Aromatic chemistry Flashcards
What is the chemical formula for Benzene
C6H6 of 6 carbons in a hexangonal planar ring
What is the structure of benzene?
Each carbon is covalently bonded to three others, each carbon has a P electron that is not bonded. The p orbitals overlap to form a Pi delocalised system.
What was the Kekule structure?
3 C-C bonds and 3 C=C bonds
What evidence is there that the delocalised system is the most appropriate
- All the Carbon-Carbon bonds were the same length
- The energy released when 1 mol of benzene is hydrogenated should 3x the energy released when 1 mol of cyclohexane is hydrogenated (C6H10) buts its much less. This is because of the stability associated with the Pi delocalised system of electrons
Draw a benzene molecule
in book
What are some facts about benzene?
- Colourless at room temperature and pressure
- Each point is a carbon with 1 H bonded
- Delocalisation is where the electrons are spread over several atoms
- Each carbon has 3Ο bonds and the 4th is an electron in a p orbital
- Benzene does NOT undergo addition reactions easily due to the stability of the π delocalised system of electrons
- Benzene undergoes ELECTROPHILIC SUBSTITUTION REACTIONS
What is the nitration of a benzene molecule?
a H is substituted for a NO2 group
What is the mechanism for the nitration of a benzene molecule?
Electrophilic substitution
What is the formula of a nitronium ion?
NO2+
How is the nitronium ion formed?
From the reaction of concentrated nitric acid and concentrated sulfuric acid
HNO3 + 2H2SO4 β NO2+ + H30+ + 2HSO4-
What are the three reactions for the formation of the nitronium ion
- HNO3 + H2SO4 β H2NO3+ + HSO4-
- H2NO3+ β NO2+ + H2O
- H2O + H2SO4 β H3O+ + HSO4-
Which is the bronsted lowry acid and base?
Sulfuric acid acts as the acid and nitric acid acts as the base
What is the equation for benzene reacting with the nitrating mixture ?
C6H6 + NO2+ β C6H5NO2 + H+
What is C6H5NO2
Nitrobenzene
What is the method of preparation of 3-nitrobenzene
- Prepare nitrating mixture
- Dissolve some methyl benzoate in concentrated sulfuric acid
- Cool the solution and nitrating mixture to below 10ΒΊC in an ice bath
- Slowly add the nitrating mixture to the methyl benzoate
- Maintain temperature below 10ΒΊC
- Stand at room temperature for 10 minutes
- Pour the mixture onto crushed ice to form the crystals
- Suction filter off the solid methyl-3-nitrobenzoate. Wash
- Dissolve in a minimum of hot ethanol
- Filter
- Allow to cool and crystallise
- Suction filter and wash crystals with a little ice cold solvent
Draw the mechanism for the nitration of methyl benzoate
in book
What are the conditions for the monohalogenation of benzene with bromine?
Using bromine in the presence of Iron or Iron(III) Bromide
Draw the mechanism for the monohalogenation of bromine with benzene
in book
What are the conditions for the alkylation and acylation of benzene
Aluminium halides e.g. aluminium chloride
What is the reaction of the alkylation of benzene
Benzene reacts with chloroethane in the presence of aluminium chloride. The aluminium chloride acts as a halogen carrier and also a catalyst
C6H6 + CH3CH2Cl β C6H5CH2CH3 + HCl
Draw the mechanism for the alkylation of benzene with chloroethane
in book
How is the electrophile formed for the alkylation
The aluminium chloride reacts with the halogen.
CH3CH2Cl + AlCl3 β CH3CH2+ AlCl4-
How is the catalyst regenerated?
AlCl4- + H+ β AlCl3 + HCl
What is the reaction for the acylation of benzene
Benzene reacts with ethanoyl chloride in the presence of aluminium chloride to form Phenylethanone
C6H6 + CH3COCl β C6H5C0CH3 + HCl
How is the electrophile formed for the acylation
CH3COCl + AlCl3 β CH3CO+ + AlCl4-
Draw the mechanism for the acylation of benzene
in book
How is the catalyst regenerated?
AlCl4- + H+ β AlCl3 + HCl
Draw the structures of Naphthalene and Anthracene
in book
What is the difference between the reactions of benzne and alkenes?
Benzene undergo electrophilic substitution reactions where as alkenes undergo electrophilic addition reactions
Why is there a difference between how benzene and alkenes react?
Benzenes undergo substitution reactions due to the stability of the pi delocalised system of electrons. The pi covalent bond in alkenes is more open to attack from eletrophiles than benzene.
