Wacker process
Overview
 
The Wacker process or the Hoechst-Wacker process (named after the chemical companies of the same name) originally referred to the oxidation of ethylene
Ethylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...

 to acetaldehyde
Acetaldehyde
Acetaldehyde is an organic chemical compound with the formula CH3CHO or MeCHO. It is one of the most important aldehydes, occurring widely in nature and being produced on a large scale industrially. Acetaldehyde occurs naturally in coffee, bread, and ripe fruit, and is produced by plants as part...

 by oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 in water in the presence of a tetrachloropalladate catalyst
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....

. The same basic reaction is currently used to produce aldehydes and ketones from a number of alkenes with the Monsanto process
Monsanto process
The Monsanto process is an important method for the manufacture of acetic acid by catalytic carbonylation of methanol. This process operates at a pressure of 30–60 atm and a temperature of 150–200 °C and gives a selectivity greater than 99%. It was developed 1960 by German BASF and...

 for producing acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...

. This chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

, a German invention, was the first organometallic and organopalladium
Organopalladium
Organopalladium chemistry is a branch of organometallic chemistry that deals with organic palladium compounds and their reactions. Palladium is often used as a catalyst in the reduction of alkenes and alkynes with hydrogen. This process involves the formation of a palladium-carbon covalent bond...

 reaction applied on an industrial scale.
Encyclopedia
The Wacker process or the Hoechst-Wacker process (named after the chemical companies of the same name) originally referred to the oxidation of ethylene
Ethylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...

 to acetaldehyde
Acetaldehyde
Acetaldehyde is an organic chemical compound with the formula CH3CHO or MeCHO. It is one of the most important aldehydes, occurring widely in nature and being produced on a large scale industrially. Acetaldehyde occurs naturally in coffee, bread, and ripe fruit, and is produced by plants as part...

 by oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 in water in the presence of a tetrachloropalladate catalyst
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....

. The same basic reaction is currently used to produce aldehydes and ketones from a number of alkenes with the Monsanto process
Monsanto process
The Monsanto process is an important method for the manufacture of acetic acid by catalytic carbonylation of methanol. This process operates at a pressure of 30–60 atm and a temperature of 150–200 °C and gives a selectivity greater than 99%. It was developed 1960 by German BASF and...

 for producing acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...

. This chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

, a German invention, was the first organometallic and organopalladium
Organopalladium
Organopalladium chemistry is a branch of organometallic chemistry that deals with organic palladium compounds and their reactions. Palladium is often used as a catalyst in the reduction of alkenes and alkynes with hydrogen. This process involves the formation of a palladium-carbon covalent bond...

 reaction applied on an industrial scale. The Wacker process is similar to hydroformylation
Hydroformylation
Hydroformylation, also known as oxo synthesis or oxo process, is an important industrial process for the production of aldehydes from alkenes. This chemical reaction entails the addition of a formyl group and a hydrogen atom to a carbon-carbon double bond...

, which is also an industrial process and also leads to aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....

 compounds. The differences are that hydroformylation promotes chain extension, and uses a rhodium
Rhodium
Rhodium is a chemical element that is a rare, silvery-white, hard and chemically inert transition metal and a member of the platinum group. It has the chemical symbol Rh and atomic number 45. It is composed of only one isotope, 103Rh. Naturally occurring rhodium is found as the free metal, alloyed...

-based catalyst system. The Wacker process is an example of homogeneous catalysis
Homogeneous catalysis
In chemistry, homogeneous catalysis is a sequence of reactions that involve a catalyst in the same phase as the reactants. Most commonly, a homogeneous catalyst is codissolved in a solvent with the reactants.-Acid catalysis:...

. The palladium complex with ethylene is reminiscent of Zeise's salt
Zeise's salt
Zeise's salt, potassium trichloroplatinate, is the chemical compound with the formula KPtCl3]·H2O. The anion of this air-stable, yellow, coordination complex contains an η2-ethylene ligand. The anion features a platinum atom with a square planar geometry.-Preparation:This compound is commercially...

