Topic 18: Organics 3 2️⃣

Colourless liquid Boiling temp of 80 degrees C Insoluble in water Carcinogenic

C6H6

Cyclic structure (hexagon) 3 double bonds

If benzene contains 3 double bonds, it should readily decolourise bromine water in an addition reaction however it does not Suggests there are no C=C bonds

If kekulé structure is correct there should be 4 isomers with the molecular formula C6H4Br2 however, only 3 are known to exist Suggests that the two isomers with bromine on adjacent carbon atoms are identical which would mean that the bonds between carbon atoms in the benzene ring are the same not different (C-C/C=C)

Evidence suggests that the bond lengths in benzene are all the same

Actual benzene has a much lower value than a theoretical structure containing 3 double bonds

Each carbon atom has one electron in a p orbital The orbitals overlap sideways so together the six electrons form a delocalised pi bond above and below the plane of carbon atoms (delocalised cloud)

Description of the bonding in a compound: Delocalised electrons forming pi bonding in a hydrocarbon ring

Benzene mixed with hydrogen, heat and high pressure, nickel catalyst Forms cyclohexane

Burns in air, WITH smoky flame

Benzene and bromine are heated under reflux, in the presence of a halogen carrier catalyst such as AlCl3 or AlBr3 or iron(III)bromide For an experiment it’s enough to add iron filings as they react with bromine to form iron(III) bromide Produces bromobenzene & hydrogen bromide

Benzene is warmed with a mixture of concentrated nitric and sulfuric acids - the nitric acid can be considered the source of NO2 (HNO3) - the sulfuric acid can be considered the catalyst (H2SO4) Produces nitrobenzene & water

Alkylation and Acylation

• use a reagent represented as XY, one of the hydrogen atoms in benzene is substituted by Y and the other product is HX • a catalyst is needed (any of… AlCl3, AlBr3, iron(II)chloride, iron(III)bromide) • anhydrous conditions needed as water would react with catalyst and sometimes organic product

Substitution of one of the hydrogen atoms from benzene by an alkyl group Reagent is a halogenoalkane Products are alkylbenzene & hydrogen chloride

Substitution of one of the hydrogen atoms in benzene by an acyl group Reagent is acyl chloride Products are a ketone (eg. phenyletnanone) and hydrogen chloride

Used to represent the benzene ring when it’s attached to another functional group

Consists of a hydroxyl group joined to a benzene ring

Occurs at room temp and without a catalyst can occur with bromine water unlike bromination of benzene Bromine water decolourises and organic product forms a white precipitate Substitution can occur 3 times HBr is formed (same) however with phenol 3HBr

• the oxygen in the OH has a lone pair and these electrons can merge with the electrons in the delocalised pi bond • electron density above and below the ring of atoms is increased (activation) • benzene can induce a dipole on bromine/bromine is polarised as it approaches the benzene ring, Br-Br bond breaks & Br+ electrophile can attack

• preparation from a halogenoalkane • preparation from nitriles

Halogenoalkane is heated with ammonia (under pressure and in a sealed container as ammonia is a gas) Alternatively can be mixed with concentrated aqueous ammonia

Nitriles can be reduced to primary amines using the reducing agent lithiumtetrahydrioaluminate [H] Reactants are mixed in dry ether to ensure no water can affect the reaction