In simple terms
A friendly intro before the formal notes — no formulas yet.
Mirror Image Molecules
Some molecules exist as a pair of 'left-handed' and 'right-handed' versions, known as enantiomers. They are mirror images of each other but cannot be perfectly overlapped, just like your hands.
Think about your hands. Your left hand is a mirror image of your right, but you can't wear a left-handed glove on your right hand. Chiral molecules are the same; they have a 'handedness' and their mirror images are distinct.
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Chiral centre: carbon with four different groups. | Sim hint: Asymmetric carbon — no plane of symmetry.
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Enantiomers are non-superimposable mirror images. | Sim hint: Rotate molecule — cannot align all groups.
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Optical activity — rotate plane-polarised light. | Sim hint: Racemic mixture: equal enantiomers — no net rotation.
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Biological significance — enzyme stereospecificity. | Sim hint: One enantiomer active, other inert/harmful.
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Full topic notes
Formal explanation with the rigour you need for the exam.
Chirality and the Chiral Centre
A molecule is described as chiral if it is non-superimposable on its mirror image. The most common cause of chirality in organic molecules is the presence of a chiral centre. A chiral centre is a carbon atom that is attached to four different atoms or groups. This carbon is also referred to as an asymmetric carbon atom. The presence of a single chiral centre in a molecule guarantees that the molecule will be chiral and will exhibit optical isomerism.
Chirality: The property of a molecule being non-superimposable on its mirror image.
Chiral Centre: A carbon atom bonded to four distinct groups.
Achiral: A molecule that is superimposable on its mirror image. It will have a plane of symmetry.
Enantiomers and Optical Activity
Molecules that are non-superimposable mirror images of each other are called enantiomers. They are a pair of optical isomers. To represent them, we use 3D drawings with wedges (bond coming out of the page) and dashed lines (bond going into the page). The two enantiomers have identical physical properties like melting point, boiling point, and solubility. Their chemical properties are also identical, except when reacting with other chiral substances.
The defining difference between enantiomers is their effect on plane-polarised light. Normal light vibrates in all planes, but when passed through a polarising filter, it emerges vibrating in only one plane. A solution of a single enantiomer will rotate this plane of light. This is called optical activity, and it is measured using a polarimeter. One enantiomer rotates the light clockwise (dextrorotatory, (+)) and the other rotates it by the exact same angle but anti-clockwise (laevorotatory, (-)).
When drawing a pair of enantiomers, draw the first molecule and then imagine a mirror next to it. Draw the reflection exactly as it would appear. The group on a wedge in the first molecule will be on a wedge in the mirror image, and the same for dashed lines. Do not swap the wedges and dashes in the mirror image.
Racemic Mixtures (Racemates)
A racemic mixture, or racemate, is a mixture containing equal amounts (an equimolar mixture) of both the (+) and (-) enantiomers of a chiral compound. Because it contains equal quantities of the two enantiomers, a racemic mixture is optically inactive. The clockwise rotation caused by one enantiomer is perfectly cancelled out by the anti-clockwise rotation caused by the other.
A classic example of racemate formation is the nucleophilic addition of hydrogen cyanide to an unsymmetrical aldehyde or ketone. Consider the reaction of propanal (CH₃CH₂CHO) with KCN followed by dilute acid. The carbonyl group is planar. The cyanide nucleophile, CN⁻, can attack the partially positive carbon atom from above the plane or from below the plane with equal probability. This leads to the formation of equal amounts of the two enantiomers of 2-hydroxybutanenitrile, resulting in a racemic mixture.
Racemic Mixture: 50% (+)-enantiomer and 50% (-)-enantiomer.
Optical Activity: None. The mixture is optically inactive due to external compensation.
Formation: Often occurs when a planar, achiral reactant is converted into a chiral product.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
Identify the chiral centre(s), if any, in the molecule 3-methylpentan-2-ol. Draw the structural formula to justify your answer.
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First, draw the structure of 3-methylpentan-2-ol: CH₃CH(OH)CH(CH₃)CH₂CH₃.
Butanone (CH₃COCH₂CH₃) reacts with HCN in the presence of a catalytic amount of KCN. (i) Name and outline the mechanism for this reaction. (ii) Explain why the product is formed as a racemic mixture and is therefore optically inactive.
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(i) The reaction is nucleophilic addition. Mechanism: Step 1: The cyanide ion (CN⁻) acts as a nucleophile and attacks the δ+ carbon of the planar carbonyl group. The π-bond in the C=O breaks. CH₃C(O)CH₂CH₃ + CN⁻ → CH₃C(O⁻)(CN)CH₂CH₃ Step 2: The intermediate anion is protonated by an HCN molecule (or H₂O) to form the product, 2-hydroxy-2-methylbutanenitrile. CH₃C(O⁻)(CN)CH₂CH₃ + HCN → CH₃C(OH)(CN)CH₂CH₃ + CN⁻
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Glossary
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Revision flashcards
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What is a chiral centre?
A carbon atom that is bonded to four different atoms or groups of atoms. It is also known as an asymmetric carbon.
Key takeaways
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Chirality: The property of a molecule being non-superimposable on its mirror image.
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Chiral Centre: A carbon atom bonded to four distinct groups.
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Achiral: A molecule that is superimposable on its mirror image. It will have a plane of symmetry.
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