Aromatic Hydrocarbons

Question 1

Aromatic hydrocarbons

Answer 1

unsaturated cyclic compounds
arenes
highly unreactive towards chemical reagents
Benzene and its homologues are better solvents as they dissolve a large number of compounds

Question 2

Reactions of benzene

Answer 2

typical example of aromatic compounds
undergoes substitution
addition
oxidation

Question 3

Naming aromatic compounds

Answer 3

benzene ring is the main functional group and the molecule is named as a substituted benzene ring - the suffix is -benzene and there are prefixes to represent any other functional groups. E.G chlorobenzene

benzene ring is not the main functional group and the molecule is named as having a phenyl group attached. Phenyl- or phen- are used as prefixes to show the molecule has a benzene ring and the suffix comes from other functional groups on the molecule. Eg -ol if it’s an alcohol or-amine if it’s an amine.

Question 4

Preparation of benzene from alkynes

Answer 4

cyclic polymerization
Ethyne is passed through a red hot iron tube at 873 K
undergoes cyclic polymerization to form benzene

Question 5

Preparation of benzene from aromatic acids

Answer 5

decarboxylation reaction
sodium salt of the benzoic acid (sodium benzoate) is heated with soda lime to produce benzene along with sodium carbonate

Question 6

Preparation of benzene from phenol

Answer 6

reduction
vapours of phenol are passed over heated zinc dust.
Zinc dust reduces them to form benzene

Question 7

Preparation of benzene from sulphonic acids

Answer 7

hydrolysis
Benzene sulphonic acid is exposed to superheated steam leading to the formation of benzene
C6H5-SO3H + H2O → C6H6 + H2SO4

Question 8

Benzene

Answer 8

boiling point of 80.1 °C
melting point of 5.5 °C
freely soluble in organic solvents
only slightly
soluble in water

Question 9

Effect of delocalised electrons on benzene

Answer 9

less reactive: than other alkenes and cycloalkenes
all the C-C bonds are the same length
less electron cloud distortion comparatively and thus less reactivity of the benzene ring

more thermodynamically stable: requires more
heat to oxidize or, in easier words, burn to produce oxygen and water

Question 10

Effect of delocalised electrons on benzene summary

Answer 10

The delocalised electrons cause stability in the dissociation of hydrogen in the carboxylic acids
The delocalised electrons change the chemical reactivity of the benzene ring
The delocalised electrons change the thermodynamic reactivity of the benzene ring
The delocalisation energy is the stabilizing energy due to the electrons being spread out over the whole covalently bonded molecule.