Chap 15B - Arenes (Benzene)

State reagents and conditions for halogenation

1. Cl2, anhydrous AlCl3 catalyst (or FeCl3 catalyst)

2. Br2, anhydrous AlBr3 catalyst (or FeBr3 catalyst)

• Fe can also be used as it is converted to iron(III) halide catalyst in situ (reaction mixture) by reaction with the halogen reagent 2Fe + 3X2 → 2FeX3

• The Lewis acid catalyst, AlCl3 or AlBr3 (or FeCl3 or FeBr3), must be dry (anhydrous) to prevent hydrolysis from taking place -> so it cannot act as a Lewis acid to generate the strong electrophile

• The Al atom in AlCl3 or AlBr3 is electron deficient as there are only six valence electrons around it

  • Al in AlCl3 and AlBr3 have energetically accessible vacant orbitals to accept a lone pair of electrons -> able to act as Lewis acids

Describe observations of halogenation

• When Cl2 is used, yellowish-green Cl2 is decolorized and white fumes of HCl are formed

• When Br2 is used, orange-red Br2 is decolorized and white fumes of HBr are formed

• Under prolonged conditions and in the presence of a higher proportion of the halogen, the 1,2-dihalide and 1,4-dihalide are formed

Describe mechanism of halogenation

Describe general reaction of halogenation

Describe reagents and conditions of nitration

• Reagents and conditions: concentrated HNO3, concentrated H2SO4 catalyst, heat at 55-60C (under reflux)

• Nitrobenzene is a pale yellow liquid with an almond smell

• Temperature for this reaction needs to be carefully controlled as high temperatures (100 C or above) will lead to formation of the di-substituted product or tri-substituted product

Describe general reaction of nitration

Describe reflux apparatus

Reflux involves heating the chemical reaction for a specific amount of time, while continually cooling the vapour produced back into liquid form, using a condenser Purpose of condenser: Prevent escape of volatile organic compounds through vaporisation -> increasing the yield of the product obtained

Describe nitration mechanism

Describe reagents and conditions of friedel-crafts alkylation

1. RCl, anhydrous AlCl3 catalyst (or FeCl3 catalyst), excess benzene

2. RBr, anhydrous AlBr3 catalyst (or FeBr3 catalyst), excess benzene

• Al in AlCl3 and AlBr3 have energetically accessible vacant orbitals and are electron-deficient molecules to accept a lone pair of electrons

• AlCl3 and AlBr3 must be anhydrous to prevent hydrolysis from taking place

• Necessary to start with excess benzene to prevent further alkylation which occurs even more readily

• Used to synthesise homologues of benzene (multi-substituted benzene)

Describe general reaction of friedel-crafts alkylation

Describe mechanism of friedel-crafts alkylation

Describe addition of hydrogen (reduction) +.general equation