Reformatsky Reaction in Organic Chemistry

What is the Reformatsky Reaction?

The name Reformatsky reaction is kept in the honour of a Russian chemist named Sergey Nikolaevich Reformatsky, who discovered this reaction in 1887. This is a reaction that takes place between a carbonyl compound and an alpha‐half ester, which can be an aldehyde, an ester, or a ketone. This reaction takes place mostly in the presence of zinc. This represents the extended reactions between the carbonyl compounds either with an alkyl zinc halide or a dialkylzinc.

Advantage of Reformatsky Reaction Process

An advantage of this reaction is that the organozinc compound isolation is not required. At the time of the reaction process, a new carbon‐carbon linkage can be created along with an organozinc halide formation and the decomposition because of the presence of dilute acids.

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Generally, the Reformatsky reaction yields are improved if the reaction is carried out in 2 steps.

• Firstly, the alpha Bromo ester can be converted into an organozinc bromide

• By reaction with the zinc compound in pure and dry dimethoxyethane.

This derivative is formed apparently almost in the quantitative yield.

Reformatsky Reaction Definition

According to the general definition, the Reformatsky reaction can be described as an organic reaction used to convert an aldehyde or ketone and α-haloester to a β-hydroxy ester with the help of acid workup and metallic zinc. Here, an inert solvent such as THF (tetrahydrofuran) or diethyl ether is often used as a reaction solvent.

The carbonyl compound’s condensation reaction, along with the alpha haloester in the presence of zinc metal, is referred to as the Reformatsky reaction.

The solvent that is most often used in this reaction is given as ether or benzene or a benzene ether mixture.

Structure of the Reagent

The THF’s complexes crystal structures of Reformatsky reagents ethyl bromozincacetate and tert-butyl bromozincacetate have been determined. These both form cyclic 8-membered dimers in the solid-state but vary in stereochemistry. The 8-membered ring in the ethyl derivative adopts a conformation of tub-shaped and contains cis THF ligands and cis Bromo groups. Whereas, in the derivative of tert-butyl, the ring exists in a chair form and the THF ligands and Bromo groups are the trans.

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Reformatsky Mechanism

Let us look at how the reaction takes place and what happens while the reaction occurs.

• Generally, the Reformatsky reaction commences either with the oxidative insertion or zinc addition into the carbon-halogen bond of an α-haloester.

• The primary purpose of using zinc is to allow the enolate generation even without using the Bronsted base, which generally condenses either with the aldehyde or ketone itself.

• After the insertion happens, the compounds get coordinated with each other leading to a dimer formation. Also, this compound further experiences a rearrangement that results in the emergence of 2 zinc enolates.

• After that, the oxygen of the aldehyde or ketone coordinates to the zinc, and a new rearrangement takes place where the 2 reagents now contain a carbon-carbon bond between them.

• Following that, an acid workup splits the oxygen bond and zinc to generate β-hydroxy ester and zinc(II) salt as the final products.

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On an important note, the α-hydroxy esters product are the essential substances that are required for natural product synthesis and in the pharmaceutical industry.

Identifying the First-order Reaction

First-order reactions are when the reaction rate at any provided time is directly proportional to the reactant’s concentration left at that specific time or the active mass.

The rate law is expressed as R=K[A] 

Let us look at some typical ways to identify it:

• These types of reactions take forever to get completed finally.

• The reaction rate decreases exponentially as the time slows down.

• The sample half-life is simply a constant value. After each half-life, the reactant amount gets halved.

• The half-life and rate constant are inversely proportional, and none of which depends on the sample’s initial concentration.

• A graph between the time and logarithm of the concentration of the reactant left forms a straight line.

Advantages of Reformatsky Reaction

A few of the significant advantages of this reaction can be listed as follows:

• The reformatsky reaction is conducted using highly hindered ketones. This reaction also facilitates the successful addition of nucleophiles to the ketone’s delta positive carbon atom.

• Reformatsky mechanism can be adapted easily for the intramolecular aldol reactions.

• The organozinc halide reagents, which are used in the Reformatsky Reaction, are considered relatively stable and are also available commercially.

• Reformatsky reaction results in the beta-hydroxy ester’s isolation.

• Another merit of the Reformatsky reaction can be given as the convenience since the reaction is an alternative to the reaction of a ketone or an aldehyde with the preferred lithium enolate of an ester.

• The yields of Reformatsky were improved with freshly prepared zinc powder, a heated column of zinc dust, acid-washed zinc, trimethylchlorosilane, and copper-zinc couple.