Chapter 25 - Aromatic Compounds Flashcards
What does the Kekule model suggest?
It suggests that benzene was based on a six members ring of carbon atoms joined by alternate single and double bonds
What evidence is there to disprove the kekule model? (Lack of reactivity)
If it contains a C=C double bond it should decolourise bromine and undergo electrophilic addition but benzene does not do either
What evidence is there to disprove the kekule model? (Length of C-C bonds)
When finding the length of the bonds by X-ray diffraction all it’s bonds were between the length of a double and a single.
What evidence is there to disprove the kekule model? (Hydrogenation enthalpies)
If the structure was cyclohexa-1,3,5-triene (kekule) it would have a hydrogenation enthalpy 3 times that of cyclohexene (which is -120kjmol) but it is -208kjmol.
Describe the structure of benzene
6 carbon atoms and 6 hydrogen atoms - hexagonal.
Therefore each carbon only used three of four electrons in bonding.
Each carbon has one electron in a p orbital and adjacent p orbitals overlap sideways and form a ring of electrons density.
This creates a pi-system which spreads over all 6 carbons.
The 6 electrons in the system are said to be delocalised.
How do you name benzene?
Halogens and nitro groups are prefixes
When a benzene ring is attached to an alkyl chain of 7 or more or a functional group the prefix phenyl is used.
Ring is numbered using lowest combination of numbers
What are the conditions needed for nitration of benzene?
50 Celsius and a H2SO4 catalyst
Reaction mechanism of the nitration of benzene
1) electrophile NO2+ is produced by reaction of conc nitric acid and sulfuric acid
2) electrophile accepts pair of electrons from benzene ring.
3) dative covalent bond formed
4) Pair of electrons transferred from carbon hydrogen bond to positive charge forming pi system.
5) Breaks down into nitrobenzene and H+ ions. H+ ions react with HSO4- to regenerate catalyst.
What are the two halogen carriers?
Al and Fe
Describe the bromination of benzene
1) bromonium Ion formed when halogen carrier reacts with bromine
2) Bromonium ion accepts a pair of electrons from benzene ring. Dative covalent bond formed.
3) Pair of electrons transferred from carbon hydrogen bond to positive charge reforming pi system.
4) Breaks down into bromobenzene and H+ ion. H+ ion reacts with FeBr4- ion to regenerate FeBr3 catalyst
Describe the alkylation reaction of benzene.
Alkyl group replaces hydrogen atoms in the presence of AlCl3
This reacts with the group e.g C2H5 + AlCl3 -> C2H5Cl
Reactivity of alkenes
Pi bond in cyclohexene contains localised electrons above and below the plane. This high electron density induces dipole in non-polar bromine and the slightly positive side makes it act like a good electrophile
Reactivity of benzene
Has delocalised pi electrons above and below the plane of the carbon atoms. It’s electrons are delocalised so the electron density is less than a double bond and insufficient to polarise bromine. No reaction can take place unless a halogen carrier is present.
Solubility of phenol vs alcohol
Phenol is less soluble in water than alcohols due to non-polar benzene ring but it does dissociate in water.
Compare the reactivity of phenols, carboxylic acids and ethanol.
Phenol is more acidic than alcohols but less acidic than carboxylic acids.
Phenols and carboxylic acids react with solutions of strong bases
Ethanol doesn’t react with strong or weak bases
Carboxylic acids are strong enough to react with weak bases.
What is phenol?
A hydroxyl -OH group attached to a benzene ring directly
Bromination of phenol
Halogen carrier not required
Bromine decolourises and a white precipitate forms
Why is phenol more reactive that benzene?
Because the lone pair of electrons from oxygen p-orbitals are donated to the pi system. This means the electron density in the benzene ring of phenol is increased and electrophile are attracted more strongly than with benzene.
What type of group is -NH2?
2 and 4 directing group
What type of group is -OH?
2,4 directing group
What type of group is -C6H5?
2,4 directing group
What type of groups are -F, -Cl, -Br, -I?
2, 4 directing groups
What happens if bromine reacts with phenylamine?
It is substituted to position 2 or 4
What happens when bromine reacts with nitrobenzene?
They react slowly together at a high temp and it requires a halogen carrier.
NO2 group deactivates the aromatic ring
Second substituent goes to position 3
What type of group is -NO2?
3, 5 directing group
What type of group is -COOH ?
It is a 3, 5 directing group
What type of group is -CHO?
It’s a 3, 5 directing group
What type of group is -CN?
It is a 3,5 directing group
Describe the alkylation of benzene
Hydrogen atoms are substituted by an alkyl group in the presence of AlCl3.
Carried out by reacting benzene with a haloalkane
What are the conditions for the nitration of phenol?
They are milder than those required for the nitration of benzene and only dilute nitric acid is required
What do all 2, 4 directing groups do? (Apart from the halogens)
They activate the benzene ring making it more susceptible to attacks from electrophiles
What do all 3 directing groups do?
They deactivate the benzene ring making it less susceptible to attacks from electrophiles
Catalyst and reagent for nitration of benzene
Catalyst = H2SO4 Reagent = HNO3
Catalyst and reagent for bromination of benzene
Catalyst = FeBr3 Reagent = Br2
Catalyst and reagent for chlorination of benzene
Catalyst = AlCl3 Reagent = Cl2
