Question

Write short notes on the following: Darzens Reaction.

Answer 1

Hint: The Darzens Reaction is the condensation of a carbonyl compound with an \[\alpha \] -halo ester in the presence of a base to form an \[\alpha \], \[\beta \] -epoxy ester. This was discovered by the chemist Auguste Georges Darzens in 1904.

Complete step by step answer:
The Darzens reaction or condensation of α‐chloroacetophenone with different aromatic aldehydes mediated by potassium fluoride on alumina at room temperature resulted in formation of good to excellent yields of trans‐ \[\alpha \], \[\beta \] ‐epoxy ketones in a period of short time. An organic chemist Auguste Georges Darzens discovered this reaction in 1904.
It involves the reaction of straight chain primary alcohols with thionyl chloride and forms choro derivatives without rearrangement.
$R - OH + SOC{l_2} + pyridine \to RCl + S{O_2} + HCl$
Mechanism of the reaction - The reaction process begins when a strong base is used to form a carbanion at the halogenated position. Because of the ester, this carbanion is a resonance-stabilized enolate, which makes it relatively easy to form. This nucleophilic structure attacks the other carbonyl component to form a new carbon–carbon bond. These first two steps are similar to a base-catalysed aldol reaction. The oxygen anion in this aldol-like product then does an intramolecular on the formerly-nucleophilic halide-bearing position, displacing the halide to form an epoxide. Thus, this reaction sequence is a condensation reaction and since there is a net loss of HCl when the two reactant molecules combine. The major role of the ester is to enable the first deprotonation to occur, and other carbonyl functional groups can be used instead. If the starting material is an \[\alpha \] -halo amide, the product is an \[\alpha \], \[\beta \] -epoxy amide. If an \[\alpha \] -halo ketone is used, the product is an \[\alpha \], \[\beta \] -epoxy ketone.

Note: Any sufficiently strong base can be used for the initial deprotonation. However, if the initial material is an ester, the alkoxide corresponding to the ester side-chain is usually chosen in order to prevent complications due to potential acyl exchange side reactions.