Benzene & Aromatic Compounds Flashcards
Give the formula of benzene
C6H6
Which 3 pieces of evidence challenge the kekulé model of benzene?
Resistance to reaction with electrophile
Enthalpy change of hydrogenation
C-C bond length
What did kekulé’s model of benzene suggest?
Benzene was an alkene
What result occurs when benzene is tested with bromine water?
Nothing happens
What are the differences in bond length between all C-C bonds? Compare it to that of benzene C-C bonds
C-C single bond (sigma bond) - LONGEST
C=C double bond (Pi bond) - SHORTEST
C-C bond in benzene - MIDDLE
Benzene carbon-carbon bonds all have the same length
Describe the bonding and structure of benzene
P-orbitals overlap above and below the ring to form a Pi bond
The Pi bond electrons are delocalised.
Sigma bonds form between C-C and C-H bonds
Benzene ring is planar with bond angle 120 degrees
C-C bonds are all of equal length
Define the term ‘electrophile’
Electron pair acceptor
In order for benzene to react with an electrophile, what must happen?
Electrophile must disrupt and remove two electrons from the ring containing 6 Pi electrons
What occurs when benzene undergoes electrophilic substitution ?
1: Two of the delocalised electrons in the ring are donated to an electrophile
2: an unstable intermediate containing only 4 Pi bond electrons and a positive charge is formed.
3: intermediate breaks down to reform the delocalised Pi bond containing 6 electrons.
Give an example of electrophilic substitution in benzene?
Nitration of benzene
Give the catalyst and conditions for nitration of benzene?
Conditions: reflux at 60 degrees with conc HNO3 + conc H2SO4 (catalyst)
What is the generally equation for nitration of benzene?
C6H6 + HNO3 –> C6H5NO2 + H2O
During nitration of benzene, what is produced if the temperature exceeds 95C?
Nitrobenzene + 1,3dinitrobenzene
Which ion is used in nitration of benzene?
Nitronium ion (NO2)
Which two compounds contribute to the production of the nitronium ion?
HNO3
H2SO4
Give the overall equation for the formation of the nitronium ion
2H2SO4 + HNO3 –> 2HSO4- + H3O- + NO2+
What happens during halogenation (bromination) of benzene?
A halogen atom is introduced into the benzene ring using a halogen carrier catalyst
What is the halogen carrier for halogenation (bromination) of benzene?
FeBr3
Or
Fe
How is the electrophile for halogenation (bromination) formed?
The catalyst and the reagent react together
Br-Br + FeBr3 –> Br+ + FeBr4-
Which two halogens react within halogenation?
Bromine
Chlorine
Give the halogen carrier for chlorine reacting with benzene?
AlCl3
What is the name of catalysts in halogenation?
Halogen carriers
Which mechanism is used in halogenation?
Electrophilic substitution
Which mechanism is the reaction of cyclohexene with bromine?
Electrophilic addition
