25 - aromatic compounds Flashcards
what are the key features of benzene?
colourless, sweet smelling, highly flammable liquid. Found naturally in crude oil. consists of hexagonal rings of 6 carbon atoms
what is Kekule’s model
suggested benzene was based on a six membered ring of carbon atoms joined by alternative single and double bonds
what are the 3 main pieces of evidence that disprove kekule’s model?
- Lack of reactivity of benzene
- length of c=c bonds in benzene
- hydrogenation enthalpies
How does lack of reactivity of benzene disprove kekule’s model?
It should decolourise bromine. However, benzene doesn’t undergo electrophilic addition reactions and it doesn’t decolourise bromine under normal conditions.
How do length of carbon carbon bonds in benzene disprove Kekule’s model?
the xray diffraction of benzene showed that all the bonds were the same lengths, between the length of single bonds and double bonds. ( was 0.139nm, single bond = 0.153nm and double bond= 0.134nm
How does benzene’s hydrogenation enthalpies disprove Kekule’s model?
If it had the structure that kekule suggested, the enthalpy change of hydrogenation would be 3x that of cyclohexane. However the enthalpy change of hydrogenation of benzene is a lot lower than expected, so benzene is a lot more stable than expected.
What is the delocalised model of benzene?
was developed by scientists who decided that the experimental evidence was enough to disprove Kekule’s structure of benzene.
What are the 6 main features of the delocalised model of benzene?
It is planar, cyclic, hexagonal hydrocarbon containing 6 c atoms and 6 h atoms. Each C uses 3/4 of it’s available electrons to bond to 2 other Cs and one H. Each C has 1 electron in a p-orbital at right angles to the plane of the bonded C and H atoms. Adjacent p-orbital electrons overlap sideways, in both directions above and below the plane of C atoms to form a ring of electron density. Overlapping p- orbitals creates a pi bond system . The 6 electrons that make up the pi bond system are delocalised.
How do you name aromatic compounds with one substituent group?
benzene ring is often parent chain. When the benzene ring is attached to an alkyl chain with a functional group or an alkyl chain with 7 or more carbon atoms, benzene is considered to be the substituent, and the prefix phenyl is used
Explain the nitration of benzene
Benzene reacts slowly with nitrogen to form nitrobenzene. The reaction is catalysed by sulfuric acid and heated to 50 degrees to obtain a good rate of reaction. A water bath can be used to maintain a steady temperature. Its electrophilic substitution
Explain the mechanism for the nitration of benzene
1.NO2+ is the electrophile in this reaction, produced from the reaction of conc nitric acid and conc sulfuric acid.
2. NO2+ accepts a pair of electrons from the benzene ring to form a dative covalent bond. The organic intermediate formed is unstable and breaks down to form nitrobenzene and an H+ ion. A stable benzene ring is formed.
3. The H+ ion reacts with the HSO4- ion from step 1 to regenerate the catalyst H2SO4.
Explain the halogenation of benzene
Halogens don’t react with benzene unless a halogen carrier is present. e.g AlCl3 and FeBr3, which can be generated in situ from the metal and halogen.
Explain the halogenation of benzene.
Electrophilic substitution - can happen at rtp in the presence of a halogen carrier.
1. Benzene is too stable to react with a non-polar Br molecule. Br+ is generated when the halogen carrier catalyst reacts with bromine.
2. The Br+ ion accepts a pair of electrons from the benzene ring to form a dative covalent bond. The organic intermediate is unstable and breaks down to form bromobenzene and an H+ ion.
3. The H+ ion reacts with the ion of the halogen carrier formed in step 1, to regenerate it.
Explain the chlorination of benzene
chlorine will react with benzene in the same way as bromine, and follows the same mechanism. The halogen carrier used is FeCl3, AlCl3 or iron metal and chlorine.
What is the alkylation of benzene?
It is the substitution of a hydrogen atom in the benzene ring by an alkyl group. It increases the length of the carbon chain.
How is the alkylation of benzene carried out?
Adding benzene with a haloalkane in the presence of AlCl3, which acts as a halogen carrier catalyst, generating the electrophile.
e.g benzene+ C2H5Cl = ethyl benzene + HCl
What is an acylation of benzene?
When benzene reacts with an acyl chloride in the presence of an AlCl3 catalyst, an aromatic ketone is formed. It is electrophilic substitution.
Compare the reactivity of alkenes and arenes
Alkenes decolourise bromine by an electrophilic addition reaction. E.g Cyclohexene + Br2 = 1,2 dibromocyclohexane. This reaction adds across the double bond in cyclohexene.
1. Pi bond in alkene has localised electrons above and below the plane of the 2 C atoms in the double bond. Has high electron density.
2. Localised electrons in the pi bond induce a dipole on the non-polar Br molecule.
3.The slight positive Br atom enables the Br molecule to act as an electrophile.
Benzene doesn’t react with bromine unless a halogen carrier is present, as benzene has delocalised pi electrons in the pi bond system above and below the ring structure. Therefore the electron density around any 2 Cs in benzene will have a lower electron density than a C=C in an alkene. When a non-polar molecule approaches benzene, there is insufficient pi electron density around any 2 Cs to polarise the bromine molecule, so no reaction takes place.
What are phenols?
They’re a type of organic chemical containing a hydroxyl -OH group directly bonded to an aromatic ring.
How can a phenol act as a weak acid?
It is less soluble in water than alcohols due to the insoluble benzene ring. When dissolved in water, phenol partially dissociates to produce protons. Therefore it is less acidic than carboxylic acids but more acidic than alcohols.
How can you distinguish between a phenol and a carboxylic acid?
A reaction with sodium carbonate, the carboxylic acid will react with sodium carbonate to produce CO2 as a gas.
Explain the reaction of a phenol with sodium hydroxide
Phenol reacts with sodium hydroxide to form the salt, sodium phenoxide, and water in a neutralisation reaction.
Explain the bromination of phenol
Phenol reacts with an aqueous solution of bromine to form a white precipitate of 2,4,6-tribromophenol. The reaction decolourises bromine water. With a phenol, a halogen carrier catalyst isn’t required and the reaction is carried out at RTP.
