Preparation of Alkenes Part 2

 Pyrolytic Eliminations

Pyrolytic elimination reactions are those in which the elimination occurs through heating and involves a cyclic transition state. These reactions occur without requiring the addition of external reagents like acids or bases. Instead, the heat provides the necessary energy to facilitate the elimination.

(i) Pyrolysis of Esters:

When an ester of a carboxylic acid (usually acetic acid) is heated at about 400°C, elimination of the carboxylic acid occurs, resulting in the formation of an alkene.

Example:

·         Mechanism: The mechanism involves a cyclic transition state where the β-hydrogen atom and the leaving group are in an antiperiplanar (anti-coplanar) geometry, facilitating the elimination.

 (ii) Chugaev Reaction:

In the Chugaev reaction, an alcohol is first converted into a xanthate ester by treating it with carbon disulfide (CS₂) and sodium hydroxide (NaOH), followed by methyl iodide (CH₃I). On heating the xanthate ester to about 200°C, pyrolytic elimination occurs to give an alkene.

Reactions Involved:

1.      Formation of the xanthate ester:

Mechanism: The reaction involves a cyclic transition state where the β-hydrogen and the leaving group are aligned in an antiperiplanar geometry, similar to the pyrolysis of esters.