Arenes ✅

Explain, in terms of delocalisation of pi electrons, the difference between benzene and alkene

1. Resonance stabilisation causes the pi electron cloud to be less reactive than C=C, hence benzene requires stronger electrophiles (positively charged—> more reactive) to react.

2. Resonance stabilisation from the delocalisation of the pi electron cloud causes benzene to undergo substitution rather than addition. Addition will destroy the stable aromatic structure of the ring.

State the reagents and conditions necessary for reactions with benzene

1. Electrophilic substitution of halogen: Br2(l), AlBr3 catalyst, r.t.p./ Cl2(g), AlCl3 catalyst, r.t.p.

2. Nitrations: concentrated HNO3(aq), concentrated H2SO4(aq), maintained at 50 degrees celcius.

3. Friedal Crafts Alkylation: CnH2n+1Cl, AlC3 catalyst, r.t.p.

4. Free radical substitution: limited Br2(g), uv light, r.t.p. / limited Cl2(g), uv light, r.t.p.

5. Complete oxidation (only if there is a benzylic H -CH): acidified KMnO4(aq), heat under reflux/ alkaline KMnO4(aq), heat under reflux, followed by H2SO4(aq).

State the role of AlBr3/ AlCl3

1. AlBr3/ AlCl3 is electron deficient. It acts as a Lewis acid to form AlCl4- (AlCl4- acts as a Lewis base), generating the electrophile.

2. AlBr3/ AlCl3 is a homogenous catalyst. It is used up in step 1 to form AlCl4- and regenerated at the end of the reaction.

State the role of H2SO4

1. H2SO4 is a stronger acid than HNO3. It acts as a Bronsted Lowry acid to donate a proton to HNO3 (HNO3 acts as a Bronsted Lowry base).

2. H2SO4 is a homogenous catalyst. It is used up in step 1 to form HSO4- and regenerated at the end of the reaction.

Predict whether halogenation will occur in the side chain or aromatic nucleus in arenes depending on reaction conditions: Explain the effect of electron withdrawing groups and electron donating groups on the reactivity of benzene

1. Electron withdrawing groups activates the benzene ring towards electrophilic substitution by increasing electron density in the ring, hence the benzene ring becomes more reactive and milder conditions are needed.

2. Electron donating groups deactivates the benzene ring towards electrophilic substitution by decreasing electron density in the ring, hence the benzene ring is less reactive and stronger conditions are needed.