Chirality
Chirality (from Greek "cheir", hand) is the geometric property of a molecule that makes it non-superimposable on its mirror image, analogous to left and right hands. A chiral molecule lacks any plane, axis, or centre of symmetry; the most common source is a tetrahedral carbon (chiral centre) bonded to four different substituents. Chirality is of enormous biological importance — enzymes, receptors, and drugs are chiral, so enantiomers often exhibit completely different pharmacological activities, which is why modern drug development rigorously controls stereochemistry.
Key Formula
[alpha]^T_D = alpha / (c × l)
LaTeX: [\alpha]^T_D = \frac{\alpha}{c \cdot l}
| Symbol | Meaning | Unit |
|---|---|---|
| [α]ᵀD | Specific optical rotation at temperature T, sodium D line | deg·mL·g⁻¹·dm⁻¹ |
| α | Observed rotation angle | degrees |
| c | Concentration of solution | g/mL |
| l | Path length of polarimeter cell | dm |
Worked Example
Problem
A solution of (S)-ibuprofen (0.500 g in 10.0 mL ethanol) placed in a 1.00 dm polarimeter cell rotates plane-polarised light by +0.895°. Calculate the specific optical rotation.
Solution
Step 1: Identify variables. α = +0.895° c = 0.500 g / 10.0 mL = 0.0500 g/mL l = 1.00 dm Step 2: Apply the specific rotation formula. [α]ᵀD = α / (c × l) [α]ᵀD = +0.895 / (0.0500 × 1.00) [α]ᵀD = +0.895 / 0.0500
Answer
[α]²⁰D = +17.9 deg·mL·g⁻¹·dm⁻¹ for this sample of (S)-ibuprofen
Key Terms in Chirality and Stereochemistry
| Term | Definition | Symbol/Notation | Example |
|---|---|---|---|
| Chiral centre | Carbon bonded to 4 different groups | * (asterisk on carbon) | C-2 of lactic acid |
| Enantiomers | Non-superimposable mirror-image pair | (R) / (S) or (+) / (−) | (R)- and (S)-alanine |
| Specific rotation | Intrinsic optical rotation of a pure compound | [α]ᵀD | +12.7 (D-glucose) |
| Racemic mixture | Equal mixture of both enantiomers | (±) or rac- | (±)-lactic acid |
| Enantiopure | ≥99% one enantiomer | ee = 99% | Natural L-amino acids |
Interactive Tools
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Related Terms
Stereoisomer
Stereoisomers are molecules that have the same molecular formula and the same connectivity (sequence of bonds) between atoms but differ in the three-dimensional arrangement of their atoms in space. There are two main types: enantiomers (non-superimposable mirror images, related by chirality) and diastereomers (stereoisomers that are not mirror images, including cis/trans and E/Z isomers). Stereoisomers can have profoundly different biological activities — the drug thalidomide is a classic example, where one enantiomer is therapeutic and the other teratogenic.
Nucleophilic Substitution
Nucleophilic substitution is a fundamental class of organic reactions in which an electron-rich species (nucleophile) attacks a carbon atom bearing a leaving group, displacing that leaving group and forming a new bond. There are two main mechanistic pathways: SN2 (concerted, bimolecular, inverts stereochemistry) and SN1 (stepwise via a carbocation intermediate, gives racemic mixture). These reactions are central to the synthesis of alcohols, ethers, amines, and other functional groups from alkyl halides.
Amine (organic)
An amine is an organic compound derived from ammonia (NH₃) by replacing one or more hydrogen atoms with organic substituents such as alkyl or aryl groups. Amines are classified as primary (RNH₂), secondary (R₂NH), or tertiary (R₃N) depending on the number of carbon-containing substituents attached to nitrogen. They are fundamental in biochemistry — amino acids, proteins, DNA bases, and many pharmaceutical drugs contain amine functional groups.
From Greek "cheir" (hand), introduced into chemistry by Lord Kelvin in 1893 to describe the handedness of molecules. The concept was foreshadowed by Louis Pasteur's 1848 separation of tartrate crystals and Jacobus van't Hoff's 1874 tetrahedral carbon theory.