Edexcel International A Level Chemistry: Chirality (YCH11) – Revision Notes

These notes condense Topic 15A: Chirality of the Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry specification (YCH11), Issue 1, September 2017, outcomes 15.1 to 15.5. You meet it in Unit 4: Rates, Equilibria and Further Organic Chemistry, one of the International A2 (IA2) units. The chirality study guide has the full explanations and longer worked examples; read it first.

Also useful: the chirality practice questions, the printable checklist, the course hub and a free diagnostic to see which topics need work.

Definitions to learn word for word

Term	Definition
Chiral	Cannot be superimposed on its own mirror image
Chiral centre (asymmetric carbon atom)	A carbon atom that carries four different atoms or groups
Optical isomers (enantiomers)	Object and non-superimposable mirror images
Optical activity	The ability of a single optical isomer to rotate the plane of polarisation of plane-polarised monochromatic light
Racemic mixture (racemate)	Equal amounts of both enantiomers mixed together; not optically active
Stereoisomers	Molecules sharing one structural formula whose atoms are arranged differently in space

15.1 and 15.2: Spotting chiral centres

Method in steps

1. Skip every CH₃, every CH₂, and every carbon in C=C or C=O.
2. For each remaining carbon, write out the four groups in full.
3. Compare whole groups. CH₃ is not C₂H₅; C₃H₇ is not C₄H₉.
4. Four different groups: mark the carbon with *.
5. Keep going: a molecule can have more than one chiral centre.

Worked reminder. 2-bromopentanoic acid, CH₃CH₂CH₂CHBrCOOH. C2 carries H, Br, C₃H₇ and COOH: four different groups, so C2 is a chiral centre and the molecule has a pair of enantiomers.

Worked reminder. 3-bromoheptane, CH₃CH₂CHBr(CH₂)₃CH₃. C3 carries H, Br, C₂H₅ and C₄H₉, so C3 is chiral. Compare 4-bromoheptane, where C4 has two propyl groups and is not chiral.

Hidden chiral centres. A chiral carbon need not carry hydrogen. In a tertiary alcohol such as 3-methylheptan-3-ol, C3 carries CH₃, C₂H₅, C₄H₉ and OH, so it is chiral.

15.2: Drawing enantiomers in 3D

Method in steps

1. Draw the chiral carbon with two plain bonds (in the plane of the paper), one solid wedge (towards you) and one hashed wedge (away from you).
2. Place the four groups, bonding each through the correct atom (the C of COOH, the N of NH₂, the O of OH).
3. Draw a vertical dashed mirror line.
4. Reflect: groups on the left move to the right and vice versa; wedges stay wedges and hashed bonds stay hashed.
5. Check that you have not simply redrawn the same molecule turned round.

For 2-bromopentanoic acid you might put C₃H₇ and COOH on plain bonds, Br on the wedge and H on the hashed bond, then reflect the whole drawing.

15.3: Optical activity in brief

• Plane-polarised light vibrates in a single plane. Monochromatic means one wavelength, for example from a sodium lamp; the angle of rotation depends on wavelength, so it must be fixed.
• A polarimeter passes plane-polarised monochromatic light through a solution of the sample. An analyser is turned to find the new plane, and the angle is compared with pure solvent.
• Under the same conditions, the two enantiomers rotate the plane by equal angles in opposite directions. The (+) enantiomer rotates clockwise as you look towards the light source; the (−) one rotates anticlockwise.
• The sign of rotation is measured. It cannot be read off a 3D drawing.

15.4: Racemic mixtures in brief

• Equal amounts of the (+) and (−) enantiomers.
• Net rotation is zero because the rotations cancel.
• The molecules in a racemic mixture are chiral. Zero rotation from an achiral compound is a different situation: that compound has no optical isomers at all.

15.5: Optical activity as mechanism evidence

Reaction	Key step	Shape at the reacting carbon	Result from a single enantiomer or planar reactant	Rotation of product
SN1 (tertiary halogenoalkane)	C–X breaks first	Planar carbocation	Nucleophile attacks either face equally: racemic mixture	Zero
SN2 (primary; secondary can also react this way)	Nucleophile attacks as C–X breaks	Attack opposite the halogen; inversion	One enantiomer only	Non-zero
HCN addition to C=O (KCN present)	CN⁻ attacks carbonyl carbon	Planar C=O	Attack from either side equally: racemic mixture (if product is chiral)	Zero

Method in steps for a “use the optical data” question

1. Check the product (and reactant) for a chiral centre. If the product is achiral, optical data give no evidence.
2. State whether the reactant is a single enantiomer (optically active) or achiral/planar.
3. Say what the measured rotation of the product shows: single enantiomer or racemic mixture.
4. Link to shape: planar intermediate or planar C=O attacked from both sides, or one-sided attack with inversion.
5. Name the mechanism. If rate data are given, add them: rate = k[RX] fits SN1; rate = k[RX][OH⁻] fits SN2.

