Aromatics

Question 1

What is the experimental evidence for a delocalised model of benzene as opposed to Kekule’s model

Answer 1

The X-ray diffraction shows that the bond lengths in the actual benzene molecule are all the same, benzene is unable to undergo a normal electrophilic addition, the enthalpy of hydrogenation of benzene is -208kJmol-1 which is more stable than the expected -360kJmol-1 as predicted by Kekule’s structure

Question 2

What are the conditions for the electrophilic substitution of nitration on a benzene

Answer 2

Concentrated HNO3, concentrated H2SO4 which acts as a base.
2H2SO4 + NO3 –> 2HSO4- + NO2+ + H3O+
monosubstitution. <50c so no more nitro groups added

Question 3

Electrophilic substitution of a halogen on a benzene ring

Answer 3

Halogen carrier like AlCl3 required to generate the positive electrophile

Question 4

Electrophilic substitution of haloalkanes or acyl chlorides

Answer 4

AlCl3 required to generate positive electrophile

Question 5

What proves phenols are weakly acidic

Answer 5

They neutralise NaOH but do not react with metal carbonates

Question 6

Electrophilic substitution of nitration on phenol

Answer 6

Dilute HNO3, room temperature. Mono-substitution position 2.

Question 7

Electrophilic substitution of bromine on phenol

Answer 7

Trisubstitution, aqueous bromine. Produces HBR

Question 8

Give examples of four electron donating groups - 2,4 directing

Answer 8

OH, NH2, X, CH2

Question 9

Give examples of four electron withdrawing groups - 3,5 directing groups

Answer 9

COOH, CO, NO2, CHO