9C. Arenes

Question 1

Arenes

Answer 1

• aromatic hydrocarbons are collectively called arenes
• benzene is the simplest arene
• all arenes contain one or more benzene-like rings
• polynuclear aromatic hydrocarbons contain two or more benzene rings, with each pair of rings sharing two adjacent carbon atoms

Question 2

Benzene

Answer 2

• consists of six carbon atoms in a ring with six hydrogen atoms, one off each carbon
• bonding between each carbon atom is neither single nor double, but a hygrid of the two + therefore very stable
• C₆H₆ benzene molecules are of planar formation, with each carbon-carbon bond of equal length + all bond angle sare 120^o

Question 3

Naming Benzene derivatives

Answer 3

• similar to alkanes + alkenes, except that benzene is generally considered the parent
• for compounds where more than one substituent is attached to the ring, two systems may be used to identify their position
  1. the carbon atoms are numbered from 1 to 6, beginning with the carbon that is bonded to the first group + continuing in such a direction as to lead to the lowest numbers for the remaining groups
  2. only applies if there are two substituents. The prefixes ortho-, meta- + para- are used to identify positions that are adjacent to the first group
• for some compounds it is easier to consider the benzene ringas the attached group + the longest carbon chain as the parent group. In this case the benzene ring is known as phenyl + the same rule as for alkanes and alkenes apply
• where there are two or more substituents on the ring + one of them imparts a common name for the compound, the compound is named as a derivativie of that common parent molecule + it is assumed that the substituent occupies ring position 1
• for substituents that don’t impart a common name, each is numbered in such a manner as to give the smallest set of numbers + then listed in alphabetical order before the ending of ‘benzene’

Question 4

Reaction of Benzene + its derivatives

Answer 4

• most characteristic reaction of aromatic compounds is aromatic substitution, whereby a hydrogen atom attached to a ring carbon is substituted by another atom or group, particularly halogens or the nitro group, -NO₂, or the sulfonic group, -SO₃H

Question 5

Aryl Group

Answer 5

• substituent group derived from an arene by removal of a hydrogen atom
• commonly represented by the symbol Ar-

Question 6

Phenyl Group

Answer 6

• substituent group derived from benzene by removal of a hydrogen atom, C₆H₅
• commonly represented by the symbol Ph-
• in molecules containing other functional groups, phenyl groups are often named as substituents

Question 7

Halogenation

Answer 7

• a hydrogen is substituted by a halogen, particularly chlorine or bromine, in the presence of a catalyst, particularly iron

Question 8

Nitration

Answer 8

nitration

• when benzene or one of its derivatives is heated with a mixture of concentrated nitric + sulfuric acids, a nito group, -NO2, replaces one of the hydrogen atoms bonded to the ring
• nitration is of particular value as the resulting -NO2 group can be reduced to a primary amino group -NH2 by catalytic reduction using hydrogen in the presence of a transition metal catalyst

Question 9

Sulfonation

Answer 9

• heating an aromatic compound with concentrated sulfuric acid results in the formation of an arenesulfonic acid, all of which are strong acids comparable in strength to sulfuric acid
• a major use of sulfonation is in the preparation of synthetic detergents