Topic 17 - Organic Chemistry (17.4 - 17.5)

Question 1

What are aromatic compounds?

Answer 1

Aromatic description of the smell of certain organic compounds but new meaning is description of bonding in a compound, delocalised electrons forming pi bonding in a hydrocarbon ring

Question 2

What is benzene?
Look at physical properties
The Kekule structure, including problems of the structure and the new model for benzene

Answer 2

Benzene at heart of every aromatic compound, described as an arene
Benzene colorless liquid, boiling temp at 80, insoluble in water. Present in crude oil and fuels obtained so petrol contains benzene. It is a carcinogen. Molecular formula = C6H6, suggests highly unsaturated
Kekule structure = Kekule dozing in fire, in flames imagined snake like molecules dancing, proposed a cyclic or ring structure for benzene.
Problem 1 = if benzene contains 3 C=C bonds, should readily decolourise bromine water in addition reaction but it doesnt decolourise it. Substitution reaction occurs so suggests no C=C bonds.
Problem 2 = As more compounds with benzene discovered, if kekule correct there should be four isomers with C4H4Br2 formula but 3 only known to exist so two isomers with bromine on adjacent carbon atoms are identical.
Problem 3 = Data about lengths of covalent bonds, bond lengths in benzene and cyclohexene suggests C-C bonds in benzene are the same and perhaps also intemrediate in character between C-C and C=C
Problem 4 = data about enthalpy changes of hydrogenation,First values for cyclohexane suggest enthalpy change for adding 1 mol of H2 to 1 mol of C=C bonds is -120kJ, Kekule could be cyclohexa-1,3,5-triene which value would be treble cyclohexene, benzene has value very much lower than theoretical structure with 3 C=C bonds would have
New Model = Benzene after all sigma bonds have formed, each C atom has one electron in p orbital, imagine formation of 3 pi bonds but instead imagine formation of one large pi bond made up of all 6 electrons. Three are in doughnut shape above the atoms and 3 are in another doughnut shape below atoms, together these 6 electrons form delocalised pi bond. New model means no individual C=C bonds so no addition reaction with bromine, there are three isomers of C6H4Br2, C-C bonds all same length, charge spread around in a species, increased stability which explains 152kJ greater stability of benzene

Question 3

Reactions of Benzene

1) Hydrogenation
2) Combustion
3) Bromination
4) Nitration
5) Friedel-Crafts reactions
- alkylation
- acylation

Answer 3

1) Mixing benzene with hydrogen and heating under pressure with nickel catalyst. C6H6 + 3H2 —> C6H12 = forms cyclohexane
2) Benzene burns in air with smoky flame, typical of many comppounds with high carbon:Hydrogen ratio
3) Benzene + Bromine heated under reflux in presence of a catalyst called a halogen carrier, usually metal-halogen compounds like aluminium chloride, aluminium bromide or iron bromide. Enough to add iron filings becasue react with some of the bromine. Products are bromobenzene and hydrogen bromide
4) Substitution of hydrogen atom by a nitro group. Nitro group formula NO2 and shouldnt be confused with nitrogen dioxide. Reaction carried out by warming benzene with mixture of conc nitric and sulfuric acids. Nitric acid considered as source of NO2 group and sulfuric acid as catalyst. Products are nitrobenzene and water
5) Friedel-Crafts reacions = using reagent by XY one of hydrogen atoms in benzene substituted by Y and product is HX - catalyst needed like aluminium chloride or other halogen carriers. Anhydrous conditions needed as water would react with catalyst or with organic product.
Alkylation = substitution of H atom for alkyl group = reagent is halogenoalkane, products are alkylbenzene like methylbenzene and hydrogen chloride
Acylation = substitution of H atom for acyl group.. reagent is acyl chloride, products a ketone phenulethanone and hydrogen chloride