Aromatics

Question 1

what are aromatic compounds?

Answer 1

molecules that contain a benzene ring

Question 2

describe the kekule structure

Answer 2

-the cyclic triene would undergo addition reactions with bromine water at room temperature
-the lengths of carbon-carbon bonds measured by x-ray diffraction were different for single and double bonds (the double bonds were shorter), therefore it was not a regular hexagon
-the enthalpy for hydrogenation is -360kJmol-1

Question 3

explain the evidence that caused scientists to question the kekule model

Answer 3

-length of carbon-carbon bonds in benzene are equal for both double and single bonds (all 0.139nm)
-the enthalpy of hydrogenation for benzene is less exothermic than expected
-benzene only reacts with bromine at high temperatures or in the presence of a halogen carrier

Question 4

describe the structure of benzene

Answer 4

-planar, perfectly hexagonal molecule
-bond angles 120
-each carbon forms 3 sigma bonds
-each carbon has 1 electron in a p orbital and adjacent p orbitals overlap to form a pi bond
-delocalised pi system, leading to a reduction in energy

Question 5

what happens in an electrophilic substitution reaction?

Answer 5

when a hydrogen atom is replaced by a species that seeks out an area of negative charge and accepts an electron pair (electrophile)

Question 6

what are the reagents and conditions needed for nitration of the benzene ring?

Answer 6

concentrated sulfuric acid, concentrated HNO3 (ratio 2:1)
50 degrees for monosubstitution
nitronium ion elecrophile

Question 7

what is the equation for formation of the nitronium ion?

Answer 7

2H2SO4 + HNO3 –> H3O+ +2HSO4- +NO2+

Question 8

what is the equation for the reforming of the H2SO4 catalyst?

Answer 8

H+ + HSO4- –> H2SO4

Question 9

what are the uses of nitration of benzene?

Answer 9

-explosives
-to make aromatic amines and pharmaceuticals

Question 10

what are the reagents and conditions needed for halogenation of the benzene ring?

Answer 10

Cl2 and AlCl3 catalyst
heat and reflex
Cl+ catalyst

Question 11

what is the equation for forming the Cl+ electrophile needed to make chlorobenzene from benzene?

Answer 11

AlCl3 + Cl2 –> AlCl4- + Cl+

Question 12

what is the equation for reforming the AlCl3 catalyst?

Answer 12

H+ + AlCl4- –> AlCl3 + HCl

Question 13

what are the reagents and conditions needed for the bromination of alkenes and explain why

Answer 13

room temp, Br2, no catalyst

localised pi bond, so high electron density, so can polarise Br2 molecule, so Br-Br is the electrophile

it is addition due to products being more stable than the alkene

Question 14

what are the reagents and conditions needed for the bromination of benzene and explain why

Answer 14

heat, Br2, AlBr3 catalyst

delocalised pi system , so cannot induce a dipole, so the catalyst forms the electrophile

it is substitution as it maintains a stable pi system