Armoatic compounds Flashcards
The kekule model
The structure of benzene is a six carbon ring which is joined by alternating single and double bonds. This was disproved
The three reasons why the kekule model was disproved
The lack of reactivity of benzene, the lengths of carbon-carbon bonds, the hydrogenation enthalpies
The lack of reactivity which disproved the kekule model
In this model the c double bond c would decolourise bromine in an electrophilic addition reaction, as the bromine is added across the double bond, however this does not happen and bromine remains brown
The length of the carbon-carbon bond which disproves the kekule model
In benzene all the bonds have the same length, between that of a single and double bond. In the kekule model you would expect the bonds to have different lengths according to whether they had a single or double bond
The hydrogenation enthalpies which disprove the kekule model
If benzene had the kekule structure you would expect the enthalpy change of hydrogenation to be three times that of cyclohexene, as benzene would have three double bonds instead of just one. However, the enthalpy change of hydrogenation was less negatie then expected
The delocalised model of benzene summary
There is the sideways overlap of p orbitals, which causes the delocalised ring of electron density to be above and below the plane of the benzene ring, forming a system of pie bonds
The delocalised model of benzene full description
Benzene is a planar, cyclic, hexagonal hydrocarbon containing six carbon atoms and six hydrogen atoms. Each carbon uses 3 of its 4 electrons to bond to two carbon atoms and a hydrogen atom. Each carbon atom has an electron in a p-orbital at a right angle to the plane of the bonded carbon and hydrogen atoms. Adjacent p-orbital electrons overlap sideways in both directions above and below the plane of the carbon atoms to form a ring of electron density. The overlapping p-orbitals form a system of pie bonds over all six carbon atoms, the six electrons in the pie bond are delocalised
Benzene with COOH
Benzoic acid
Benzene with NH2
Phenyl amine
Benzene with CHO
Benzaldehyde
Naming aromatic compounds- prefex benzene
Alkyl groups, halogens, nitro (NO2),
Naming aromatic compounds - prefex phenyl
An alkyl chain with a functional group, an alkyl chain with 7 or more carbon atoms
What type of reactions does benzene and its derivatives undergo
A substitution reaction in which a hydrogen atom is replaced by another atom or group of atoms
Nitration of benzene
Benzene reacts with nitric acid to form nitrobenzene, this is catalysed by sulphuric acid and refluxed at fifty degrees, this is done by using a water bath.
start HNO3 + H2SO4 –> NO2+ + H2O + HSO4-
end H+ + HSO4- –> H2SO4
Halogenation of benzene
It is catalysed by a halogen carrier i.e. AlCl3, this can be generated in situ from the halogen and the metal. The reaction is done at room temperature and pressure. The electrophile Br+ is generated when the halogen carrier reacts with the halogen.
start Br2 + FeBr3 –> FeBr4- + Br+
end H+ + FeBr4- –> FeBr3 + HBr
Alkylation reactions
This is the substitution of a hydrogen with an alkyl chain. This reaction is done with a haloalkane i.e. C2H5Cl in the presence of AlCl3, which acts as a halogen carrier catalyst generating C2H5+
Alcylation reactions
When Benzene reacts with an alcyl chloride in the presence of an AlCl3 catalyst, an aromatic ketone is formed and HCl
Comparing the reactivity of alkenes with arenes
Unlike alkenes benzene does not react with bromine unless a halogen carrier is present. They can not undergo electrophilic addition because benzene has the delocalised electrons spread over and above the plane of the carbon atoms. The electron density between any two adjacent carbon atoms in benzene is less then in alkenes where the electrons are localised. When a non-polar molecule such as bromine approaches the benzene ring there is insufficient pie electron density around any two carbon atoms to polarise the bromine molecule
What is a phenol
Benzene bonded with an OH
Acidity of phenol
More acidic then alcohol as it can react with strong bases such as NaOH, but less acidic then carboxylic acids as it can not react with weak bases such as sodium carbonate. It is defined as a weak acid meaning it partially dissociates to produce protons.
Soloubility of phenol
Less soluble then alcohol in water due to the presence of a non-polar benzene ring
Bromination of phenol
Phenol reacts with bromine water (an aqueous soloution of bromine) to form a white precipitate of 2,4,6 tribromophenol. Bromine water is decolourised. No catalyst is needed
Nitration of phenol
Phenol readily reacts with dilute nitric acid at room temperature. A mixture of 2-nitrophenol and 4-nitrophenol is produced. This is in contrast to the reaction with bezene where concentrated nitric acid where a sulphuric acid catalyst is used.
Why is phenol more reactive then benzene
The increased reactivity is caused by the lone pair of electrons from the oxygen being donated into the pie system of the benzene ring, thus increasing the electron density. The increased electron density attracts electrophiles more strongly then benzene. For bromine, the electron density of the ring structure is sufficient to polarise bromine molecules so no halogen carrier is needed
Further substitutions with bromine to phenyl amine
The bromine will join on the 2nd, 4th, and 6th position. This is because NH2 activates the ring so the aromatic ring reacts readily with electrophiles. No need for a halogen carrier
Causes further substitution to the second and 4th position with phenyl amine
NH2 and OH
Causes further substitution to the third position with phenyl amine
NO2
Reaction of NO2 with phenyl amine
The NO2 deactivates the aromatic ring so the ring reacts less readily with electrophiles. Nitrobenzene reacts much more slowly with bromine, requiring a halogen carrier catalyst and high temperatures. The benzene ring in nitrobenzene is less susceptible to electrophilic addition then benzene itself
Why does the delocalised model of benzene account for its stability
The delocalised model has the pi bond electron density spread over and above the benzene ring instead of localised. As in the kekule model there are concentrated areas of electron density, meaning they more easily undergo electrophillic addition
