Arenes Flashcards
(34 cards)
What are arenes?
hydrocarbons that possess at least one benzene ring
Why is benzene stable?
The continuous overlap of six unhybridised p orbitals results in the delocalisation of π electrons above and below the ring, this resonance structure gives benzene extra stability
Why do benzenes not undergo addition reaction and what reactions do they undergo instead?
Benzene is less susceptible to addition reactions because it will destroy the aromatic character, it is more likely to undergo substitution reaction to retain the stable delocalised π electron system
What reactions do benzene undergo?
- Combustion
- Electrophilic substitution
What kind of flame do benzene combust in and why?
Sooty flame due to its high carbon content
What types of electrophilic substitution reactions of benzene are there?
- Nitration
- Halogenation
- Friedel-craft alkylation
- Friedel-craft acylation
What are the reagents and conditions for nitration of benzene?
conc. HNO3, conc. H2SO4, maintained at 55 degree celsius
What is the first step of nitration of benzene?
HNO3 + 2H2SO4 ⇌ NO2⁺ + 2HSO4⁻ + H3O⁺
What are the reagents and conditions of halogenation of benzene?
X2, Fe or anhydrous FeX3 or anhydrous AlX3
Why is F2 and I2 not used for the halogenation of benzene?
- F2 is too reactive and will give low yields of mono-substituted products
- I2 is too unreactive towards benzene ring
What is the first step of halogenation of benzene?
If FeCl3 used: FeCl3 + Cl2 → FeCl4⁻ + Cl⁺
If Fe used: Fe + 3/2Cl2 → FeCl3, FeCl3 + Cl2 → FeCl4⁻ + Cl⁺
If AlCl3 used: AlCl3 + Cl2 → AlCl4⁻ + Cl⁺
What does Fe, FeX3 and anhydrous AlX3 act as in halogenation of benzene and why are they needed?
They are catalysts which acts a Lewis acid which accepts electron pair from Cl⁻ because halogens are not very strong electrophiles and this converts them to more reactive electrophiles to attack stable π electron system of benzene ring
What are the reagents and conditions of Friedel-crafts alkylation?
RCl, anhydrous AlCl3 or FeCl3
What is the first step of Friedel-crafts alkylation?
Using AlCl3: AlCl3 + CH3CH2Cl → AlCl4⁻ + CH3CH2⁺
*note that it is the second C of CH2⁺ that attacks the benzene ring
What are the reagents and conditions of Friedel-crafts acylation?
RCOCl, anhydrous AlCl3
What is the first step of Friedel-crafts acylation?
AlCl3 + RCOCl → AlCl4⁻ + RCO⁺
Why is methylbenzene a useful organic solvent?
It is preferred over benzene since its fumes are considerably less toxic than benzene
Why is the boiling point of methylbenzene higher than benzene?
Methyl benzene has a greater number of electrons thus more energy is required to overcome the stronger id-id interactions
Why is melting point of methylbenzene lower than benzene?
methylbenzene is less symmetrical, thus it has less efficient packing and lower energy is required to overcome the less extensive id-id
What reactions do methylbenzene undergo?
- side chain reactions: free radical substitution, oxidation
- electrophilic substitution of benzene ring
What are the reagents and conditions of oxidation of methylbenzene under acidic conditions?
acidified KMnO4, heat under reflux
What are the observations when methylbenzene undergo oxidation under acidic condition?
Purple KMnO4 turns colourless, when reaction mixture is cooled, white precipitate of benzoic acid is formed
(benzoic acid is insoluble in cold water but soluble in hot water)
What are the reagents and conditions of oxidation of methylbenzene under alkaline conditions?
alkaline KMnO4, heat under reflux
What is the side chain after methylbenzene undergo oxidation under alkaline conditions?
COO⁻
