Difficulty: Intermediate | Prerequisites: Chapters 1–3 (bonding, molecular geometry, conformations, cis-trans isomerism)
Chirality is the idea that certain molecules exist as non-superimposable mirror images of each other, the same way your left and right hands are mirror images but cannot be perfectly overlapped. This chapter builds on what you already know about molecular shape (sp3 geometry, cis-trans isomerism) and adds a new layer: how the 3D arrangement of groups around a single carbon can create molecules with identical formulas and connectivity but different spatial configurations. Understanding chirality is essential for later work in reaction mechanisms, biochemistry, and pharmacology, where the "handedness" of a molecule can determine whether it functions as a medicine or a toxin. If you are comfortable with tetrahedral geometry and Newman projections, you are ready for this material.
Some molecules are "handed" (chiral), meaning they and their mirror images cannot be superimposed. These mirror-image pairs are called enantiomers. You identify chirality by looking for sp3 carbons bonded to four different groups, checking for planes or centres of symmetry, and measuring optical rotation with a polarimeter. Enantiomers share most physical properties but rotate plane-polarised light in equal and opposite directions.
Chirality (handedness)
A geometric property where an object cannot be superimposed on its mirror image. Think of it as the molecular version of left-handedness vs. right-handedness.
Chiral centre (chirality centre, stereogenic centre, stereocentre)
A tetrahedral (sp3-hybridised) carbon bonded to four different atoms or groups. In simple terms, this is the atom that makes a molecule handed.
Enantiomers
Stereoisomers that are non-superimposable mirror images of each other. Think of them as a pair of molecular "left and right hands." Enantiomer is a relative term: you cannot point to a single molecule and call it "an enantiomer" without naming what it is the mirror image of.
Stereoisomers
Molecules with the same connectivity but different spatial arrangements of atoms. Enantiomers and diastereomers are both types of stereoisomer.
Achiral
A molecule that can be superimposed on its mirror image. In simple terms, the molecule has no handedness.
Plane of symmetry (mirror plane)
An imaginary plane that divides a molecule into two identical halves. If a molecule has one, it is achiral.
Centre of symmetry
A point in a molecule through which a line drawn to any atom on one side reaches an identical atom at equal distance on the other side. A molecule with a centre of symmetry is achiral.
Optical activity
The ability of a chiral substance to rotate the plane of plane-polarised light. Think of it as the physical measurement that proves a substance is handed.
Polarimeter
The instrument used to measure optical rotation. It passes plane-polarised light through a sample and detects whether the plane has been rotated.
Dextrorotatory (+)
Rotates polarised light clockwise (to the right). The prefix (+) is placed before the compound name.
Levorotatory (–)
Rotates polarised light counterclockwise (to the left). The prefix (–) is placed before the compound name.
Racemic mixture (±)
A 50:50 mixture of both enantiomers. Optically inactive because the equal and opposite rotations cancel out.
Homochiral (enantiopure)
A sample consisting entirely of one enantiomer.
Enantiomeric excess (ee)
The percentage by which one enantiomer exceeds the other in a mixture. Calculated as: % major enantiomer minus % minor enantiomer.
Specific rotation [α]
A physical constant for a chiral substance, calculated from the observed rotation, the concentration, and the path length of the polarimeter tube.
Elemental composition → Empirical formula → Molecular formula → Constitution/Connectivity → Configuration
You already know cis-trans prefixes describe relative configurations. Chirality adds a new dimension: the spatial arrangement around a single tetrahedral carbon.
A geometric figure is chiral if its mirror image cannot be superimposed on itself.
The most common source of chirality in organic chemistry: a carbon bonded to four different groups.
Example: bromochlorofluoromethane (CHBrClF). Build two models or open two 3D viewers side by side, and you will see they relate like a left hand and a right hand.
Because mirror-image molecules differ only in spatial arrangement, they are stereoisomers. Specifically, they are enantiomers: stereoisomers related as non-superimposable mirror images.
"Enantiomer" is a relative word, like "cousin." You cannot label a single isolated molecule as "an enantiomer" without saying what it is the enantiomer of.
Given four different groups (w, x, y, z) on a chiral carbon:
Swapping any two groups converts the molecule into its enantiomer.
Swapping any three groups returns the original structure.
This is a useful shortcut for checking your work with models or drawings.
The surest test is mirror-image analysis: draw or build the mirror image and attempt to superimpose. If it fails, the molecule is chiral.
Physical models (or virtual 3D viewers like MolView) are strongly recommended.
A tetrahedral (sp3-hybridised) carbon with four different substituents.
Those groups can be atoms (H, Br, F), groups (CH3, CH2CH3, OH), or any combination of elements.
Different isotopes of the same element count as different substituents (e.g. deuterium vs. protium).
Carbons with double or triple bonds are sp2 or sp hybridised, not sp3. They cannot be chirality centres.
A carbon bonded to two or more identical groups is not a chirality centre.
Everything covered so far in this section applies to molecules with one chirality centre. Molecules with two or more centres introduce additional complexity (covered in Sections 4.10–4.12).
If you can find an imaginary plane that divides the molecule into two identical halves, the molecule is achiral.
This is the quickest test. Start here before trying anything else.
If a line drawn through a central point of the molecule reaches identical atoms at equal distances on both sides, the molecule is achiral.
Less common than a plane of symmetry, but equally valid.
Check for a plane of symmetry first (easiest to spot).
If none found, check for a centre of symmetry.