Must-know distinctions

• Chiral molecules vs an optically active sample. A racemic sample is made of chiral molecules but shows no net rotation.
• Racemic mixture vs achiral compound. Both give a zero reading; only one has enantiomers.
• Inversion vs change of sign. SN2 inverts the arrangement in space. It does not let you predict whether the product is (+) or (−).
• Geometric vs optical isomerism. Geometric (E/Z) isomers come from restricted rotation about C=C (see the alkenes guide); optical isomers come from a chiral centre.
• Planar carbocation vs planar carbonyl. Both lead to racemic products, but SN1 is substitution and HCN addition is nucleophilic addition.

Quick self-test

1. Define the term chiral centre.
2. Does bromochlorofluoromethane, CHBrClF, have a chiral centre? Give a reason.
3. Identify the chiral centre in hexan-2-amine, CH₃CH(NH₂)CH₂CH₂CH₂CH₃, and list its four groups.
4. Explain why pentan-3-ol, CH₃CH₂CH(OH)CH₂CH₃, has no optical isomers.
5. How many chiral centres are there in 2-hydroxybutanoic acid, CH₃CH₂CH(OH)COOH?
6. Define optical activity.
7. A solution of one enantiomer rotates the plane of polarisation by −9.0°. Give the rotation for (a) the other enantiomer at the same concentration and conditions, (b) a racemic mixture of the same total concentration.
8. Why must the light used in a polarimeter be monochromatic?
9. A single enantiomer of a tertiary halogenoalkane is hydrolysed. The product shows zero rotation, although it is chiral. Which mechanism does this support, and why?
10. 2-chloro-2-methylhexane, (CH₃)₂CCl(CH₂)₃CH₃, is hydrolysed to 2-methylhexan-2-ol. Can optical activity data give evidence for the mechanism? Explain.
11. Nonanal reacts with HCN in the presence of KCN to give 2-hydroxydecanenitrile, CH₃(CH₂)₇CH(OH)CN. Is the product sample optically active?
12. State what a racemic mixture is.

Answers

1. A carbon atom that carries four different atoms or groups.
2. Yes. The carbon carries H, Br, Cl and F: four different atoms.
3. C2: H, NH₂, CH₃ and C₄H₉.
4. C3 carries two identical ethyl groups, so it is not a chiral centre, and no other carbon is chiral.
5. One: C2, with H, OH, C₂H₅ and COOH.
6. The ability of a single optical isomer to rotate the plane of polarisation of plane-polarised monochromatic light.
7. (a) +9.0°, equal and opposite. (b) 0°, as the rotations cancel.
8. The angle of rotation depends on the wavelength, so a single wavelength gives a fixed, comparable value.
9. SN1. The C–X bond breaks to give a planar carbocation; the nucleophile attacks either face with equal probability, giving a racemic mixture.
10. No. C2 in both the reactant and the product carries two methyl groups, so neither is chiral and both would give zero rotation whatever the mechanism.
11. No. The new chiral centre forms by CN⁻ attack on either side of the planar C=O with equal probability, giving a racemic mixture.
12. Equal amounts of the two enantiomers of a compound mixed together; it does not rotate the plane of polarisation.

Where marks are usually lost

• Defining optical activity without “plane-polarised” or “monochromatic”, or saying light is “bent” instead of the plane being rotated.
• Overlooking a chiral carbon that carries no hydrogen, as in a tertiary alcohol or halogenoalkane.
• Marking a carbon as chiral even though it carries two identical chains, such as two propyl groups.
• Drawing the second “enantiomer” as the same molecule rotated, instead of a mirror image.
• Bonding OH or COOH to the chiral carbon through the wrong atom in a 3D diagram.
• Saying a racemic product “has no chiral centre”.
• Writing “racemic, so SN1” without the planar carbocation and attack from both faces with equal probability.
• Claiming that SN2 turns a (+) reactant into a (−) product.
• Using optical data as evidence when the product is achiral, so the data cannot distinguish the mechanisms.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry (XCH11/YCH11) specification, Issue 1, September 2017, published by Pearson Education Limited. Topic 15A: Chirality, outcomes 15.1 to 15.5.