6.1.1 - Aromatic Compounds Flashcards
What was Kekule’s idea?
A structure which had 6 carbons in a ring with an alternating double and single bonds.
What was the issue with bromine water and Kekule’s idea?
The substance did not react with bromine water like an alkene, so no addition reaction was occurring.
What was the issue with the bonds and Kekule’s idea?
Bond lengths for double and single bonds are different but in benzene they are all equal forming a regular hexagon.
What was the issue with enthalpy of hydrogenation and Kekule’s idea?
The enthalpy of hydrogenation isn’t 3 times the hydrogenation of cyclohexene, which suggested it is more stable than Kekule’s structure.
What is a pi bond?
Sideways overlap of p orbitals above and below the nucleus.
What are sigma bonds?
Head on overlap of orbitals directly between the bonding atoms.
What is the actual structure of benzene?
6 pi electrons forming a delocalised pi bond above and below the ring.
Describe the carbons in benzene.
Each carbon atom is bonded to 2 other carbon atoms and 1 hydrogen atom. These 3 bonds are called sigma bonds.
What is formed around each carbon?
A planar hexagonal ring with a trigonal planar shape around each carbon.
What is left in the structure?
A 4th electron on each carbon in a p-orbital above and below the plan of carbon atoms.
What do the pi orbitals do?
Overlap sideways in both directions to form a delocalised pi system. The 6 electrons are now spread above and below the ring, they are delocalised so there is less repulsion between them, making benzene more stable.
What are arenes?
Hydrocarbons containing one or more benzene rings.
What is an aromatic compound?
Any compound with a benzene ring.
In the nitration of benzene what is added to the benzene ring?
NO2
What are the reagents and conditions for the nitration of benzene?
Conc HNO3 in the presence of conc H2SO4 catalyst. Reflux at 50C.
What is reflux?
Continuous process of evaporation and condensation to prevent volatile reagents escaping.
What may happen if the temperature is increased in the nitration of benzene?
Further substitution.
What is added to the benzene ring during the halogenation of benzene?
A halogen
What are the reagents and conditions for the halogenation of benzene?
A halogen in the presence of a halogen carrier catalyst, (e.g AlCl3 or iron chloride or iron metal when adding chlorine).
What happens during alkylation?
When a hydrogen is replaced by an alkyl group.
What are the reagents and conditions for alkylation?
Haloalkane in the presence of a halogen carrier catalyst.
How can you limit multiple substitutions occurring?
Use excess benzene.
What is an acyl group?
An alkyl R group attached to a C=O
What are the conditions for acylation?
Halogen carrier catalyst, reflux 60C for 30minutes, anhydrous conditions.
Why is acylation an important reaction in synthesis?
A new C-C bond has been formed to the aromatic ring.
What mechanism occurs with alkanes?
Free radical substitution.
What mechanism occurs with alkenes?
Electrophilic addition.
What mechanism occurs with haloalkanes?
Nucleophilic substitution
What mechanism occurs with benzene?
Electrophilic addition
What mechanism occur with aldehydes and ketones?
Nucleophilic addition
Describe the mechanism for electrophilic substitution?
- Electrophile is attracted to the electron rich pi system.
- Electrophile accepts a pair of pi electrons from the ring.
- Unstable intermediate formed (pi system temporarily disrupted).
- H+ leaves, putting 2 electrons back into the ring.
What is the equation for the generation of the electrophile in nitration?
HNO3 + H2SO4 -> HSO4- + NO2+ + H2O
What is the equation for the generation of the electrophile in the halogenation of benzene using bromine?
Br2 + AlBr3 -> AlBr4- + Br+
What is the equation for the generation of the electrophile in the alkylation of benzene using chloroethane?
CH3CH2Cl + AlCl3 -> AlCl4- + CH3CH2+
What is the equation for the generation of the electrophile in the alkylation of benzene with ethanoyl chloride?
CH3COCl + AlCl3 -> AlCl4- + CH3CO+
What does a halogen carrier catalyst do?
A halogen carrier catalyst takes a halogen atom from the reagent and carries it by forming a covalent bond to the halogen atom.
How does reaction of benzene and an alkene differ?
Alkenes decolourise when Br2 is added, whereas benzene does not unless a catalyst is added.v
How is the pi bond different in a cycloalkene than benzene?
In alkenes the pi bond is localised (shared between 2 atoms) but in benzene there is a delocalised pi system (shared between more than 2 atoms).
Compare electron density of alkenes and benzene.
Alkenes have a higher electron density so are more attractive to electrophiles and so polarise more strongly. As benzene has a lower electron density, it is less attractive to electrophiles and so polarises them less strongly.
Why do alkenes and benzenes react by different mechanisms?
They are different because substitution keeps the delocalised pi system intact, whereas addition would permanently disrupt it, so the extra stability would be lost.
What is a phenol?
When an -OH group is directly bonded to a benzene.
What is the formula for a phenyl group?
C6H5
What is the formula for a phenol group?
C6H5OH
What bonding does phenol contain?
Hydrogen and so slightly soluble in water.
What is phenol at room temperature?
Solid
Is the benzene ring polar?
No.
What is phenol?
A very weak acid.
What is the equation for the dissociation of phenol?
C6H5OH H+ + C6H5O-
Phenol + NaOH?
Sodium phenoxide (O-Na+) + H2O
Why is sodium phenoxide more soluble in water than phenol?
It is an ionic salt.
Sodium phenoxide + HCl?
Phenol + NaCl
Sodium + phenol equation?
2Na + 2C6H5OH -> 2C6H5O-Na+ + H2
Why doesn’t phenol react with carbonates?
Phenol is too weak of an acid.
Bromine + phenol reagents + conditions?
Bromine water, no catalyst, room temperature.
What observations are there when bromine reacts with phenol?
Decolourises bromine water, and white precipitate forms.
What is formed when phenol reacts with bromine?
2,4,6-tribromophenol + HBr
What are the reagents and conditions when nitric acid reacts with phenol?
Dilute nitric acid without a catalyst.
What is produced when phenol reacts with dilute nitric acid?
Mixture of products: 2-nitrophenol and 4-nitrophenol + H2O
What is produced when concentrated nitric acid is reacted with phenol?
2,4,6-trinitrophenol.
What shows phenol is more reactive than benzene?
Phenol decolourises bromine water and reacts with dilute nitric acid without a catalyst.
Why is phenol more reactive than benzene?
There is a lone pair of electrons in a p orbital on the oxygen atom which is able to overlap with the delocalised pi system. This leads to higher electron density in the ring, resoling in the electrophile being attracted more strongly and polarised more strongly.
Where and why do -OH and -NH2 groups bond?
They have a 2,4-directing effect as they are both electron donating groups.
Where and why do -NO2 groups bond?
Nitro groups have a 3-directing effect due to it being an electron withdrawing group.
