Topic 18
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Everything from this topic in one place, for the last read before an exam. If anything here is unfamiliar, follow the link back to the full page.
Numbers to know
| Quantity | Value |
|---|---|
| ΔH hydrogenation, cyclohexene | −120 kJ mol⁻¹ |
| ΔH hydrogenation, Kekulé benzene (predicted) | 3 × (−120) = −360 kJ mol⁻¹ |
| ΔH hydrogenation, benzene (measured) | −208 kJ mol⁻¹ |
| extra stability of benzene | 360 − 208 = 152 kJ mol⁻¹ |
| C–C single / benzene C–C / C=C | 0.154 / 0.139 / 0.134 nm |
| bond angle in benzene | 120° |
| alkene C=C infrared range | 1669–1645 cm⁻¹ (check your Data Booklet) |
| nitration temperature | about 50 °C, below 55 °C |
The evidence and the model
| Evidence | Kekulé predicts | Observed |
|---|---|---|
| enthalpy of hydrogenation | −360 kJ mol⁻¹ | −208 kJ mol⁻¹ |
| bond lengths | alternating 0.154 and 0.134 nm | six equal bonds of 0.139 nm |
| infrared | alkene C=C peak | no peak in 1669–1645 cm⁻¹ |
Model: three σ bonds per carbon in a plane at 120°; one p-orbital per carbon at right angles to the ring; sideways overlap with both neighbours; six delocalised electrons above and below the plane. The delocalised model.
Definitions to learn word for word
- Arene: a hydrocarbon that contains a benzene ring.
- Delocalised electrons: electrons shared between more than two atoms.
- Electrophile: an electron pair acceptor.
- Acyl chloride: a compound RCOCl; ethanoyl chloride is CH₃COCl.
The reactions
| Reaction | Reagents and conditions | Electrophile | Product |
|---|---|---|---|
| combustion | oxygen in air | none | CO₂ and H₂O; soot gives a smoky flame |
| bromination | Br₂, FeBr₃, room temperature, dry | Br⁺ | bromobenzene |
| nitration | conc. HNO₃ and conc. H₂SO₄, about 50 °C | NO₂⁺ | nitrobenzene |
| sulfonation | fuming sulfuric acid, warm | not required | benzenesulfonic acid |
| Friedel-Crafts alkylation | halogenoalkane, AlCl₃, reflux, dry | carbocation, such as CH₃CH₂⁺ | alkylbenzene |
| Friedel-Crafts acylation | acyl chloride, AlCl₃, reflux, dry | acylium ion, such as CH₃CO⁺ | aromatic ketone |
| phenol + bromine water | Br₂(aq), room temperature, no catalyst | none | 2,4,6-tribromophenol, white precipitate |
Electrophiles and catalysts
| Reaction | Electrophile forms by | Catalyst regenerated by |
|---|---|---|
| bromination | ||
| nitration | ||
| acylation | ||
| alkylation |
Methods
- Mechanism: arrow from the ring to E⁺ → horseshoe over five carbons with + inside and H and E on the sixth → arrow from C–H into the ring → product and H⁺. Electrophilic substitution.
- Benzene against an alkene: localised against delocalised, high against lower electron density, polarise Br₂.
- Phenol against benzene: lone pair delocalised into the ring, electron density up, bromine polarised more easily.
- Evidence answers: say what Kekulé predicts, what is measured, what it shows.
- Smoky flame: high C : H ratio, incomplete combustion, soot.
The ten traps
- Drawing the circle and double bonds together in one benzene ring.
- Comparing −208 with −120 instead of with −360 for the stability argument.
- Saying benzene has shorter double and longer single bonds, or "average" bonds.
- Confusing the C=C (about 1650) and C=O (about 1700) infrared regions.
- Putting delocalised electrons "in the plane" of the ring.
- Saying benzene "cannot react" with bromine instead of "resists addition".
- Starting the first curly arrow on a carbon atom rather than on the ring.
- Making the horseshoe cover the carbon that carries H and E.
- Forgetting to regenerate the catalyst, or giving Cl₂ instead of HCl as a by-product.
- For phenol, giving "decolourised" without "white precipitate", or "OH donates electrons" without the lone pair.
Mixed test
Try these with no notes, then check the answers.
- Predict the enthalpy of hydrogenation of the Kekulé structure of benzene, given −120 kJ mol⁻¹ for cyclohexene. What does the measured −208 kJ mol⁻¹ show?
- Describe how X-ray diffraction and infrared data show that benzene is not cyclohexa-1,3,5-triene.
- Describe how the π bonds in benzene are formed.
- Explain why benzene does not decolourise bromine water but cyclohexene does.
- Outline the mechanism for the nitration of benzene, including the formation of the electrophile.
- State the reagents and conditions, and the name of the product, for the reaction of benzene with ethanoyl chloride.
- 3.90 g of benzene gives 4.38 g of phenylethanone in a Friedel-Crafts acylation. Calculate the percentage yield.
- Explain why phenol reacts with bromine water but benzene does not, and give the observations.
Answers cover these until you have tried the questions
- Predicted: 3 × (−120) = −360 kJ mol⁻¹. The measured value is 152 kJ mol⁻¹ less exothermic, so benzene is 152 kJ mol⁻¹ more stable than the Kekulé structure, because its π electrons are delocalised.
- X-ray diffraction: all six C–C bonds are the same length, 0.139 nm, between the single (0.154 nm) and double (0.134 nm) values, not alternating. Infrared: no absorption in the alkene C=C range 1669–1645 cm⁻¹, so no localised C=C bonds.
- Each carbon has a p-orbital at right angles to the ring, holding one electron. Each p-orbital overlaps sideways with the p-orbitals on both neighbours, giving π bonding round the whole ring: six delocalised electrons above and below the plane.
- In cyclohexene the π electrons are localised between two carbons, so electron density is high and polarises Br₂, which is attacked (addition). In benzene they are delocalised over six carbons, so electron density is lower and does not polarise Br₂ enough.
- Electrophile: (concentrated acids, about 50 °C). Curly arrow from the ring to the nitrogen of NO₂⁺. Intermediate: horseshoe over five carbons, + inside, H and NO₂ on the sixth. Curly arrow from the C–H bond into the ring, losing H⁺. Product nitrobenzene. Catalyst regenerated:
- Ethanoyl chloride, anhydrous AlCl₃ catalyst, heat under reflux, dry. The product is phenylethanone.
- n(C₆H₆) = 3.90 ÷ 78.0 = 0.0500 mol. Theoretical mass of C₆H₅COCH₃ = 0.0500 × 120.0 = 6.00 g. Percentage yield = 4.38 ÷ 6.00 × 100 = 73.0%.
- A lone pair on oxygen is partly delocalised into the ring, so the ring has higher electron density and polarises bromine easily, with no catalyst. Observations: orange bromine water is decolourised and a white precipitate (2,4,6-tribromophenol) forms.