25.1 Introducing Benzene & 25.2 Electrophilic Substitution Reactions of Benzene

Question 1

What can aromatic hydrocarbons be called?

Answer 1

• Arenes

Question 2

What is the molecular formula of benzene?

Answer 2

• C6H6

Question 3

What was the original model of benzene called, and what did it look like?

Answer 3

• The Kekulé model
• A ring of carbon atoms with alternating double and single bonds between them

Question 4

What 3 pieces of evidence disprove Kekulé’s model?

Answer 4

• The bond lengths
• The enthalpy change of hydrogenation
• The lack of reactivity

Question 5

How do the bond lengths disprove Kekulé’s model?

Answer 5

• The C–C bond lengths are all the same
• The bond length is between the length of single and double bonds

Question 6

How does the enthalpy change of hydrogenation disprove Kekulé’s model?

Answer 6

• 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)

Question 7

How does benzene’s lack of reactivity disprove Kekulé’s model?

Answer 7

• It doesn’t decolourise bromine water, which shows it has no double bonds

Question 8

What is the correct structure of benzene?

Answer 8

• 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

Question 9

How do the 2 models of benzene differ?

Answer 9

• 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

Question 10

What are monosubstituted compounds?

Answer 10

• Compounds with one substituent group

Question 11

How are monosubstituted aromatic compounds named?

Answer 11

• 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

Question 12

What are the exceptions when naming monosubstituted aromatic compounds?

Answer 12

• Benzoic acid (benzene bonded to COOH)
• Phenylamine (benzene bonded to NH2)
• Benzaldehyde (benzene bonded to CHO)

Question 13

How do you name aromatic compounds with more than one substituent group?

Answer 13

• 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

Question 14

What is the NH2 group called as a prefix?

Answer 14

• Amino

Question 15

What is the structure of an ester? What is their functional group?

Answer 15

• Alkyl chain, oxygen atom, C=O, alkyl chain
• Their functional group is COO

Question 16

How are esters named?

Answer 16

• 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

Question 17

Why do arenes and alkenes react differently with bromine, for example?

Answer 17

• 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

Question 18

What mechanism does benzene undergo? Give 4 examples.

Answer 18

• Electrophilic substitution
• Nitration
• Halogenation
• Alkylation
• Acylation

Question 19

What is the overall equation for the nitration of benzene?

Answer 19

• Benzene + nitric acid -> nitrobenzene + water

Question 20

What are the conditions for the nitration of benzene?

Answer 20

• A concentrated sulfuric acid catalyst is used

Question 21

Describe the mechanism for the nitration of benzene.

Answer 21

• 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

Question 22

What is required for benzene to react with a halogen? Give examples.

Answer 22

• A halogen carrier catalyst
• AlCl3, AlBr3, FeCl3 and FeBr3

Question 23

What is the overall equation for the halogenation of benzene?

Answer 23

• Benzene + X2 -> halobenzene + hydrogen halide

Question 24

Describe the mechanism for the halogenation of benzene, using bromine.

Answer 24

• 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

Question 25

What is the overall equation for the alkylation of benzene?

Answer 25

• Benzene + haloalkane -> alkyl benzene + hydrogen halide
• There is a halogen carrier catalyst

Question 26

What is an acyl chloride?

Answer 26

• An organic compound with the functional group -COCl

Question 27

How are acyl chlorides named?

Answer 27

• They have the suffix -oyl chloride (number not needed as they are at the end of the chain)

Question 28

What is the overall equation for the acylation of benzene?

Answer 28

• Benzene + acyl chloride -> phenylketone + hydrogen chloride
• There is a halogen carrier catalyst (iron (III) chloride/ aluminium chloride)

Question 29

What’s special about the alkylation and acylation of benzene?

Answer 29

• They increase the number of carbon atoms in a compound by forming carbon to carbon bonds
• They are therefore called Friedel-Crafts reactions