S) HOOC-CHOH-CHOH-COOH (2R,3S) HOOC-CHOH-CHOH-COOH (2S,3R)
HOOC-CHOH-CHOH-COOH (2R,3R) HOOC-CHOH-CHOH-COOH (2S,3S) HOOC-CHOH-CHOH-COOH (2R,3S) HOOC-CHOH-CHOH-COOH (2S,3R)

(2R,3S) and (2S,3R) are one pair of enantiomers, while (2R,3R) and (2S,3S) are another pair of enantiomers. (2R,3R) is a diastereomer of (2R,3S) and (2S,3R), and vice versa.

Racemic mixture

A racemic mixture, or racemate, is a mixture containing both enantiomers of a chiral molecule in equal amounts. In such a mixture, the optical activity cancels out because one enantiomer rotates light in one direction, while the other rotates it by the same amount in the opposite direction. The result is an optically inactive mixture despite the presence of chiral molecules.

RC*-A + SC*-A ⇒ Racemate
RC*-A + SC*-A ⇒ Racemate

Racemic mixtures are commonly found in the chemical industry, particularly in the production of synthetic drugs, where both enantiomers are initially produced in equal quantities. Strategies such as chiral resolution or asymmetric synthesis are often needed to separate the enantiomers or specifically synthesize one over the other since often only one enantiomer is biologically active or has the desired effect.

Applications and importance of chirality

Chirality is an extremely important concept in biological systems and the pharmaceutical industry. The two enantiomers of a chiral drug can have very different effects in the body. One may have a beneficial effect while the other may be ineffective or even harmful.

Medicines

Many drugs are chiral, and their efficacy and safety depend largely on their stereochemistry. For example, one enantiomer of the drug thalidomide was found to be a potent sedative, while the other caused severe birth defects. This tragic case underscores the importance of rigorous chirality evaluation in drug design and administration.

Flavour and fragrance industry

Chirality also affects the taste and aroma of substances. For example, the two enantiomers of carvone have different odors. One smells like mint, while the other smells like caraway. These differences are due to the chiral nature of receptor sites in the olfactory system, which interact differently with each enantiomer.

(R)-carvone: Spearmint smell (S)-carvone: Caraway smell
(R)-carvone: Spearmint smell (S)-carvone: Caraway smell

Biological molecules

In nature, most biomolecules such as sugars and amino acids are chiral and exist primarily in one enantiomeric form. For example, the amino acids in proteins are predominantly L-enantiomers. Homochirality of biological molecules plays an important role in the structure and function of proteins and, by extension, in the physiology of living organisms.

Conclusion

Understanding chirality and optical activity is important in the field of organic chemistry. These concepts not only affect the physical and chemical behavior of molecules but also have profound implications in a variety of fields such as pharmaceuticals, flavors, fragrances, and biology. By mastering the principles of chirality, chemists can devise better syntheses of enantiomerically pure compounds and advance developments in many scientific and industrial fields.


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