Wednesday, January 16, 2008

Ch.24 Benzene - Core Points for Revision




Electrophile Substitution Reactions
--- Sulphonation
--- Friedel-Crafts Alkylation
--- Friedel-Crafts Acylation

Effect of --, m- and p- directing groups in mono-substituted benzenes

1. Benzene has the molecular formula C6H6. It has hexagonal ring of six carbon atoms with three double bonds in alternate positions.

Arenes are the aromatic hydrocarbons which contain one or more hexagonal rings of carbon atoms with double bonds in alternate positions.

2. Preparation of benzene and its homologues

1. From alkynes: acetylene and other alkynes polymerise at high temperatures to give benzene and other arenes.

3C2H2 gives C6H6

Benzene was first synthesized by Berthelot by passing acetylene through red hot iron tube.

2 Decarboxylation of aromatic acids: by heating sodium benzoate with soda lime

Decarboxylation: Removal carboxyl group

3. From phenol: by distillation of phenol with zinc.

3. Physical properties

i) colour less liquids up to eight carbon atoms
ii) aromatic hydrocarbons are insoluble in water ut soluble in organic solvents.
iii) They are inflammable and burn with sooty flame

4. Chemical properties

Even though double bonds are present, benzene is quite stable and does not undergo common addition reactions undergone by alkenes.

Benzene and other arenes undergo following types of reactions.

1. substitution
2. addition
3. oxidation

5. Halogenation

benzene will react with a mixture of Cl-2 and FeCl-3.

The output is a combination of benzene with Cl, Cl diplacing one hydrogen atom from benzene(Chlorobenzene).

6. Nitration
A mixture of nitric acid and sulphuric acid is the nitrating agent.

7. Sulphonation

The product is a combination Benzene and SO-3H that displaced one hydrogen atom from benzene.

For sulphonation we require excess of H-2SO-4 along with SO-3.

8. Friedel-Crafts Alkylation

Benzene reacts with a combination of alkyl halide and AlCl-3. AlCl-3 acts as a Lewis acid.

The alkyl group replaces one hydrogen atom in benzene.

9. Friedel-Crafts Acylation

Acylation is the term given to substituting an acyl group such as CH-3CO- into another molecule. An acyl group is a hydrocarbon group attached to a carbon-oxygen double bond.

The most commonly used example of an acyl group is the ethanoyl group, CH3CO-.

10. Effect of o-, m- and p- directing groups in mono-substituted benzenes

In planning syntheses based on substitution reactions of mono-substituted benzenes, you must be able to predict in advance which of the available positions of the ring are most likely to be substituted.

Basically, three problems are involved in the substitution reactions of aromatic compounds: (a) proof of the structures of the possible isomers, o, m, p, that are formed; (b) the percentage of each isomer formed, if the product is a mixture; and (c) the reactivity of the compound being substituted relative to some standard substance, usually benzene.

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