25.1 Introducing Benzene & 25.2 Electrophilic Substitution Reactions of Benzene Flashcards
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?
- 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
- It 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
Describe 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
Describe 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
What is the overall equation for the alkylation of benzene?
- Benzene + haloalkane -> alkyl benzene + hydrogen halide
- There is a halogen carrier catalyst
What is an acyl chloride?
- An organic compound with the functional group -COCl
How are acyl chlorides named?
- They have the suffix -oyl chloride (number not needed as they are at the end of the chain)
What is the overall equation for the acylation of benzene?
- Benzene + acyl chloride -> phenylketone + hydrogen chloride
- There is a halogen carrier catalyst (iron (III) chloride/ aluminium chloride)
What’s special about the alkylation and acylation of benzene?
- They increase the number of carbon atoms in a compound by forming carbon to carbon bonds
- They are therefore called Friedel-Crafts reactions
