Chapter 25 Aromatic compounds

Question 1

Describe Kekule’s model of benzene

Answer 1

A hexagonal ring of six carbon atoms, with each carbon atom joined together with alternating single and double bonds. Each carbon atom is attached to 2 carbon atoms and 1 hydrogen atom

Question 2

What are the three main pieces of evidence that disproved kekule’s model?

Answer 2

• Benzene contains C==C but doesn’t decolourise bromine water
• All the lengths of the carbon-carbon bonds via xray diffraction proved that they are the same
• Hydrogenation enthalpies, should be 3x more than cyclohexene but alot lower than it should be

Question 3

What type of hydrocarbon is benzene?

Answer 3

Aromatic or arene

Question 4

Describe the delocalised model of benzene (6 marks)

Answer 4

• Benzene exists as a planar, cyclic, hexagonal hydrocarbon containing six carbon and six hydrogen atoms
• Each carbon atom uses three of its available four electrons in bonding to two other carbon atoms and to one hydrogen atom
• Each carbon atom has one electron in a p-orbital at right angles 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
• This overlapping of the p-orbitals creates a system of Pi-bonds which spread over all six of the carbon atoms in the ring structure

Question 5

What type of substitution reaction does benzene undergo?

Answer 5

Benzene undergoes electrophilic substitution reactions in which a hydrogen atom on the benzene ring is replaced by another atom or group of atoms

Question 6

What are the requirements for benzene to form nitrobenzene?

Answer 6

Benzene must react with nitric acid with H2SO4 present at 50c. Water is produced

Question 7

When can a halogen react with benzene?

Answer 7

Halogens can only react with benzene when a halogen carrier is present

Question 8

What are some common halogen carriers?

Answer 8

AlCl3
FeCl3
AlBr3
FeBr3

Question 9

What are phenols?

Answer 9

Phenols are organic compounds containing an -OH functional group directly bonded to a benzene ring

Question 10

What are some common uses of phenol?

Answer 10

Disinfectants, detergents, plastics, paints and even aspirin

Question 11

Why is phenol considered a weak acid?

Answer 11

Phenol has a non-polar benzene ring attached to it so it doesn’t dissolve well in a polar solvent like water

When phenol is dissolved in water, phenol will only partially dissociate forming a phenoxide ion and a H+ ion, this makes phenol a weak acid

Question 12

Describe what happens with the bromination of phenol

Answer 12

Phenol will react with bromine water to form 2,4,6-tribromophenol and this reaction will decolorise bromine water and doesn’t require a halogen carrier

Question 13

Describe what happens with the nitration of phenol

Answer 13

Phenol will react with nitric acid to form a mixture of two products, 2-nitrophenol and 4-nitrophenol

Question 14

Explain why phenol is more reactive than benzene

Answer 14

Phenol is more reactive than benzene because of the lone pair of electrons from its hydroixde group that is added to the pi orbital ring in phenol making its electron density higher than benzenes and therefore phenol will more easily react with electrophiles

Question 15

Describe what the -NH2 group does to an aromatic ring

Answer 15

The -NH2 group activates the ring as the aromatic ring reacts more readily with electrophiles and directs the second substituent to position 2 or 4

Question 16

Describe what the -NO2 group does to an aromatic ring

Answer 16

The -NO2 group deactivates the ring as the aromatic ring reacts less readily with electrons and directs the second substituent to position 3

Question 17

Unlike alkenes, why does benzene not react with bromine unless a halogen carrier is present

Answer 17

Benzene has delocalised Pi-electrons spread above and below the plane of the carbon atoms in the ring structure and the electron density around any two carbon atoms in the benzene ring is less than that in a C==C double bond in an alkene

So when a non-polar molecule such as bromine approaches the benzene ring this is insufficient Pi-electron density around any two carbon atosm to polarise the bromine molecule

This is why a halogen carrier is required otherwise the reaction cannot take place

Question 18

Describe the alkylation of benzene

Answer 18

The alkylation of benzene is the substitution of a hydrogen atom in the benzene ring by an alkyl group

This reaction is carried out by reacting benzene with a haloalkane in the presence of AlCl3

Question 19

What is alkylation?

Answer 19

The processs of increasing the number of carbon atoms in a compound by forming carbon-carbon bonds

Question 20

Describe the acylation of benzene

Answer 20

The acylation of benzene is a substitution reaction when benzene reacts with an acyl chloride in the presence of an AlCl3 catalyst, an aromatic compound is formed