25 - aromatic compounds

Question 1

what are the key features of benzene?

Answer 1

colourless, sweet smelling, highly flammable liquid. Found naturally in crude oil. consists of hexagonal rings of 6 carbon atoms

Question 2

what is Kekule’s model

Answer 2

suggested benzene was based on a six membered ring of carbon atoms joined by alternative single and double bonds

Question 3

what are the 3 main pieces of evidence that disprove kekule’s model?

Answer 3

1. Lack of reactivity of benzene
2. length of c=c bonds in benzene
3. hydrogenation enthalpies

Question 4

How does lack of reactivity of benzene disprove kekule’s model?

Answer 4

It should decolourise bromine. However, benzene doesn’t undergo electrophilic addition reactions and it doesn’t decolourise bromine under normal conditions.

Question 5

How do length of carbon carbon bonds in benzene disprove Kekule’s model?

Answer 5

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

Question 6

How does benzene’s hydrogenation enthalpies disprove Kekule’s model?

Answer 6

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.

Question 7

What is the delocalised model of benzene?

Answer 7

was developed by scientists who decided that the experimental evidence was enough to disprove Kekule’s structure of benzene.

Question 8

What are the 6 main features of the delocalised model of benzene?

Answer 8

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.

Question 9

How do you name aromatic compounds with one substituent group?

Answer 9

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

Question 10

Explain the nitration of benzene

Answer 10

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

Question 11

Explain the mechanism for the nitration of benzene

Answer 11

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.

Question 12

Explain the halogenation of benzene

Answer 12

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.

Question 13

Explain the halogenation of benzene.

Answer 13

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.

Question 14

Explain the chlorination of benzene

Answer 14

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.

Question 15

What is the alkylation of benzene?

Answer 15

It is the substitution of a hydrogen atom in the benzene ring by an alkyl group. It increases the length of the carbon chain.

Question 16

How is the alkylation of benzene carried out?

Answer 16

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

Question 17

What is an acylation of benzene?

Answer 17

When benzene reacts with an acyl chloride in the presence of an AlCl3 catalyst, an aromatic ketone is formed. It is electrophilic substitution.

Question 18

Compare the reactivity of alkenes and arenes

Answer 18

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.

Question 19

What are phenols?

Answer 19

They’re a type of organic chemical containing a hydroxyl -OH group directly bonded to an aromatic ring.

Question 20

How can a phenol act as a weak acid?

Answer 20

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.

Question 21

How can you distinguish between a phenol and a carboxylic acid?

Answer 21

A reaction with sodium carbonate, the carboxylic acid will react with sodium carbonate to produce CO2 as a gas.

Question 22

Explain the reaction of a phenol with sodium hydroxide

Answer 22

Phenol reacts with sodium hydroxide to form the salt, sodium phenoxide, and water in a neutralisation reaction.

Question 23

Explain the bromination of phenol

Answer 23

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.

Question 24

Explain the nitration of phenol

Answer 24

Phenol reacts readily with dilute nitric acid at room temp. A mixture of 2-nitrophenol and 4-nitrophenol is formed. The reaction also produces water.

Question 25

Compare the reactivity of phenol and benzene

Answer 25

Bromine and nitric acid react more readily with phenol than they do with benzene. Phenol is nitrated with dilute nitric acid rather than needing conc nitric and sulfuric acids as with benzene. The increased reactivity is caused by the lone pair of electrons from the p-orbital of the -OH group being donated into the pi bond system of phenol, and therefore the electron density of the benzene ring in phenol is increased. The increased electron density attracts electrophiles more strongly than with benzene.

Question 26

Give an example of an activating group on an aromatic ring

Answer 26

Bromine reacts rapidly with phenylamine. The -NH2 group activates the ring as the aromatic ring reacts more readily with electrophiles.

Question 27

Give an example of a deactivating group on an aromatic ring

Answer 27

Nitrobenzene reacts slowly with bromine, requiring both a halogen carrier catalyst and a high temperature. The benzene ring in nitrobenzene is less susceptible to electrophilic substitution than benzene its self. The -NO2 group deactivates the aromatic ring as the ring reacts less readily with electrophiles.

Question 28

What is a directing group?

Answer 28

Different groups can be attached to a benzene ring. Different groups can have a directing effect on any second substituent on the benzene ring.
All 2- and 4- directing groups are activating groups, except halogens.
All 3- directing groups are deactivating groups