, K[PtCl3(C2H4)] which is a heterogeneous catalyst
Heterogeneous catalysis
In chemistry, heterogeneous catalysis refers to the form of catalysis where the phase of the catalyst differs from that of the reactants. Phase here refers not only to solid, liquid, vs gas, but also immiscible liquids, e.g. oil and water. The great majority of practical heterogeneous catalysts...

.

History

The development of the chemical process now known as the Wacker process began in 1956 at Wacker Chemie
Wacker Chemie
Wacker Chemie AG is a worldwide operating company in the chemical business, founded 1914. The company is controlled by the Wacker-family holding more than 50 percent of the shares. The corporation is operating more than 25 production sites in Europe, Asia, and the Americas...

. At the time, many simple aliphatic compounds were produced from acetylene
Acetylene
Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...

 (as calcium carbide
Calcium carbide
thumb|right|Calcium carbide.Calcium carbide is a chemical compound with the chemical formula of CaC2. The pure material is colorless, however pieces of technical grade calcium carbide are grey or brown and consist of only 80-85% of CaC2 . Because of presence of PH3, NH3, and H2S it has a...

) but the construction of a new oil refinery in Cologne
Cologne
Cologne is Germany's fourth-largest city , and is the largest city both in the Germany Federal State of North Rhine-Westphalia and within the Rhine-Ruhr Metropolitan Area, one of the major European metropolitan areas with more than ten million inhabitants.Cologne is located on both sides of the...

 by Esso
Esso
Esso is an international trade name for ExxonMobil and its related companies. Pronounced , it is derived from the initials of the pre-1911 Standard Oil, and as such became the focus of much litigation and regulatory restriction in the United States. In 1972, it was largely replaced in the U.S. by...

 close to a Wacker site, combined with the realization that ethylene
Ethylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...

 would be a cheaper raw-material prompted Wacker to investigate its potential uses. As part of the ensuing research effort, a reaction of ethylene and oxygen over palladium on carbon
Palladium on carbon
Palladium on carbon, often referred to as Pd/C, is a form of palladium used for catalysis. It is usually used for catalytic hydrogenations in organic chemistry...

 in a quest for ethylene oxide
Ethylene oxide
Ethylene oxide, also called oxirane, is the organic compound with the formula . It is a cyclic ether. This means that it is composed of two alkyl groups attached to an oxygen atom in a cyclic shape . This colorless flammable gas with a faintly sweet odor is the simplest epoxide, a three-membered...

 unexpectedly gave evidence for the formation of acetaldehyde (simply based on smell). More research into this ethylene to acetaldehyde conversion resulted in 1957 in a gas-phase reaction patent using a heterogeneous catalyst. In the meanwhile Hoechst AG
Hoechst AG
Hoechst AG was a German chemicals then life-sciences company that became Aventis Deutschland after its merger with France's Rhône-Poulenc S.A. in 1999...

 joined the race and after a patent filing forced Wacker into a partnership called Aldehyd GmbH. The heterogeneous process ultimately failed due to catalyst inactivation and was replaced by the water-based homogeneous system for which a pilot plant was operationally in 1958. Problems with the aggressive catalyst solution were solved by adopting titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....

 (newly available for industrial use) as construction material for reactors and pumps. Production plants went into operation in 1960.

Reaction mechanism

The reaction mechanism
Reaction mechanism
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in...

 for the industrial Wacker process (olefin oxidation via palladium(II) chloride) has received significant attention for several decades, and parts of it are still a contentious subject. A modern formulation is described below:
The catalytic cycle
Catalytic cycle
A catalytic cycle in chemistry is a term for a multistep reaction mechanism that involves a catalyst . The catalytic cycle is the main method for describing the role of catalysts in biochemistry, organometallic chemistry, materials science, etc. Often such cycles show the conversion of a...

 can also be described as follows:
[PdCl4]2 − + C2H4 + H2O → CH3CHO + Pd + 2 HCl + 2 Cl
Pd + 2 CuCl2 + 2 Cl → [PdCl4]2− + 2 CuCl
2 CuCl + ½ O2 + 2 HCl → 2 CuCl2 + H2O


Note that all catalysts are regenerated and only the alkene and oxygen are consumed. Without copper(II) chloride
Copper(II) chloride
Copper chloride is the chemical compound with the formula CuCl2. This is a light brown solid, which slowly absorbs moisture to form a blue-green dihydrate. The copper chlorides are some of the most common copper compounds, after copper sulfate....

 as an oxidizing agent
Oxidizing agent
An oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...

