Let's look into Benzene in A-Level Chemistry to see how it is different from other hydrocarbon!
Benzene
C6H6– first discovered by Michael Faraday in 1825 is colourless, sweet smelling, highly flammable.
Found naturally in crude oil, petrol and cigarette smoke.
Made of hexagonal ring of 6 carbons with each carbon bonded to 2 carbons and 1 hydrogen
The R group is H in benzene.
Kekulé Model
1865 Friedrich August Kekulé suggested that benzene is a 6 carbon ring with alternating double and single bonds between the carbons.
Evidence to disprove the Kekulé Model
Lack of reactivity of benzene –benzene does not decolourise bromine under normal conditions. It also does not undergo electrophilic addition.
Lengths of carbon-carbon bonds –using x-ray diffraction it was found all bonds were 0.139 nm the length between a double bond 0.134nm and a single 0.153 nm
Hydrogenation enthalpies –if benzene had the Kekulé model it would have the hydrogenation enthalpy x3 of cyclohexene. However, it is only -208 kJ mol-1 not -306kJ mol-1, suggesting that Benzene is more stable.
Delocalisation
The 6 carbons are arranged in a hexagoanl planar structure. Each carbon forms 3 bonds, leaving one delocalised electron in the p orbital. The six p orbitals delocalise to form an extended pi system above and below the plane of the ring. This gives the molecule stability.
Benzene derivatives
Nitration of Benzene
Benzene + nitric acid (sulfuric acid catalyst at 50°) creates nitrobenzene and water
H is replaced with NO2
Nitric acid and sulfuric acid react to for NO2+electrophile
Then the H+ and HSO4 react to form H2SO4
Halogenation of Benzene
Halogens do not react with benzene unless a halogen carrier is present e.g. AlCl3
Bromination of Benzene
Room temperature in the presence of a halogen carrier on hydrogen on benzene is replaced with Br
Electrophilic substitution
Alkylation of Benzene
H replaced with alkyl group
Benzene + haloalkane (in the presence of halogen carrier) creates and alkylbenzene plus an acid
Sometimes called a Friedel-Crafts Reaction.
Acylation Reactions
Benzene + acyl chloride (in the presence of AlCl3) ---> aromatic ketone +HCl
Drafted by Eunice (Chemistry)