OxfordAQA A-Level Chemistry: Optical isomerism (9620) – Revision Notes

These notes condense section 3.3.7, Optical isomerism (subsection 3.3.7.1), of OxfordAQA’s International AS and A-level Chemistry specification (9620), Version 5.3, for International AS and A-level exams from May/June 2020. It is International A2 content, so it is International A-level only. Unit 4 assesses sections 3.3.7 to 3.3.16, and the synoptic Unit 5 can draw on any content. For full explanations and worked examples, read the optical isomerism study guide.

When you have revised, test yourself with the practice questions. The course hub, the printable checklist and the free diagnostics help you plan the rest of your revision.

What the specification asks for

Section 3.3.7.1 says you should be able to:

• draw the structural formulas and displayed formulas of enantiomers
• understand how racemic mixtures (racemates) are formed and why they are optically inactive.

It also states that optical isomerism is limited to molecules with a single chiral centre. The section lists no practical activity.

Key terms

Term	What to write
Stereoisomers	Same structural formula; the atoms differ in how they are arranged in space
Optical isomerism	Stereoisomerism that results from chirality in a molecule
Asymmetric carbon atom (chiral centre)	Carbon with four different atoms or groups attached
Enantiomers (optical isomers)	Mirror-image forms that cannot be superimposed; they affect plane-polarised light differently
Plane-polarised light	Light with all its vibrations in one plane
Optically active	Turns the plane of plane-polarised light
Racemic mixture (racemate)	Both enantiomers mixed in equal amounts

Spotting a chiral centre

Never chiral: a carbon in C=C, C=O or C≡N; any CH₃ or CH₂; any carbon in a benzene ring.

Check fully: every CH carbon and every carbon with no hydrogen. Follow each of its four groups to the end of the chain before deciding two groups are the same.

Quick contrasts

Compound	Carbon checked	Its four groups	Chiral?
Butane-1,2-diol, HOCH₂CH(OH)CH₂CH₃	C2	H, OH, CH₂OH, CH₂CH₃	Yes
1-phenylethanol, C₆H₅CH(OH)CH₃	the C–OH carbon	H, OH, CH₃, C₆H₅	Yes
1-chloro-2-methylpropane, (CH₃)₂CHCH₂Cl	C2	H, CH₃, CH₃, CH₂Cl	No
3-methylhexan-3-ol, CH₃CH₂C(OH)(CH₃)CH₂CH₂CH₃	C3	OH, CH₃, C₂H₅, C₃H₇	Yes, with no H on it

In butane-1,2-diol, C1 is a CH₂ group, so C2 is the only chiral centre. In 1-phenylethanol, none of the ring carbons can be chiral.

Drawing enantiomers: method in steps

1. Mark the asymmetric carbon with an asterisk.
2. Give it two flat bonds, a wedge (pointing at you) and a hashed bond (pointing away).
3. Put one of the four groups on each bond.
4. Draw a vertical mirror line.
5. Redraw the molecule as its reflection: left and right swap; the wedge and hashed bond keep their groups.
6. For structural formulas, write the groups as CH₃, OH, COOH and so on.
7. For displayed formulas, keep the same arrangement at the chiral carbon but show every bond in every group, including C=O and O–H.

Worked reminder: butane-1,2-diol. C2 carries H, OH, CH₂OH and C₂H₅. With CH₂OH and C₂H₅ in the plane, OH on the wedge and H on the hashed bond:

              OH          :          OH
              ▼           :          ▼
   HOCH2  --  C  --  C2H5 : C2H5 --  C  --  CH2OH
              ⋮           :          ⋮
              H           :          H

(Key: ▼ wedge, out of the page; ⋮ hashed, into the page; plain line, flat on the page.)

Plane-polarised light: the facts

• One enantiomer rotates the plane of plane-polarised light one way.
• The other rotates it by the same angle the opposite way, under the same conditions.
• Otherwise their physical properties match: identical melting points, boiling points and densities.
• A sample with only one enantiomer is optically active. A racemate is not.

Racemates

Definition: equal amounts of the two enantiomers.

Why optically inactive (three points):

1. The two enantiomers turn the plane through equal angles.
2. One turns it clockwise and the other anticlockwise.
3. With equal numbers of each, the turning effects cancel: no net rotation.

How one forms (four points):

1. The carbon that becomes chiral is planar in the reactant or the intermediate.
2. The attacking species can reach either side of that plane.
3. Neither side is favoured: the chance of attack is the same for both.
4. So the two enantiomers form in equal amounts.

Worked reminder: KCN with 2-methylpropanal (section 3.3.8.1). 2-methylpropanal, (CH₃)₂CHCHO, gives 2-hydroxy-3-methylbutanenitrile, (CH₃)₂CHCH(OH)CN. C2 carries H, OH, CN and CH(CH₃)₂, so it is chiral. The C=O carbon is planar, so the cyanide ion has an equal chance of adding from above or below, and the product is a racemate. C3 carries two CH₃ groups and is not chiral.