 Pd(0) metal (resulting from reductive elimination of Pd(II) in the final step) would precipitate out and the reaction would come to a halt (the stoichiometric reaction without catalyst regeneration was discovered in 1894). Air, pure oxygen, or a number of other oxidizers can then oxidise the resultant CuCl
Copper(I) chloride
Copper chloride, commonly called cuprous chloride, is the lower chloride of copper, with the formula CuCl. The substance is a white solid sparingly soluble in water, but very soluble in concentrated hydrochloric acid...

 back to CuCl2, allowing the cycle to repeat.

The initial stoichiometric reaction was first reported by Phillips
and the Wacker reaction was first reported by Smidt et al.

Mechanism summary

Early mechanistic studies from the 1960s elucidated much of the oxidation process. Several interesting key points were found, and these observations are generally not considered controversial:
  1. There is no H/D exchange seen in this reaction. Experiments using C2D4 in water generate CD3CDO, and runs with C2H4 in D2O generate CH3CHO. Thus, keto-enol tautomerization is not a possible mechanistic step.
  2. There is a negligible kinetic isotope effect
    Kinetic isotope effect
    The kinetic isotope effect is the ratio of reaction rates of two different isotopically labeled molecules in a chemical reaction. It is also called "isotope fractionation," although this term is somewhat broader in meaning...

     with fully deuterated reactants (k H/k D=1.07). Hence, it is inferred that hydride transfer is not a rate-determining step
    Rate-determining step
    The rate-determining step is a chemistry term for the slowest step in a chemical reaction. The rate-determining step is often compared to the neck of a funnel; the rate at which water flows through the funnel is determined by the width of the neck, not by the speed at which water is poured in. In...

    .
  3. A significant competitive isotope effect with C2H2D2, (k H/k D= ~1.9), suggests that the rate determining step should be prior to oxidized product formation.
  4. Running the reaction under higher concentrations of chloride and copper(II) chloride
    Copper(II) chloride
    Copper chloride is the chemical compound with the formula CuCl2. This is a light brown solid, which slowly absorbs moisture to form a blue-green dihydrate. The copper chlorides are some of the most common copper compounds, after copper sulfate....

     result in the formation of a new product, chlorohydrin.


Based on these observations, it is generally accepted the rate-determining step occurs before a series of hydride rearrangements. Many experimental and theoretical investigations have sought to identify the rate determining step, but a unifying interpretation of these experiments has been difficult to attain.

Most mechanistic studies on the Wacker process have focused on identifying whether nucleophilic attack occurred via an external (anti-addition) pathway or via an internal (syn-addition) pathway. Using arguments supported by kinetics experiments, Henry inferred the mechanism for nucleophilic attack would be by an internal (syn-) pathway. Later, stereochemical studies by Stille and coworkers yielded chemical products that indicated the Wacker process proceeds via anti-addition; however, since these experiments were run under conditions significantly different from industrial Wacker process conditions, the conclusions were disputed. Contemporary stereochemical studies using normal industrial Wacker conditions (except with high chloride and high copper chloride concentrations) also yielded products that inferred nucleophilic attack was an anti-addition reaction. The published results of these two independent stereochemical studies were accepted by the chemistry community as proof that the standard reaction process occurs via an anti-addition step, and many reference texts have not renewed their presentation of the Wacker process mechanism since this point.

Later kinetic studies by Henry and coworkers used isotopically substituted allyl alcohols at standard industrial conditions (with low-chloride concentrations) to probe the reaction mechanisms. Those results showed nucleophilic attack is a slow process, while the proposed mechanisms explaining the earlier stereochemical studies assumed nucleophilic attack to be a fast process.

