Benzene New Flashcards
Evidence against Kekule’s structure of Benzene
- Lack of reactivity
- C=C should decolourise Bromine water
- Doesn’t undergo electrophilic addition reactions
- Length of C-C bonds
- X-ray diffraction measures bond - all are 0.139nm whereas C-C should be longer than C=C
- Hydrogenation enthalpies
- enthalpy change should be 3x cyclohexene
- actual value is much lower
Describe the delocalised model of Benzene
- Planar, cyclic hexagonal C6H6
- C uses 3 electrons to bond 1 hydrogen and 2 Carbons
- C has 1 electron delocalised in p-orbital
- pi-bond system above and below ring is delocalised
Electrophile substitution reactions of benzene
- They typically react with electrophile
- Electrophiles are electron deficient centres are ready to accept a pair of electrons
- Benzene acts as a nucloephile
Steps in nitration of benzene
- HNO3 + H2SO4 —> NO2+ + HSO4- + H2O
- Requires diagrams, look in textbook
- H+ + HSO4- —> H2SO4
What reactions does benzene undergo?
Electrophilic subsititution
- Nitration of benzene
Conditions for nitration of benzene
- sulfuric acid catalyst
- 50-60 Degrees Celsius
If temperatures increase above 50 Degrees Celsius during nitration of benzene, what could form?
- Dinitrobenzene and 2H2O could form instead of nitrobenzene
- There would be increased substitution reactions that would occur
- So 2 NO2 would react instead of 1
What is required for halogenation of benzene?
A halogen carrier catalyst needs to be present
- In order to generate the electrophile
Examples of halogen carrier catalysts for halogenation of benzene. Where are they generated/created?
- AlCl3
- AlBr3
- FeCl3
- FeBr3
- They are generated in situ (in the reaction vessel) from the metal and the halogen
What type of reaction are bromination and halogenation of benzene examples of?
- Electrophilic substitution
Bromination of benzene mechanism
1.
What is alkylation of benzene?
Conditions required?
- The substitution of a hydrogen atom in the benzene ring by an alkyl group
- increases the number of carbon atoms in a compound by forming C-C bonds
- Sometimes called Friedel-Crafts alkylation after chemists who first did reaction
Conditions:
- React benzene with a haloalkane
- Halogen carrier catalyst, AlCl3, to generate the electrophile
What is an acylation reaction?
Conditions?
- When benzene reacts with an acyl chloride in the presence of AlCl3, an aromatic ketone is formed
Conditions:
- Halogen Carrier catalyst, AlCl3, to generate electrophile
What is formed during the reaction of benzene with ethanoyl chloride?
- Uses of product?
- Phenylethanone
- Useful in the perfume industry
Product of reaction of benzene with chloroethane
- Ethylbenzene and HCl
Comparing reactivity of alkenes with arenes
- Alkenes are much more reactive than arenes, they decolorisation bromine water
- Arenes, hoewever, require a halogen carrier catalyst to bond with bromine
Phenol def
A type of organic chemical containing a hydroxyl functional group (-OH) bonded directly to an aromatic ring
What do phenols dissociate into in solution
A phenoxide ion and H+ ion
How strong an acid are phenols?
They are weak acids
- more acidic than alcohols
- less acidic than carboxylic acids
How to distinguish between phenols and carboxylic acids
- Carboxylic acids react with the weak sodium carbonate base to produce CO2 gas
- Phenols don’t react with sodium carbonate, so no CO2 gas will be produced
Reaction of phenol with sodium hydroxide
Produces sodium phenoxide and water
What reactions do phenols undergo?
Electrophilic substitution reactions:
- bromination
- nitration
Bromination of phenol
- Phenol reacts with an aqueous solution of bromine
- Forms a white precipitate of 2,4,6 tribromophenol
- Reaction decolourises bromine water
Conditions:
- no halogen carrier catalyst required
- room temperature
Nitration of phenol
What is the major and minor product?
- Phenol reacts with dilute nitric acid at room temperature
- Nitric acid provides an NO2- ion for electrophilic substitution
- a mixture of 2-nitrophenol and 4-nitrophenol is formed, as well as water
- 2-nitrophenol is the major product, whereas 4-nitrophenol is the minor product