Worked reminder: bromine with propene (section 3.3.4.2). Propene and Br₂ give 1,2-dibromopropane, CH₂BrCHBrCH₃. C2 carries H, Br, CH₃ and CH₂Br, so it is chiral. The intermediate carbocation is planar at C2, so Br⁻ adds to either face equally and the product is a racemate. So the product molecules are chiral, yet the sample shows no optical activity.

Must-know distinctions

• Chiral molecule vs optically active sample. A racemate is made of chiral molecules but is not optically active.
• Enantiomer vs mirror image. Every molecule has a mirror image. Only a chiral molecule’s mirror image cannot be superimposed on it.
• Optical vs E–Z isomerism. Optical: a carbon with four different groups. E–Z: a C=C that cannot rotate, where both carbons carry two unlike groups.
• Aldehyde or unsymmetrical ketone vs symmetrical ketone with KCN then dilute acid. Aldehydes and unsymmetrical ketones lead to a racemate. With a symmetrical ketone, the new C–OH carbon carries two identical groups, so the product is not chiral. Methanal gives no chiral product either, because it has two hydrogens on its C=O carbon.
• Structural vs displayed formula. Both must show the 3D arrangement at the chiral carbon. Only the displayed formula shows every bond.

Quick self-test

1. Define the term asymmetric carbon atom.
2. State what is meant by a racemic mixture.
3. Does 3-chloro-2-methylhexane, (CH₃)₂CHCHClCH₂CH₂CH₃, have a chiral centre? If so, give the four groups.
4. Is 1-chloro-2-methylpropane chiral? Explain.
5. Explain why a solution of one enantiomer rotates the plane of plane-polarised light but a racemate does not.
6. Hexanal, CH₃(CH₂)₄CHO, reacts with KCN followed by dilute acid. Name the product and state whether it is a racemate.
7. How many stereoisomers does 1-phenylethanol have?
8. A sample contains 0.0250 mol of racemic 2-bromohexane, C₆H₁₃Br. Calculate the mass of each enantiomer in it. (Ar: H = 1.0, C = 12.0, Br = 79.9)
9. Which carbon in 2-hydroxy-2,4-dimethylpentanenitrile, (CH₃)₂CHCH₂C(OH)(CH₃)CN, is chiral?
10. But-2-ene, CH₃CH=CHCH₃, shows stereoisomerism. Which kind, and does it show optical isomerism?

Answers

1. A carbon carrying four different atoms or groups.
2. A mixture of equal amounts of the two enantiomers of a compound.
3. Yes. C3 carries H, Cl, CH(CH₃)₂ and CH₂CH₂CH₃. (C2 carries two CH₃ groups, so it is not chiral.)
4. No. C2 carries two identical CH₃ groups, C1 is a CH₂ and the other carbons are CH₃.
5. Each enantiomer rotates the plane by the same angle in opposite directions. A single enantiomer gives a rotation. A racemate contains the two in a 1:1 ratio, so the two rotations cancel out.
6. 2-hydroxyheptanenitrile, CH₃(CH₂)₄CH(OH)CN. Yes: C2 carries H, OH, CN and C₅H₁₁, and the two faces of the flat C=O group are equally open to cyanide attack.
7. Two: one pair of enantiomers.
8. Mr of C₆H₁₃Br = 6(12.0) + 13(1.0) + 79.9 = 164.9. Each enantiomer is 0.0125 mol, so mass = 0.0125 × 164.9 = 2.06 g.
9. C2: it carries OH, CN, CH₃ and CH₂CH(CH₃)₂. C4 carries two CH₃ groups.
10. E–Z isomerism. It has no carbon with four different groups, so it does not show optical isomerism.

Where marks are usually lost

• Forgetting the word “non-superimposable” when describing enantiomers.
• Leaving “equal amounts” out of the racemate definition.
• Saying a racemate is inactive because it “contains no chiral molecules”. The molecules are chiral; the rotations cancel.
• Writing “the light is rotated” instead of “the plane of plane-polarised light is rotated”.
• Putting an asterisk on a CH₂ or C=O carbon, or on a carbon in a benzene ring.
• Missing a chiral centre with no hydrogen on it, such as C3 in 3-methylhexan-3-ol.
• Drawing a “displayed formula” that still shows CH₃ or COOH as groups.
• Drawing the second enantiomer with all bonds flat, or with two groups swapped as well as reflected.
• Explaining racemate formation without “planar”, “either side” and “equally likely”.

Related pages

The introduction to organic chemistry guide covers stereoisomers and E–Z isomerism (3.3.1.3). The transition metals guide covers optical isomers of octahedral complexes (3.2.5.3).

Official syllabus

OxfordAQA, International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards: section 3.3.7 Optical isomerism (3.3.7.1), International A2 content.