Perkin’s Reaction Mechanism

A Perkin reaction produces a beta unsaturated aromatic acid (‘beta unsaturated’ means it has a double bond), a carboxylic acid group by aldol condensation of an aromatic aldehyde group (meaning it has -CHO), and an acid anhydride in the presence of an alkali salt of the acid, which acts as a base catalyst to speed up the reaction. At 180°C, the aldehyde is heated with an excess of acid anhydride to complete the reaction. Under the reaction circumstances, dehydration usually occurs, resulting in an anhydride. Excess aldehyde is removed using steam distillation, and the resultant unsaturated acid is obtained via anhydride hydrolysis with dilute HCl.

Mechanism of perkin reaction

The following steps constitute the widely accepted mechanism of the Perkin reaction:

• The carboxylate ion abstracts a proton to generate the resonance stabilized carbanion, which is a species containing carbon with a negative charge.

• The nucleophilic attachment of the carbanion to the carbonyl carbon atom of the aldehyde results in the formation of a tetrahedral intermediate.

• Acetic acid generated during the procedure protonated the tetrahedral intermediate.

• The removal of a water molecule from a hydroxy derivative.

• The unsaturated molecule is hydrolyzed, which is the addition of water to the unsaturated acid.

Applications

• It is used in the laboratory to synthesize cinnamic acid. Cinnamon and shea butter both include cinnamic acids, which are naturally occurring unsaturated aromatic carboxylic acids.

• We may employ this reaction to create – and -unsaturated aromatic acids, which are commonly used in the pharmaceutical industry.

Perkin condensation

The Perkin reaction occurs when aromatic aldehydes react with alkanoic anhydrides in the presence of an alkanoate. This reaction is analogous to aldol condensation.

The carbanion is formed in the reaction by carboxylate removing an Alpha hydrogen atom from acid anhydride (anion of the corresponding acid of the acid anhydride). The carbanion then reacts with the aromatic aldehyde to produce alkoxide anion. The acetyl group is subsequently transferred from carboxyl oxygen to alkoxy oxygen via a cyclic intermediate to produce a more stable anion. The loss of a good leaving group from the position to give an anion of the Alpha, beta unsaturated acid arises from the removal of an Alpha hydrogen from this anion by carboxylate. This produces Alpha,-unsaturated acid when acidified.

For example, in the presence of sodium acetate, PhCHO reacts with excess acetic anhydride to produce cinnamic acid, which is then acidified (3-phenylpropanoic acid). 

Perkin condensation mechanism

The acetate ion is there to extract a proton from the anhydride’s -carbon, forming a carbanion that then attacks the carbonyl group of the aldehyde. The product then removes a proton from the acid to generate an aldol-type molecule. In the presence of heated acetic anhydride, the latter dehydrates.

Example of Perkin’s reaction

The electron withdrawing nitro group is present in 4-nitrobenzaldehyde.It makes carbonyl carbon more positively charged.As a result, in the Perkins condensation reaction, 4-nitro benzaldehyde is the most reactive towards nucleophile.The positive charge on the carbonyl carbon reduces when electron releasing groups are present in aromatic aldehydes.As a result, such compounds are less reactive to nucleophiles in the Perkins condensation reaction.

What is reaction mechanism?

A reaction mechanism in chemistry is the step-by-step sequence of elementary reactions that results in total chemical change.A chemical mechanism is a theoretical hypothesis that attempts to define in detail what happens at each stage of a chemical process. In most circumstances, the specific processes of a reaction are not visible. The proposed mechanism was chosen because it is thermodynamically feasible and has experimental support in isolated intermediates (see the following section) or other quantitative and qualitative properties of the reaction. It also specifies which bonds are broken (and in what order) and which bonds are created for each reactive intermediate, activated complex, and transition state (and in what order).A thorough mechanism must also explain why the reactants and catalyst were chosen, the stereochemistry seen in reactants and products, the products generated, and the quantities of each.

Reaction intermediates

Reaction intermediates are chemical entities that are not reactants or products of the overall chemical reaction, but are transitory products and/or reactants in the mechanism’s reaction steps. They are frequently unstable and short-lived (although can sometimes be separated). Free radicals or ions are frequently used as reaction intermediates.The kinetics (relative rates of the reaction steps and the overall rate equation) are explained in terms of the energy required to convert the reactants to the proposed transition states (molecular states that corresponds to maxima on the reaction coordinates, and to saddle points on the potential energy surface for the reaction). 

Conclusion

A proper reaction mechanism is a critical component of accurate predictive modeling. Detailed mechanisms for many combustion and plasma systems are either unavailable or must be developed. Even when information is accessible, discovering and compiling relevant data from many sources, reconciling discrepant numbers, and extrapolating to new conditions can be challenging without expert assistance. Rate constants and thermochemical data are frequently unavailable in the literature, necessitating the employment of computational chemistry techniques or group additivity methods to get the requisite parameters. Computational chemistry tools can also be used to compute potential energy surfaces for reactions and identify likely processes.