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Optical Isomerism: Chiral Centres and Enantiomers

Optical isomers are stereoisomers that are mirror images of each other but cannot be placed on top of each other. They happen when a carbon atom has four different groups: a chiral centre. The two forms, called enantiomers, have the same physical and chemical properties except that they rotate plane-polarised light by the same angle in opposite directions, and they can act differently in the body. A 50 : 50 mix (racemic mixture) has no overall effect on the light.

🎬 Step-by-step story

  1. Look at the left molecule. One carbon in the middle holds four different groups: H, OH, CH₃ and COOH. A carbon with four different groups is called a chiral centre. We mark it with a star.
  2. A mirror appears. On the right is the mirror image: the same four groups, but in mirror order. Like your left and right hands.
  3. Drag the slider to turn the right molecule. You can line up two groups, but the other two are then swapped. They never fit on top of each other. These two molecules are enantiomers.
  4. Shine plane-polarised light (light vibrating in one flat plane) through each one. The left form turns the plane clockwise. The right form turns it by the same angle anticlockwise.
  5. Now mix them 50 : 50. One turns the light one way, the other turns it back. The net rotation is 0°. This mix is a racemic mixture: optically inactive.
  6. Your turn. Change CH₃ into H or OH. Now two groups are the same. Is the carbon still chiral? Turn the mirror image and see.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

How do I spot a chiral centre fast?

Skip CH₃, CH₂ and C=C carbons. For each remaining carbon, list its four groups fully. Four different = chiral. Step 1 shows the four coloured groups.

Isn't the mirror image just the same molecule turned around?

No. Turn it in step 3: two groups can match, but then the other two are swapped. Only a real mirror swap makes them match.

If enantiomers are so alike, how can we tell them apart?

By plane-polarised light: they turn it by the same angle in opposite directions. Step 4.

Why is a racemic mixture optically inactive when each molecule is chiral?

For every molecule turning the light clockwise, another turns it anticlockwise by the same amount. They cancel. Step 5.

Why do reactions at C=O give racemic mixtures?

C=O is flat, so the attacking group comes from above or below with equal chance, making both enantiomers equally. Step 5 readout.

What happens if two groups are the same?

The molecule then has a mirror plane and matches its mirror image, so it is not chiral. Try it in free play.

Stereoisomerism and chiral centres

Stereoisomers have the same structural formula but a different arrangement of atoms in space. Two kinds are studied: E/Z (geometric) isomerism around C=C, and optical isomerism.

A chiral centre (asymmetric carbon) is a carbon atom bonded to four different atoms or groups. It is often shown with an asterisk, C*. A molecule with one chiral centre always has two optical isomers.

How to find one: look at each sp³ carbon. Ignore any carbon with two H atoms (CH₂, CH₃) or a double bond. For the rest, check that all four groups are different, looking along the whole chain, not just the first atom. Example: in butan-2-ol, CH₃CH(OH)CH₂CH₃, carbon 2 holds H, OH, CH₃ and C₂H₅: chiral.

Enantiomers and plane-polarised light

The two optical isomers are enantiomers: non-superimposable mirror images. To draw them, draw the carbon with two bonds in the plane, one wedge (towards you) and one dash (away), then draw its reflection in a mirror line.

Ordinary light vibrates in all planes. A polarising filter lets through light vibrating in one plane only: plane-polarised light. When it passes through a solution of one enantiomer, the plane is turned:

A substance that rotates the plane is optically active. Enantiomers have the same melting point, boiling point and reactions with non-chiral reagents. They can react differently with other chiral molecules, such as enzymes and receptors in the body.

Racemic mixtures

A racemic mixture (racemate) contains equal amounts of both enantiomers. Their rotations cancel, so it is optically inactive.

Racemic mixtures often form in reactions that create a chiral centre from a planar group:

Drug makers may need to separate the enantiomers or use chiral catalysts to make just one, because the two may have different effects in the body.

Try it: the hand and mirror test

Make a model with a ball (or a potato) and four toothpicks with four different coloured sweets. Make a second model as its mirror image in a real mirror. Try to turn one so that all four colours match the other. You cannot. Now make two sweets the same colour: suddenly the models match. In the 3D, free play lets you do the same thing.

Key formulas and definitions

Worked examples

1. Does propan-2-ol, CH₃CH(OH)CH₃, have a chiral centre?

Step 1: Carbon 2 holds H, OH, CH₃ and CH₃. Step 2: Two groups are the same (CH₃). Answer: no chiral centre, no optical isomers.

2. Identify the chiral centre in 2-hydroxypropanoic acid (lactic acid), CH₃CH(OH)COOH.

Step 1: Carbon 1 (COOH) has a C=O: not chiral. Carbon 3 (CH₃) has three H: not chiral. Step 2: Carbon 2 holds H, OH, CH₃, COOH: four different groups. Answer: carbon 2 is the chiral centre.

3. Which of these is chiral: 1-chlorobutane or 2-chlorobutane?

Step 1: 1-chlorobutane: C1 has two H; other carbons are CH₂/CH₃. No chiral centre. Step 2: 2-chlorobutane: C2 holds H, Cl, CH₃, C₂H₅. Answer: 2-chlorobutane is chiral.

4. One enantiomer of a compound rotates plane-polarised light by +14°. What does the other enantiomer do, and the racemic mixture?

Step 1: Enantiomers rotate by the same amount in opposite directions: −14°. Step 2: A 50 : 50 mix: +14° and −14° cancel. Answer: −14°; racemic mixture 0°.

5. Explain why HCN addition to propanal gives an optically inactive product even though the product has a chiral centre.

Step 1: The C=O carbon and its groups lie in a flat plane. Step 2: CN⁻ can attack from above or below with equal chance. Step 3: Each side gives a different enantiomer, so 50 : 50 forms. Answer: a racemic mixture forms; rotations cancel.

6. How many stereoisomers at most can a molecule with 2 chiral centres have?

Step 1: Each chiral centre can be in 2 arrangements. Step 2: 2 × 2 = 2² = 4. Answer: up to 4 (fewer if the molecule has internal symmetry).

Common mistakes

Practice quiz

1. A chiral centre is a carbon bonded to:
2. Enantiomers differ in:
3. A racemic mixture contains:
4. Which molecule is chiral?
5. HCN adding to ethanal gives a racemic mixture because:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What is optical isomerism?

A type of stereoisomerism where two molecules are non-superimposable mirror images (enantiomers), usually because a carbon has four different groups.

What is the difference between chiral and optically active?

Chiral describes a molecule that differs from its mirror image. Optically active describes a sample that rotates plane-polarised light. A racemic mixture is made of chiral molecules but is optically inactive.

Do E/Z isomers rotate plane-polarised light?

Not because of the double bond. E/Z isomerism is geometric; only chiral molecules rotate the light.

Where this is taught

England (GCSE, A level)Year 133.3 Organic chemistry

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