Vivian Marlowe
Ketones are organic compounds containing carbonyl groups (C=O). They can be prepared in a variety of ways, including the oxidation of secondary alcohols. The oxidizing agent used in these reactions is typically a solution of sodium or potassium dichromate(VI) acidified with dilute sulfuric acid. During oxidation, both C-O and O-H bonds are broken to form C=O bonds. The removal of the alcohol resulting from the reduction of a cheap aldehyde or ketone used as the oxidant is another method. This addresses the issue of alcohol being less volatile than the corresponding carbonyl compound.
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What You'll Learn
Oxidation of secondary alcohols
The oxidation of alcohols is a common transformation of organic compounds, often used in the synthesis of complex molecules. Alcohols can be oxidized into a variety of carbonyl compounds, depending on the nature of the alcohol and the oxidizing agent used.
Secondary alcohols can only be oxidized to form ketones. This reaction typically occurs in the presence of a strong oxidizing agent, such as chromic acid (H2CrO4), which is formed from chromium trioxide (CrO3) or sodium dichromate (Na2Cr2O7) in the presence of sulfuric acid. This is also known as the Jones reagent. Potassium permanganate (KMnO4) is another strong oxidizing agent that is usually used in a basic aqueous solution. Both of these agents can oxidize primary alcohols to carboxylic acids and secondary alcohols to ketones. However, it is important to control the temperature and concentrations carefully to avoid overoxidation, which may cleave carbon-carbon bonds.
There are also milder oxidizing agents that can be used to convert secondary alcohols to ketones. These include pyridinium chlorochromate (PCC), pyridinium dichromate (PDC), and the Dess-Martin (DMP) oxidation. These milder reagents stop the oxidation process once the carbonyl group is formed, preventing the formation of carboxylic acids.
Several methods and catalysts have been developed to selectively oxidize secondary alcohols to ketones. One method involves the use of a silica gel-supported TEMPO catalyst, which can be easily prepared and reused multiple times. Another approach is a ternary hybrid catalyst system that enables the acceptorless dehydrogenation of secondary alcohols to ketones under visible light irradiation at room temperature, yielding high amounts of the desired product without producing side products. Additionally, the use of fluorinated alcohols as solvents has been shown to influence the reaction by donating strong hydrogen bonds while being weak acceptors. Specifically, 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) has been used as an activating solvent for the aerobic oxidation of secondary alcohols to ketones, with excellent yields and selectivity.
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Friedel-Crafts acylations
The Friedel-Crafts acylation reaction involves the addition of an acyl group to an aromatic ring. This reaction is commonly used in aromatic chemistry to prepare aryl ketones. It is named after Charles Friedel and James Mason Crafts, who developed it in 1877.
In the Friedel-Crafts acylation reaction, a Lewis acid catalyst such as AlCl3 is typically employed. The reaction between the Lewis acid and the chlorine atom of the acid chloride forms a complex, and the C-Cl bond cleavage of the complex yields an acylium ion with a positive charge on the carbon. This acylium ion then acts as an electrophile, reacting with the arene to produce the monoacylated product (aryl ketone).
The Friedel-Crafts acylation reaction has several advantages over the related alkylation reaction. It offers better control over the reaction products, and the acylium cation is stabilised by resonance, preventing rearrangement. Additionally, the ketone product is always less reactive than the original molecule due to the electron-withdrawing effect of the carbonyl group, ensuring that multiple acylations do not occur.
However, the Friedel-Crafts acylation reaction also has some limitations. It only yields ketones because formyl chloride decomposes into CO and HCl under these conditions. Certain functional groups, such as alcohols and amines, may be incompatible with the reaction conditions, limiting the range of substrates.
In certain cases, Friedel-Crafts acylation can be carried out with catalytic amounts of milder Lewis acids, such as Zn(II) salts, or a Brønsted acid catalyst using the anhydride or carboxylic acid as the acylation agent. The resulting ketone can be reduced to the corresponding alkane substituent by Clemmensen or Wolff-Kishner reduction.
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Using acyl chlorides
Acyl chlorides are highly reactive and can be converted into other acyl compounds through nucleophilic substitution. This reactivity allows them to be used to create ketones in the laboratory.
