M6 Aromatic Compounds

Question 1

Describe the properties of benzene

Answer 1

• A colourless, sweet smelling, highly flammable liquid.
• Found naturally in crude oil, is a component of petrol, and also found in cigarette smoke.
• Classified as a carcinogen.

Question 2

What is Kekule’s model of benzene?

Answer 2

Based on a six membered ring of carbon atoms joined by alternate single and double bonds

Question 3

What is the evidence to disprove Kekule’s model?

Answer 3

1. The lack of reactivity of benzene:
   If benzene contained the C=C bonds in his model, it should decolourise bromine in an electrophilic addition reaction. However:
   - benzene does not undergo electrophilic addition reactions
   - benzene does not decolourise bromine under normal conditions
   This lead scientists to suggest that benzene cannot have any C=C bonds in it’s structure.
2. The lengths of the carbon-carbon bonds in benzene:
   Using X-ray diffraction, you can measure bond lengths in a molecule. When benzene was discovered all of the bonds were 0.139nm in length. This bond length was between the length of a single bond (0.152nm) and a double bond (0.134nm).
3. Hydrogenation enthalpies:
   If benzene did have the Kekule structure, then it would be expected to have an enthalpy change of hydrogenation that is 3 times one of cyclohexene.
   - When cyclohexene is hydrogenated, one double bond reacts with hydrogen. The enthalpy change of hydrogenation is -120kJ/mol.
   - As the Kekule structure is predicted to contain three double bonds and the expected enthalpy change for reacting three double bonds with hydrogen would be 3 x -120 = -360 kJ/mol.
   - The actual enthalpy change of hydrogenation of benzene is only -208 kJ/mol. Therefore 152 kJ/mol less energy is produced than expected. Therefore the actual structure of benzene is more stable than the theoretical Kekule model of benzene.

This information lead scientists to propose the delocalised model of benzene.

Question 4

Describe the delocalised model of Benzene

Answer 4

• Benzene is a planar, cyclic, hexagonal hydrocarbon containing 6 carbon atoms and 6 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 all over six of the carbon atoms in the ring structure - attracting electrophiles
• The six electrons occupying this system of pi-bonds are said to be delocalised.

Question 5

How to name aromatic compounds with one substituent group?

Answer 5

• Aromatic compounds with one substituent group are monsubstituted.
• In aromatic compounds, the benzene ring is often considered to be the parent chain.
• Alkyl groups, halogens and nitro groups are all considered prefixes to benzene.
• When a benzene ring is attached to an alkyl chain with a functional group or to an alkyl chain with seven or more carbon atoms, benzene is considered to be a subsituent.
• In compounds with more than one substituent on the benzene ring, the ring is numbered. The substituent groups are listed in alphabetical order using the smallest numbers possible.

Question 6

Describe electrophilic substitution of benzene

Answer 6

• Benzene reacts with electrophiles in which a hydrogen atom on the benzene ring is replaced by an electrophile (E+)

Typical equation:
H E
| |
benzene + E+ –> benzene + H+

Question 7

Describe nitration of benzene

Answer 7

• Benzene reacts slowly with nitric acid to form nitrobenzene.
• The reaction is catalysed by concentrated sulfuric acid and heated to 50 degrees Celsius to obtain a good rate of reaction.
• In nitration, one of the hydrogen atoms on the benzene ring is replaced by a nitro (-NO2) group.
  NO2
  |
  benzene + HNO3 –> benzene
  + H2O

Question 8

What will happen during nitration of benzene if temperature rises above 50 degrees Celsius?

Answer 8

Further substitution reactions may occur leading to the production of dinitrobenzene.

benzene + 2 H2O –> dinitrobenzene + 2 H2O

Question 9

Uses of nitrobenzene

Answer 9

Important starting material in the preparation of dyes, pharmaceuticals and pesticides. It can be used as a starting material in the preparation of paracetamol.

Question 10

What is the electrophile in nitration of benzene?

