Perkin Reaction: Definition & Mechanism

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  • 0:04 The Perkin Reaction
  • 1:31 Perkin Reaction Procedure
  • 1:55 Perkin Reaction Mechanism
  • 2:40 Perkin Reaction…
  • 4:48 Lesson Summary
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Lesson Transcript
Instructor: Saranya Chatterjee

Saranya has a masters degree in Chemistry and in Secondary Education. She has taught high school, AP chemistry for 2 years and is teaching undergraduate college chemistry for 3 years.

This lesson defines an organic condensation reaction which is popularly called the Perkin reaction. It also discusses the mechanism of the reaction and talks about its uses and applications.

The Perkin Reaction

A Perkin reaction is an organic condensation reaction developed by English chemist William Henry Perkin in the 19th century and is used to make cinnamic acids. Before going into the details of Perkin reaction, we need to know what a cinnamic acid is and what an aldol condensation reaction is.

Cinnamic acids are carboxylic acids that occur naturally and can be found in things like cinnamon (as the name would suggest) and shea butter. The term aldol, on the other hand, is an abbreviation of 'aldehyde' and 'alcohol.'

When the enolate of an aldehyde or a ketone reacts at the alpha-carbon, which is the carbon next to the carbonyl carbon, with the carbonyl of another molecule under basic or acidic conditions to obtain beta-hydroxy aldehyde, or ketone, beta means the second carbon atom next to the carbonyl group, this reaction is called 'aldol condensation reaction.' Often, dehydration of the hydroxy aldehyde/ketone leads to the unsaturated aldehyde or ketone as shown in the diagram.

Aldol condensation reaction
aldol condensation reaction

A Perkin reaction yields a beta unsaturated aromatic acid ('beta unsaturated' meaning it contains a double bond), containing carboxylic acid group by the aldol condensation of an aromatic aldehyde group (meaning it contains -CHO), and an acid anhydride in the presence of an alkali salt of the acid, which acts as the base catalyst in order to accelerate the reaction.

Perkin Reaction Procedure

The reaction is carried out by heating the aldehyde with an excess of acid anhydride at 180°C. Usually dehydration takes place under the conditions of the reaction and an anhydride results. Excess of aldehyde is removed by distillation in steam and the resulting unsaturated acid is obtained by hydrolysis of the anhydride with dilute HCl.

Perkin Reaction Mechanism

The generally accepted mechanism of the Perkin reaction is represented by the following steps:

  1. Involves the abstraction of proton by the carboxylate ion to form the resonance stabilized carbanion, which is a species containing carbon with a negative charge.
  2. Involves the nucleophilic addition of the carbanion to the carbonyl carbon atom of the aldehyde to form a tetrahedral intermediate.
  3. The tetrahedral intermediate is protonated by the acetic acid formed in the process.
  4. Involves the elimination of water molecule from the hydroxy derivative.
  5. Hydrolysis, which is the addition of water, of the unsaturated compound to the unsaturated acid occurs.

Perkin Reaction Application and Uses

By the Perkin condensation reaction, aryl (a substituent derived from an aromatic ring) substituted acrylic acid can be prepared. Acrylic acid is an organic compound with the formula CH2=CHCOOH. It's the simplest unsaturated carboxylic acid, consisting of a vinyl group connected directly to a carboxylic acid terminus. This colorless liquid has a characteristic acrid or tart smell. It becomes a single compound when mixed with water, alcohols, ethers, and chloroform. // //

These substituted acrylic acids, through catalytic hydrogenation can be converted to beta-aryl derivatives of saturated carboxylic acids, which may then be converted to different compounds. As for example Ar.CH=CH.COOH can be hydrogenated under nickel catalyst to yield Ar.CH2 CH2 COOH which can be reduced by lithum aluminium hydride (LiAlH4) to get ArCH2 CH2 OH.

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