Subsequent stereochemical studies by Patrick M. Henry and coworkers observed different stereoisomers of products that indicated that both pathways occur and are dependent on chloride concentrations. However, these studies too are disputed since allyl-alcohols may be sensitive to isomerization reactions, and different stereoisomers may be formed from those reactions and not from the standard Wacker process.

In summary, experimental evidence seems to support that syn-addition occurs under low-chloride reaction concentrations (< 1 mol
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...

/L
Litér
- External links :*...

, industrial process conditions), while anti-addition occurs under high-chloride (> 3mol
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...

/L
Litér
- External links :*...

) reaction concentrations, probably due to chloride ions saturating the catalyst and inhibiting the inner-sphere mechanism. However, the exact pathway and the reason for this switching of pathways is still unknown.

Further complicating the Wacker process mechanism is questions about the role of copper chloride in the mechanism. Most mechanistic theories so far presented have invoked Occam's razor
Occam's razor
Occam's razor, also known as Ockham's razor, and sometimes expressed in Latin as lex parsimoniae , is a principle that generally recommends from among competing hypotheses selecting the one that makes the fewest new assumptions.-Overview:The principle is often summarized as "simpler explanations...

, and assumed copper does not play a role in the olefin oxidation mechanisms. Yet, experiments by Stangl and Jira found chlorohydrin formation was dependent on copper chloride concentrations. Work by Hosokawa and coworkers yielded a crystallized product containing copper chloride, indicating it may have a non-innocent role in olefin oxidation. Finally, experiments by Anderson and Sigman yielded a different kinetic rate law for olefin oxidation without copper than reactions using industrial conditions with low concentrations of copper co-catalysts. While these works complicate the picture of the Wacker process mechanism, one should probably infer that this and related chemistry can be sensitive to reaction conditions, and multiple different reaction pathways may be in play.

Another key step in the Wacker process is the migration of the hydrogen from oxygen to chlorine and formation of the C-O double bond. This step is often regarded to proceed through a so-called β-hydride elimination with a four-membered cyclic transition state
Transition state
The transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest energy along this reaction coordinate. At this point, assuming a perfectly irreversible reaction, colliding reactant molecules will always...

:
In silico
In silico
In silico is an expression used to mean "performed on computer or via computer simulation." The phrase was coined in 1989 as an analogy to the Latin phrases in vivo and in vitro which are commonly used in biology and refer to experiments done in living organisms and outside of living organisms,...

 studies argue that the transition state
Transition state
The transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest energy along this reaction coordinate. At this point, assuming a perfectly irreversible reaction, colliding reactant molecules will always...

 for this reaction step is unfavorable and an alternative reductive elimination reaction mechanism
Reaction mechanism
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in...

 is in play. The proposed reaction steps are likely assisted by water molecule in solution acting as a catalyst.

Wacker-Tsuji oxidation

The so-called Wacker-Tsuji oxidation is the laboratory scale version of the above reaction, for example the conversion of 1-decene
Linear alpha olefin
Linear Alpha Olefins or Normal Alpha Olefins are olefins or alkenes with a chemical formula CxH2x, distinguished from other mono-olefins with a similar molecular formula by linearity of the hydrocarbon chain and the position of the double bond at the primary or alpha position.Linear alpha olefins...

 to 2-decanone with palladium(II) chloride
Palladium(II) chloride
Palladium chloride, also known as palladium dichloride, are the chemical compounds with the formula PdCl2. PdCl2 is a common starting material in palladium chemistry – palladium-based catalysts are of particular value in organic synthesis. It is prepared by chlorination of...

 and copper(II) chloride
Copper(II) chloride
Copper chloride is the chemical compound with the formula CuCl2. This is a light brown solid, which slowly absorbs moisture to form a blue-green dihydrate. The copper chlorides are some of the most common copper compounds, after copper sulfate....

 in a water / dimethylformamide
Dimethylformamide
Dimethylformamide is an organic compound with the formula 2NCH. Commonly abbreviated as DMF , this colourless liquid is miscible with water and the majority of organic liquids. DMF is a common solvent for chemical reactions...

solvent mixture in the presence of air:


Some of the related chemistry on Wacker-like oxidations and aminations have been reviewed by Stahl and coworkers.
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