Answer 10

Nitronium ion - NO2 +

Question 11

Describe the mechanism for nitration of benzene

Answer 11

1. Generation of the electrophile:
   HNO3 + H2SO4 –> NO2+ + H2SO4- +H2O
   Concentrated nitric acid and concentrated sulfuric acid react to produce a nitronium ion (the electrophile).
2. Electrophilic attack:
   - The electrophile (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 the organic product, nitrobenzene and the H+ ion. A stable benzene ring is reformed.

benzene with curly arrow to NO2+ electrophile –> intermediate (benzene with horseshoe shape and positive charge, and H and NO2 poking out end, curly arrow from H bond to positive charge) –> nitrobenzene + H+

3. Regeneration of the catalyst:
   H+ + HSO4- –> H2SO4

Question 12

Describe halogenation of benzene

Answer 12

• Halogens do not react with benzene unless a catalyst called a halogen carrier is present.
• Common halogen carriers include AlCl3, AlBr3 and FeBr3, which can be generated in situ from the metal and the hydrogen.

Question 13

Describe bromination of benzene

Answer 13

• At room temperature and pressure and in the presence of a halogen carrier, benzene reacts with bromine in an electrophilic substitution reaction.
• In bromination, one of the hydrogen atoms on the benzene ring is replaced by a bromine atom.
  FeBr3/AlBr3
  benzene + Br2 —–> bromobenzene + HBr

Question 14

Describe the mechanism for bromination of benzene

Answer 14

1. Generation of the electrophile:
   Benzene is too stable to react with a non-polar bromine molecule. The electrophile is the bromonium ion (Br+) which is generated when the halogen carrier catalyst reacts with bromine.
   Br2 + FeBr3 —> FeBr4- + Br+
2. Electrophilic attack:
   - The bromonium 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 the organic product bromobenzene, and a H+ ion.
   benzene with curly arrow to Br+ –> intermediate (horseshoe shape with positive charge, extended bonds with H and Br, curly arrow from H bond to positive charge) –> bromobenzene + H+
3. Regeneration of the electrophile
   H+ + FeBr4- —> FeBr3 + HBr

Question 15

Describe chlorination of benzene

Answer 15

• Chlorine will react with benzene in the same way as bromine following the same mechanism.
• The halogen carrier is FeCl3, AlCl3 or iron metal and chlorine, which react to make FeCl3.
  FeCl3 or AlCl3
  benzene + Cl2 ——-> chlorobenzene + HCl

Question 16

Describe alkylation reactions of benzene

Answer 16

• The alkylation of benzene is the substitution of a hydrogen atom in the benzene ring by an alkyl group.
• The reaction is carried out by reacting benzene with a haloalkane in the presence of AlCl3, which acts as a halogen carrier catalyst, generating the electrophile.
• Alkylating increases the number of carbon atoms in a compound by forming carbon-carbon bonds.
• The reaction is sometimes called a Friedel-Crafs alkylation after the two chemists who first carried out the reaction. AlCl3
  eg. benzene + C2H5Cl ——> ethylbenzene + HCl

Question 17

Describe acylation reactions of benzene

Answer 17

• When benzene reacts with acyl chloride in the presence of an AlCl3 catalyst, an aromatic ketone is formed.
• This is called an acylation reaction and is another example of electrophilic substitution. The reaction forms carbon-carbon bonds and is useful in organic synthesis.
• Ethanocyl chlorine (CH3COCl) is the first member of the acyl chloride homologous series. Phenylethanone in produced in the reaction between benzene and ethanoyl chloride:
  AlCl3
  benzene + ethanoyl chloride ——> phenylethanone + HCl

Question 18

Compare the reactivity of alkenes with arenes?

Answer 18

• Alkenes decolourise bromine by an electrophilic addition reaction. Eg. the reaction between cyclohexene and bromine, in this reaction bromine adds across the double bond in the cyclohexene:
  • The pi-bond in The alkene contains localised electrons above and below the plane of the two carbon atoms in the double bond. This produces an area of high electron density.
  • The localised electrons in the pi-bond induce a dipole in the non-polar bromine molecule making one bromine atom of the Br2 molecule slightly positive and the other slightly negative.
  • The slightly positive bromine atom enables the bromine molecule to act like an electrophile.
• Unlike alkenes, benzene does not react with bromine unless a halogen carrier catalyst is present. This is because benzene has delocalised pi-electrons spread above and below the plane of the carbon atoms in the ring structure.
• 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.
• When a non-polar molecule such as bromine approaches the benzene ring there is insufficient pi-electron density around any two carbon atoms to polarise the bromine molecule. This prevents any reaction taking place (without a catalyst).