One method of preparing ketones from acyl chlorides involves the use of Grignard reagents. Grignard reagents are nucleophilic and can react with acyl chlorides to form ketones. However, the high reactivity of Grignard reagents makes isolating the ketone intermediate difficult. This is because the addition of a second equivalent of the Grignard reagent rapidly occurs, leading to the formation of a mixture of starting material, ketone, and alcohol.
Organocuprates, also known as Gilman or lithium dialkyl cuprates, are another class of reagents that can be used to prepare ketones from acyl chlorides. Organocuprates are less reactive than Grignard reagents, and the reaction stops once the ketone is formed. This is because the alkyl groups in organocuprates are connected to copper rather than magnesium, making their carbanionic character less pronounced. The copper atom in organocuprate reagents also changes the reaction mechanism, making it more complex than the typical "addition-elimination" sequence seen in nucleophilic acyl substitutions.
Another method of preparing ketones from acyl chlorides is through Friedel-Crafts acylation. This reaction involves the acylation of a benzene ring with an acid chloride in the presence of a Lewis acid catalyst, such as aluminum chloride (AlCl3). This reaction is popular and is known as the Friedel-Craft acylation reaction.
In addition to these methods, ketones can also be prepared from acyl chlorides through cross-coupling reactions. For example, a dual Ni/photoredox system can be used to generate acyl radicals from aldehydes, which can then be cross-coupled with benzylic and allylic pyridinium salts to provide ketones. Visible light photoredox/nickel dual catalysis also enables the cross-coupling of acyl chlorides with potassium alkyltrifluoroborates, leading to the synthesis of unsymmetrical alkyl ketones.
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Dehydrogenation of alcohol
For example, the reaction of ethyl alcohol with Cu as a catalyst produces ethanal. However, aldehydes can be further oxidised to form carboxylic acids. To prevent this, an excess of alcohol can be used so that there is insufficient oxidising agent to carry out the second oxidation.
Another method involves the use of chromyl chloride (CrO2Cl2). The reaction of toluene with chromyl chloride forms a chromium complex, which, upon hydrolysis, produces benzaldehyde. This is known as the Etard reaction.
A more recent method involves the use of a ternary hybrid catalyst system comprising a photoredox catalyst, a thiophosphate organocatalyst, and a nickel catalyst. This system enables the acceptorless dehydrogenation of aliphatic secondary alcohols to ketones under visible light irradiation at room temperature with high yield and without producing side products, except for H2 gas.
Additionally, the dehydrogenation of alcohol mixtures in the C3 and C4 alcohol boiling ranges has been investigated for the production of esters and ketones. The reaction was conducted in the presence of a Cu/Zn/Al2O3 catalyst at temperatures ranging from 219-278 °C for the propanol-rich feed and 219-300 °C for the butanol-rich feed, both at atmospheric pressure. The yield of ketones increased with temperature.
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Using copper-catalysed oxidative coupling
The use of copper-catalysed oxidative coupling is an efficient method for forming ketones from alcohols. This method offers a highly selective aerobic oxidation of alcohols to aldehydes and ketones under mild conditions.
One notable example of this process involves the use of Cu(NO3)2·3H2O and TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) or 4-HO-TEMPO as catalysts. This system enables the oxidation of various alcohols, including allenols, alkenols, propargylic alcohols, benzylic alcohols, and aliphatic alcohols, in acetonitrile or 1,2-dichloroethane. The reaction proceeds at room temperature, forming aldehydes or ketones with excellent selectivity.
Another approach to copper-catalysed oxidative coupling involves the use of NHPI (N-hydroxyphthalimide) in conjunction with a copper(II) catalyst. This method selectively oxidises propargylic substrates to the corresponding conjugated carbonyl products, including ketones. The addition of the copper catalyst enhances reactivity and allows for lower reaction temperatures.
Furthermore, copper-catalysed aerobic oxidation reactions have demonstrated significant potential for the selective oxidation of organic substrates. These reactions avoid the use of precious metals and offer advantages such as the use of O2 as a stoichiometric oxidant and increased functional group compatibility.
In some cases, copper-catalysed oxidative coupling can be achieved through direct hydroxylation by metal oxo insertion into the benzylic C–H bond, mimicking the oxidation of alkanes by cytochrome P450. The subsequent metal-catalysed degradation leads to the formation of alcohol and ketone products.
Overall, the use of copper-catalysed oxidative coupling provides an efficient, selective, and mild method for forming ketones from alcohols, with the added benefit of avoiding the use of expensive and toxic catalysts typically associated with aerobic oxidations.
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