Armoatic compounds

Question 1

The kekule model

Answer 1

The structure of benzene is a six carbon ring which is joined by alternating single and double bonds. This was disproved

Question 2

The three reasons why the kekule model was disproved

Answer 2

The lack of reactivity of benzene, the lengths of carbon-carbon bonds, the hydrogenation enthalpies

Question 3

The lack of reactivity which disproved the kekule model

Answer 3

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

Question 4

The length of the carbon-carbon bond which disproves the kekule model

Answer 4

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

Question 5

The hydrogenation enthalpies which disprove the kekule model

Answer 5

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

Question 6

The delocalised model of benzene summary

Answer 6

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

Question 7

The delocalised model of benzene full description

Answer 7

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

Question 8

Benzene with COOH

Answer 8

Benzoic acid

Question 9

Benzene with NH2

Answer 9

Phenyl amine

Question 10

Benzene with CHO

Answer 10

Benzaldehyde

Question 11

Naming aromatic compounds- prefex benzene

Answer 11

Alkyl groups, halogens, nitro (NO2),

Question 12

Naming aromatic compounds - prefex phenyl

Answer 12

An alkyl chain with a functional group, an alkyl chain with 7 or more carbon atoms

Question 13

What type of reactions does benzene and its derivatives undergo

Answer 13

A substitution reaction in which a hydrogen atom is replaced by another atom or group of atoms

Question 14

Nitration of benzene

Answer 14

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

Question 15

Halogenation of benzene

Answer 15

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

Question 16

Alkylation reactions

Answer 16

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+

Question 17

Alcylation reactions

Answer 17

When Benzene reacts with an alcyl chloride in the presence of an AlCl3 catalyst, an aromatic ketone is formed and HCl

Question 18

Comparing the reactivity of alkenes with arenes

Answer 18

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

Question 19

What is a phenol

Answer 19

Benzene bonded with an OH

Question 20

Acidity of phenol

Answer 20

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.

Question 21

Soloubility of phenol

Answer 21

Less soluble then alcohol in water due to the presence of a non-polar benzene ring

Question 22

Bromination of phenol

Answer 22

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

Question 23

Nitration of phenol

Answer 23

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.

Question 24

Why is phenol more reactive then benzene

Answer 24

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

Question 25

Further substitutions with bromine to phenyl amine

Answer 25

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

Question 26

Causes further substitution to the second and 4th position with phenyl amine

Answer 26

NH2 and OH

Question 27

Causes further substitution to the third position with phenyl amine

Answer 27

NO2

Question 28

Reaction of NO2 with phenyl amine

Answer 28

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

Question 29

Why does the delocalised model of benzene account for its stability

Answer 29

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