If neither is present, the molecule is likely chiral, but confirm with the superimposing test (draw the mirror image and try to overlap it with the original).
Optical activity is the ability of a chiral substance to rotate the plane of plane-polarised light.
The instrument is called a polarimeter.
The light source uses a single wavelength, typically 589 nm (the sodium D-line).
A light source emits unpolarised, single-wavelength light.
A polarising filter restricts the beam to one plane.
The polarised beam passes through a tube containing the chiral sample (in solution or as a pure liquid).
If the plane of light rotates, the substance is optically active. All achiral substances are optically inactive.
Enantiomers rotate light by the same magnitude but in opposite directions.
Clockwise rotation = dextrorotatory, labelled (+).
Counterclockwise rotation = levorotatory, labelled (–).
A racemic mixture (±) contains equal amounts of both enantiomers, so the rotations cancel: net rotation = zero.
Tube length: Doubling the path length doubles the observed rotation (more molecules encountered).
Concentration: Higher concentration means more chiral molecules in the beam's path.
Enantiomeric ratio (er): The ratio of the major enantiomer to the minor enantiomer. Example: 80:20 means 80% of one enantiomer, 20% of the other.
Enantiomeric excess (ee): % major minus % minor. For an 80:20 ratio: ee = 80% – 20% = 60%.
ee can also be calculated from specific rotation: ee = ([α] of sample / [α] of pure enantiomer) × 100. This value equals the optical purity.
Specific rotation:
[\alpha] = \frac{100\alpha}{c \cdot l}Where:
[α] = specific rotation (a physical constant for the substance)
α = observed rotation (degrees)
c = concentration (g per 100 mL of solution)
l = path length of polarimeter tube (dm)
Enantiomeric excess from specific rotation:
ee = \frac{[\alpha]_{\text{sample}}}{[\alpha]_{\text{pure}}} \times 100Notation convention: [α]_D^25 means the specific rotation measured at the sodium D-line (589 nm) and 25 °C.
Enantiomeric excess from composition:
ee = \%_{\text{major}} - \%_{\text{minor}}Pharmaceutical drugs often exist as enantiomers, and only one form may be therapeutically active. The other can be inactive or even harmful. This is why chirality matters in drug design and manufacturing.
Polarimetry is a standard quality-control technique in the food and pharmaceutical industries to verify the purity and identity of chiral compounds.
Students often think any carbon with four bonds is a chirality centre. It is not. The four substituents must all be different, and the carbon must be sp3 hybridised.
Students often assume that (+) means R and (–) means S. There is no fixed relationship between the sign of optical rotation and the R/S configuration. The only way to know the sign is to measure it experimentally.
Students sometimes refer to a single molecule as "an enantiomer" on its own. Enantiomer is a relational term: molecule A is the enantiomer of molecule B. One molecule alone cannot be an enantiomer.
A common error is thinking that a racemic mixture has no chiral molecules. It does. It contains equal amounts of both enantiomers, and their optical rotations cancel, but the individual molecules are still chiral.
⚠️ Identifying chirality centres is one of the most frequently tested skills in this chapter. Be able to scan a structure and quickly locate sp3 carbons with four different groups.
⚠️ Expect calculation problems using the specific rotation formula. Know how to solve for any of the three variables (observed rotation, concentration, path length) given the other two.
⚠️ ee calculations appear regularly. Practise converting between enantiomeric ratio and enantiomeric excess in both directions.
⚠️ Know the difference between achiral (superimposable on its mirror image) and chiral (not superimposable). Be able to identify planes and centres of symmetry.
True or false: A carbon with a double bond can be a chirality centre. (False, it must be sp3.)
Fill in the blank: A molecule that can be superimposed on its mirror image is called ______. (Achiral.)
True or false: Enantiomers have different boiling points. (False, their ordinary physical properties are identical.)
Fill in the blank: A 50:50 mixture of enantiomers is called a ______ mixture. (Racemic.)
True or false: Swapping two groups on a chirality centre converts a molecule to its enantiomer. (True.)
Q: What structural feature is the most common source of chirality in organic molecules?
A: A tetrahedral (sp3) carbon bonded to four different atoms or groups.
Q: A sample has a specific rotation of +30.0°. The pure enantiomer has a specific rotation of +50.0°. What is the enantiomeric excess?
A: ee = (30.0 / 50.0) × 100 = 60%.
Q: If a mixture has an enantiomeric ratio of 90:10, what is the ee?
A: ee = 90% – 10% = 80%.
Q: You observe a rotation of +4.5° using a 1 dm tube and a concentration of 3.0 g/100 mL. What is the specific rotation?
A: [α] = (100 × 4.5) / (3.0 × 1) = +150°.
Q: A molecule has a plane of symmetry. Is it chiral or achiral?
A: Achiral. A plane of symmetry means the molecule is superimposable on its mirror image.
Q: You swap two substituents on a chirality centre. What have you produced?
A: The enantiomer of the original molecule.
This material connects directly to Chapter 3's treatment of conformations and cis-trans isomerism. Cis-trans isomers are a specific type of stereoisomer, and chirality extends that idea to tetrahedral centres. Later in the course, reaction mechanisms (SN1, SN2, addition reactions) will depend heavily on whether stereochemistry is preserved or inverted, making these concepts essential groundwork. In biochemistry, enzyme-substrate interactions rely on chiral recognition: only one enantiomer of a substrate fits a given enzyme's active site.
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