1. Chiral Carbons, Asymmetry & Enantiomers
Stereoisomers possess the same structural formula and the same connectivity of atoms, but their atoms are arranged differently in three-dimensional space. In Unit 2, you studied geometric E/Z isomerism. In Unit 4, we study optical isomerism.
- Chiral Centre (Asymmetric Carbon): A carbon atom that is bonded to four different atoms or groups of atoms. Marked on structural diagrams with an asterisk (C*).
- Enantiomers: A pair of non-superimposable mirror image stereoisomers.
- Chirality: The geometric property of a molecule that makes it non-superimposable on its mirror image (just like human left and right hands).
Physical and Chemical Properties of Enantiomers
Enantiomers share identical physical properties (same melting point, boiling point, density, solubility in achiral solvents) and identical chemical reactivity towards achiral reagents.
They differ in only two fundamental ways:
- Their effect on the plane of plane-polarised light (one rotates it clockwise, the other rotates it anticlockwise).
- Their interaction with other chiral molecules (such as biological enzymes, antibody receptors, and cellular proteins).
2. Tetrahedral Enantiomer Mirror Diagram
The diagram below displays the two optical isomers of 2-hydroxypropanoic acid (lactic acid) reflected across a mirror plane:
3. Polarimetry & Optical Activity
Standard light consists of electromagnetic waves oscillating in all possible planes perpendicular to the direction of propagation. Passing monochromatic light through a polaroid filter (polariser) allows only waves vibrating in a single plane to pass through, creating plane-polarised light.
- Dextrorotatory (+ or d): Rotates the plane of polarised light clockwise to the right.
- Laevorotatory (- or l): Rotates the plane of polarised light anticlockwise to the left by the exact same numerical angle.
4. Racemic Mixtures (Racemates) & Planar Carbonyls
A classic exam question asks why nucleophilic addition of cyanide to ethanal or butanone produces an optically inactive product. You must state:
- The carbonyl group (C=O) is planar around the carbonyl carbon (trigonal planar geometry, 120° bond angles).
- The incoming nucleophile (e.g. CN⁻ or :H⁻) has an equal 50% probability of attacking from above the plane or from below the plane.
- Attack from above produces one enantiomer, while attack from below produces the mirror-image enantiomer in equal amounts.
- An equimolar racemic mixture is formed, which exhibits zero net optical rotation.
The full KCN mechanism is on the Aldehydes and Ketones page.
5. Biological Significance & Pharmaceutical Drug Design
Enzymes and cellular drug receptors are constructed from chiral L-amino acids and have complex 3D chiral binding pockets (lock-and-key model). Consequently, two enantiomers of a drug molecule often have profoundly different pharmacological activities:
Beyond the OxfordAQA specification: the (R)/(S) naming system and the thalidomide and ibuprofen details in this table are not required.
| Pharmaceutical Drug | Active Enantiomer Effect | Inactive / Harmful Enantiomer Effect | Synthetic & Regulatory Strategy |
|---|---|---|---|
| Thalidomide | (R)-enantiomer: effective morning sickness remedy and sedative. | (S)-enantiomer: severe teratogen causing catastrophic fetal limb malformations. | Withdrawn in 1961; now used again, under strict controls, for some cancers and leprosy. Racemises in the body. Highlighted the need for chirality screening of modern medicines. |
| Ibuprofen | (S)-enantiomer: potent anti-inflammatory COX inhibitor. | (R)-enantiomer: pharmacologically inactive. | Sold as racemate because body naturally converts inactive (R) to active (S) in liver. |
6. Worked Examples
(a) Butan-2-ol: CH₃-CH(OH)-CH₂-CH₃
(b) Pentan-3-ol: CH₃-CH₂-CH(OH)-CH₂-CH₃
(c) 2-Chlorobutane: CH₃-CH(Cl)-CH₂-CH₃
Solution:
- (a) Butan-2-ol: Carbon-2 is bonded to four distinct groups: -H, -OH, -CH₃, and -CH₂CH₃ (ethyl). Therefore, Carbon-2 is a chiral centre and butan-2-ol exhibits optical isomerism.
- (b) Pentan-3-ol: Carbon-3 is bonded to -H, -OH, and two identical ethyl groups (-CH₂CH₃). Because two groups are identical, Carbon-3 is achiral. Pentan-3-ol has no optical isomers.
- (c) 2-Chlorobutane: Carbon-2 is bonded to -H, -Cl, -CH₃, and -CH₂CH₃. Four distinct groups; therefore, Carbon-2 is a chiral centre.
Step-by-step examination model answer:
- The starting material propanal (CH₃CH₂CHO) has a planar carbonyl group (C=O) at the reaction centre.
- The cyanide nucleophile (CN⁻) has an equal probability of attacking the carbonyl carbon from either side (above or below the plane).
- Attacking from one side yields the (+) enantiomer, while attacking from the opposite side yields the (-) enantiomer.
- Because both attack pathways are equally likely, an equimolar (50:50) racemic mixture is produced.
- The optical rotations of the two enantiomers cancel out completely, leaving the mixture optically inactive.
Go deeper
These UK AQA pages cover the same ideas in more detail. They may include content that is not in 9620.
7. Practice Questions
Try each question, then open the answer to check your mark points.
Question 1 (2 marks): State what is meant by a chiral carbon atom. Identify the chiral carbon in 2-hydroxypropanoic acid, CH₃CH(OH)COOH.
Show Answer & Mark Points
- A carbon atom bonded to four different atoms or groups (1)
- Carbon-2, the carbon bonded to the OH group (1)
Question 2 (2 marks): How do two enantiomers of the same compound differ in their effect on plane-polarised light?
Show Answer & Mark Points
- Each enantiomer rotates the plane of plane-polarised light (1)
- By the same amount but in opposite directions (1)
Question 3 (3 marks): Which of these compounds has optical isomers: propan-2-ol, butan-2-ol or 2-chloropropane? Explain your answer.
Show Answer & Mark Points
- Butan-2-ol only (1)
- Its carbon-2 is bonded to four different groups: H, OH, CH₃ and C₂H₅ (1)
- In the other two, the central carbon has two identical CH₃ groups, so it is not chiral (1)