Chapter 25 - Aromatic Compounds
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What is the molecular, empirical and skeletal formula of benzene?
C6H6 CH Hexagon with three double bonds = Kekulé model (disproved) Hexagon with a circle in the middle = delocalised model
What was the evidence to disprove Kekulé’s model?
• Lack of reactivity • Carbon-carbon bond lengths • Hydrogenation enthalpies
Explain how the lack of reactivity of benzene disproves the Kekulé model?
• if benzene contain C=C bonds, it should react in a similar way to alkenes and discolouring bromine water • benzene does not undergo electrophilic addition reactions expected of C=C bonds and does not de colourise bromine under normal conditions
Explain how the carbon-carbon bond lengths disprove the Kekulé model?
X-ray diffraction studies showed that the carbon-carbon bond lengths in benzene are all the same length which are between the C-C and C=C lengths.
Explain how the hydrogenation enthalpies disproves the Kekulé model?
• the hydrogenation enthalpy of benzene is less exothermic than expected for the Kekulé model • this means that Benzene is more stable (un reactive) than expected for the Kekulé model
What is the subsistent group NO2 named?
Nitro (nitrogen is bonded to the ring)
What is the rule for naming compounds with benzene bonded to an alkyl group and halogens?
The alkyl group is priority so it is assumed to be at position 1 and the other substituent groups are named relative to the alkyl group
What is -C6H5?
A way of representing a phenyl group
When is benzene considered a substituent?
Ring is attached to: - an alkyl chain with a functional group - alkyl chain with 7 or more carbon atoms
What prefix is used for benzene?
Phenyl
What exceptions are there for naming benzene?
• Connected to a carboxylic acid = benzoic acid • Connected to an amine group = phenyl amine • Connected to an aldehyde group = benzaldehyde • Connected to an alcohol group = phenol
Why is benzene so stable?
Its delocalisation
Why does benzene not take part in addition reactions?
As the pi system would break if it did making it less stable which is energetically unfavourable
What are the conditions for nitration of benzene?
• presence of concentrated sulfuric acid catalyst • reaction with concentrated nitric acid
What is the overall reaction for nitration of benzene?
Benzene + nitric acid -> (sulfuric acid catalyst) nitro benzene + water
What are the 3 general steps for substitution reactions with benzene?
Step 1: generate very reactive electrophile (catalyst + reactant) Step 2: substitution (curly arrows) Step 3: regenerating the catalyst
What are the conditions for halogenation of benzene?
• halogen carrier must be present as halogens do not react with benzene otherwise (AlCl3, FeBr3)
What are halogen carriers?
Generated in a reaction between the metal and halogen
What is the overall reaction for halogenation (bromine) of benzene?
Benzene + Br2 -> bromobenzene + HBr
What are the conditions for the alkylation of benzene (Friedel-Crafts)?
• uses a haloalkane and a halogen carrier catalyst to replace one of the hydrogen atoms on benzene with an alkyl group
What is the overall reaction of alkylation of benzene to form the organic product ethylbenzene?
Benzene + chloroethane -> ethyl benzene + hydrogen chloride
What are the conditions for the acylation of benzene?
• benzene reacts with an acyl chloride • presence of an AlCl3 catalyst • aromatic aldehyde/ketone is formed
What is the overall reaction of ethanoyl chloride with benzene?
Benzene + ethanoyl chloride -> phenylethanone + HCl
Describe the bonding and structure of benzene?
P orbitals overlap above and below the ring to form pi-bonds. The electrons are delocalised to form a pi-system