6.1.1 Benzene
How do you name an aromatic compound?
- Alkyl groups, halogens and nitro groups are all prefixes to benzene (e.g. ethylbenzene) - If benzene is attached to an alkyl group with a functional group or more than 7 carbons, benzene becomes the substituent
Describe the delocalised model of benzene
- Each carbon is single-bonded to 2 other carbons in a planar ring and 1 hydrogen - Each carbon atom has 1 electron in a p-orbital perpendicular to the plane of the ring - Adjacent p-orbitals overlap sideways, above and below the plane of the carbon atoms to form a ring of electron density - This is a system of pi bonds (a delocalised pi system) containing 6 delocalised electrons spread over all 6 carbons in the ring structure
What is the evidence against the Kekulé model of benzene?
- Lack of reactivity / resistance to reaction / does not undergo electrophilic addition reactions (so doesn't decolourise bromine water under normal conditions) (suggests that there are no C=C double bonds) - All carbon-carbon bonds are the same length (140pm) instead of 3 single C-C bonds (154 pm) and 3 double C=C bonds (134 pm) as expected. The bond length is between the length of a single and double bond (suggests a different type of bond) - the Kekulé model of benzene was expected to have an enthalpy change of hydrogenation 3x that of cyclohexene (3 x -120 = -360 kJ mol⁻¹) but its actual enthalpy change of hydrogenation is only -208 kJ mol⁻¹. This is far less exothermic so more energy is needed to break its bonds then expected so benzene is more stable than the proposed molecule
Explain the shape of a benzene molecule
- Planar - Hexagon/6 carbon ring/120° bond angle - C-C bonds equal in length / C-C bond lengths between single and double bond
Name this molecule
1,3-dimethylbenzene
Name this molecule
Benzaldehyde
Name this molecule
Benzoic acic
What does the molecular formula of benzene suggest about the molecule's saturation?
CₙHₙ shows that there are probably many double / triple bonds (normal saturation is CₙH₂ₙ₊₂
How did Kekulé adapt his model of benzene to explain its lack of reactivity?
He said that benzene was constantly flipping between 2 different forms (isomers) by switching bonds It was resonating in such a rapid equilibrium that there was not enough time for it to react with other molecules
What was the first proposed structure of benzene?
In 1865, Kekulé proposed the structure of benzene. - He said it was a planar (flat) ring of 6 carbon atoms with alternating single and double bonds. - Each carbon was bonded to 1 hydrogen
Why would the enthalpy change of hydrogenation of cyclohexa-1,3-diene be different from the enthalpy change of hydrogenation of cyclohexa-1,4-diene?
In cyclohexa-1,3-diene: - Double bonds separated by 1 single bond / alternating - Allows some delocalisation / overlap of p orbitals (double bonds close enough to allows some delocalisation)
Name this molecule
Phenol
Name this molecule
Phenylamine
Name this molecule
Phenylethanone
Explain the bonding in and the shape of a benzene molecule. Compare the stability of benzene with that of the hypothetical cyclohexa-1,3,5-triene molecule. Use the data in your answer. Hydrogenation: of cyclohexene: ∆H = -120 kJ mol⁻¹ of benzene: ∆H = -208 kJ mol⁻¹ [6]
Stage 1 Bonding 1a) Each C has three (covalent) bonds 1b) Spare electrons (in a p orbital) overlap (to form a π cloud) 1c) delocalisation Stage 2 Shape 2a) Planar 2b) Hexagon/6 carbon ring/120° bond angle 2c) C-C bonds equal in length / C-C bond lengths between single and double bond Stage 3 Stability 3a) Expected ∆H o hydrogenation of cyclohexatriene = -360 kJ mol -1 3b) ∆H o hydrogenation benzene (is less exothermic) by 152 kJ mol -1 3c) Benzene lower in energy than cyclohexatriene / Benzene is more stable
How do you name an aromatic compound with more than 1 group?
Start your numbering from the suffix functional group (the group that takes precedence) (e.g. carboxylic acid)
What are compounds with benzene rings called?
arenes / aromatic compounds
What is the IUPAC name of the Kekulé model of benzene?
cyclohexa-1,3,5-triene
What is the prefix for benzene?
phenyl
