Organic Chemistry III
Notes for every section, eight new exam-style questions with full worked solutions, and online recall cards for your review days. Nothing here is copied from the book.
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18.1 Benzene: two models of its bonding
Benzene, C6H6, is a planar hexagon of six carbon atoms, each bonded to one hydrogen. Bond angles are 120°.
Kekulé model: a ring of alternating C=C and C–C bonds.
Delocalised model: each carbon uses three electrons in σ bonds (two C–C, one C–H). The fourth electron is in a p orbital above and below the plane. The six p orbitals overlap sideways to form a ring of delocalised π electrons above and below the ring. It is drawn as a hexagon with a circle inside.
Check yourself: How many electrons are in benzene's delocalised π system?
Six (one from each carbon)
18.2 Evidence for the delocalised model
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18.3 Why benzene resists bromination
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18.4 Reactions of benzene
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18.5 The electrophilic substitution mechanism
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18.6 Phenol and bromine water
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18.7 Amines, amides and amino acids: recognising them
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18.8 Amines as bases
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18.9 Other reactions of butylamine
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18.10 Making amines
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18.11 Amides and polyamides
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18.12 Amino acids
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18.13 Proteins and the peptide bond
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18.14 Core Practical 15: analysing unknowns
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18.15 Deducing formulae from data
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18.16 Grignard reagents: making the carbon chain longer
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18.17 Planning a synthesis
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18.18 Hazards and practical techniques
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18.19 Core Practical 16: the preparation of aspirin
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Extra worked questions
Cover the solution, answer on paper, then mark yourself against each step. Each step shows the mark it earns.
All 8 worked solutions are free for members. The first one is open to everyone.
Bond lengths in benzene
X-ray studies show all the carbon–carbon bonds in benzene are 0.139 nm long. Explain how this supports the delocalised model rather than the Kekulé model.
Try it first, then show the worked solution
| Kekulé | Kekulé benzene would have two bond lengths: 0.154 nm (C–C) and 0.134 nm (C=C) | 1 |
|---|---|---|
| Delocalised | All bonds are equal and intermediate in length, as the π electrons are shared evenly round the ring | 1 |
Answer: One intermediate bond length fits delocalisation.
Examiner tip: Quote the numbers; it makes the comparison clear.
Nitrating benzene
Give the reagents and conditions for the nitration of benzene, and write an equation for the formation of the electrophile.
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Phenol and bromine water
Bromine water is added to aqueous phenol. State what is seen, write an equation, and explain why benzene does not react under the same conditions.
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Friedel–Crafts acylation
Benzene reacts with ethanoyl chloride in the presence of AlCl3. Write an equation for the formation of the electrophile, name the organic product, and show how the catalyst is regenerated.
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Ranking base strength
Put ammonia, ethylamine and phenylamine in order of base strength, strongest first, and explain the order.
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Nylon-6,6
Nylon-6,6 is made from hexane-1,6-diamine and hexanedioic acid. (a) Name the type of polymerisation. (b) Describe the repeat unit. (c) Explain why nylon fibres are strong.
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Two uses of a Grignard reagent
Bromoethane is converted into a Grignard reagent, which is then reacted separately with (a) carbon dioxide and (b) propanone, each followed by dilute acid. Give the conditions for making the reagent, and name both products.
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Identifying an unknown alcohol
0.250 g of compound X, containing C, H and O only, burns completely to give 0.595 g CO2 and 0.304 g H2O. Its molecular ion is at m/z = 74. X has a broad infrared absorption near 3350 cm−1 and is not oxidised by warm acidified dichromate. Identify X.
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Video walkthroughs
Narrated walkthroughs of exam-style questions from this topic of the book, showing where each mark comes from. Try the question first, then watch. Captions are available in the player.
How stable is benzene?
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Bromination of benzene
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Base strength of amines
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A synthesis and a Grignard reaction
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Identifying an amine from its data
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Premium includes a walkthrough video for the key questions in every topic. Each one is narrated step by step, with captions.
Recall cards
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