Chapter 25: Benzene
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What can aromatic hydrocarbons be called?
• Arenes
What is the molecular formula of benzene?
• C6H6
What was the original model of benzene called, and what did it look like?
• The Kekulé model • A ring of carbon atoms with alternating double and single bonds between them
What 3 pieces of evidence disprove Kekulé’s model?
• The bond lengths • The enthalpy change of hydrogenation • The lack of reactivity
How do the bond lengths disprove Kekulé’s model?
• The C–C bond lengths are all the same • The bond length is between the length of single and double bonds
How does the enthalpy change of hydrogenation disprove Kekulé’s model?
• According to Kekulé’s structure, benzene should have triple the hydrogenation enthalpy of cyclohexene • However, benzene has a hydrogenation enthalpy less exothermic than expected (cyclohexene’s is -120, so you would expect -360, but it’s actually -208)
How does benzene’s lack of reactivity disprove Kekulé’s model?
• It doesn’t decolourise bromine water, which shows it has no double bonds
What is the correct structure of benzene?
• Each carbon atoms has 3 sigma bonds; one with a hydrogen atom and 2 with other carbon atoms • The remaining electrons are in p-orbitals (above and below the plane of the ring) • There is then a sideways overlap of the p-orbitals (all of them overlap sideways in both directions), which creates a delocalised ring of electron density above and below the plane of the benzene ring
How do the 2 models of benzene differ? What does this mean for the second model?
• In Kekulé’s model the π-bonds are localised • In the actual model there is a system of delocalised π-bonds • This structure is more stable
What are monosubstituted compounds?
• Compounds with one substituent group
How are monosubstituted aromatic compounds named?
• Benzene is the parent chain if it is bonded to: alkyl groups with less than 7 carbons, halogens and nitro groups (NO2) • They therefore become prefixes • If the benzene ring is bonded to an alkyl chain with a functional group or one with seven or more carbons • Benzene then becomes the prefix phenyl • There are also some exceptions
What are the exceptions when naming monosubstituted aromatic compounds?
• Benzoic acid (benzene bonded to COOH) • Phenylamine (benzene bonded to NH2) • Benzaldehyde (benzene bonded to CHO)
How do you name aromatic compounds with more than one substituent group?
• The ring is numbered to give the lowest possible numbering • See if it is based off of methylbenzene, chlorobenzene or nitrobenzene- if it is, these are on carbon number one, but don’t need to be numbered • The substituent groups are then listed in alphabetical order
What is the NH2 group called as a prefix?
• Amino
What is the structure of an ester? What is their functional group?
• Alkyl chain, oxygen atom, C=O, alkyl chain • Their functional group is COO
How are esters named?
• The part with an alkyl chain and an oxygen atom is from the alcohol • The part with C=O and an alkyl chain is from the acid • The name of the alcohol is changed so that it end in -yl • The name of the acid is changed so that it ends in -oate • The alcohol’s name goes first
Why do arenes and alkenes react differently with bromine, for example?
• In alkenes, there is high electron density due to the localised π-bond, so it can polarise the Br-Br bond • However, in arenes there is lower electron density, so it cannot polarise the Br-Br bond • Benzene therefore only reacts with bromine when there’s a halogen carrier present as it is too stable
What mechanism does benzene undergo? Give 4 examples.
• Electrophilic substitution • Nitration • Halogenation • Alkylation • Acylation
What is the overall equation for the nitration of benzene?
• Benzene + nitric acid -> nitrobenzene + water
What are the conditions for the nitration of benzene?
• A concentrated sulfuric acid catalyst is used • 50°C
Draw the mechanism for the nitration of benzene.
• Step 1: the electrophile is formed • HNO3 (conc) + H2SO4 (conc) -> NO2+ + HSO4- + H2O • Step 2: The nitronium ion (the electrophile) reacts with benzene • NO2+ accepts a pair of electrons from the delocalised ring to form a dative covalent bond • This forms an unstable intermediate that breaks down to form nitrobenzene and a H+ ion • Step 3: sulfuric acid (the catalyst) is reformed • H+ + HSO4- -> H2SO4
What is required for benzene to react with a halogen? Give examples.
• A halogen carrier catalyst • AlCl3, AlBr3, FeCl3 and FeBr3
What is the overall equation for the halogenation of benzene?
• Benzene + X2 -> halobenzene + hydrogen halide
Draw the mechanism for the halogenation of benzene, using bromine.
• Step 1: the electrophile is formed • Br2 + FeBr3 -> FeBr4- + Br+ • Step 2: Br+ (the electrophile) reacts with benzene • Br+ reacts with benzene and accepts a pair of electrons from the delocalised ring to form a dative covalent bond • This forms an unstable intermediate that breaks down to form bromobenzene and a H+ ion • Step 3: regeneration of catalyst • H+ + FeBr4- -> FeBr3 + HBr