6.1.1 - Aromatic Compounds

Question 1

Kekulé model of benzene

Answer 1

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

Question 2

evidence to disprove the Kekulé model of benzene

Answer 2

• resistance to reaction → does not undergo electrophilic addition reactions or react with/decolourise bromine
• bond lengths → all bond lengths the same
• enthalpy change of hydrogenation → less exothermic than expected

Question 3

delocalised model of benzene

Answer 3

• benzene = planar, cyclic, hexagonal hydrocarbon containing 6 C atoms and 6 H atoms
• each C atom 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 the C atoms to form a ring of electron density
• overlapping of p-orbitals creates system of π-bonds which spread over all 6 C atoms in ring structure
• 6 electrons occupying this system of π-bonds said to be delocalised

Question 4

nitration of benzene

Answer 4

• electrophilic substitution
• benzene reacts with concentrated nitric acid in the presence of a concentrated sulfuric acid catalyst
• nitronium ion electrophile → accepts electron pair from benzene ring
• forms nitrobenzene and water

HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄^- + H₂O
H⁺ + HSO₄^- → H₂SO₄

Question 5

bromination of benzene

Answer 5

• electrophilic substitution
• benzene reacts with bromine in the presence of a halogen carrier (AlBr₃ or FeBr₃)
• bromonium ion (Br⁺) electrophile → accepts electron pair from benzene ring
• forms bromobenzene and HBr

Br₂ + FeBr₃ → FeBr₄^- + Br⁺
H⁺ + FeBr₄^- → FeBr₃ + HBr

Question 6

chlorination of benzene

Answer 6

• electrophilic substitution
• benzene reacts with chlorine in the presence of a halogen carrier (AlCl₃ or FeCl₃)
• bromonium ion (Br⁺) electrophile → accepts electron pair from benzene ring
• forms chlorobenzene and HCl

Cl₂ + FeCl₃ → FeCl₄^- + Cl⁺
H⁺ + FeCl₄^- → FeCl₃ + HCl

Question 7

alkylation of benzene

Answer 7

• electrophilic substitution
• benzene reacts with a haloalkane in the presence of a halogen carrier (AlCl₃)
• increases number of C atoms in a compound
• forms e.g. ethylbenzene and HCl

Question 8

acylation of benzene

Answer 8

• electrophilic substitution
• benzene reacts with an acyl chloride in the presence of a halogen carrier (AlCl₃)
• increases number of C atoms in a compound
• forms an aromatic ketone and HCl

Answer 9

• alkenes react with bromine by electrophilic addition
• benzene only reacts with bromine if a halogen carrier is present
• benzene has delocalised π-electrons spread above and below the plane of the C atoms in the ring structure
• the electron density around any 2 C atoms in the benzene ring is less than that in a C=C double bond in an alkene
• therefore benzene cannot polarise a bromine molecule, preventing a reaction

Question 10

weak acidity of phenols

Answer 10

• partially dissociates when dissolved in water (forming phenoxide ion and a proton) = weak acid
• more acidic than alcohols
• less acidic than carboxylic acids
• only CAs react with sodium carbonate to produce CO₂
• phenols do not react with carbonates
• phenols do react with sodium hydroxide (NaOH) to form sodium phenoxide and water in a neutralisation reaction

Question 11

bromination of phenol

Answer 11

• electrophilic substitution
• phenol reacts with bromine water
• halogen carrier not required
• forms white precipitate of 2,4,6-tribromophenol and HBr
• bromine water decolourised

Question 12

nitration of phenol

Answer 12

• electrophilic substitution
• phenol reacts with dilute nitric acid
• forms mixture of 2-nitrophenol and 4-nitrophenol (and water)

Question 13

relative ease of electrophilic substitution of phenol compared with benzene

Answer 13

• caused by lone pair of electrons from oxygen p-orbital of -OH group being donated into the π-system of phenol
• electron density of benzene ring in phenol is increased, attracting electrophiles more strongly than with benzene
• aromatic ring in phenol is more susceptible to electrophilic attack than in benzene

Question 14

directing effects

Answer 14

• OH group and NH₂ group are 2- and 4- directing (activating groups/electron-donating)
• NO₂ group is 3- directing (deactivating group/electron-withdrawing)