Mason Delacroix
The oxidation of secondary alcohols to ketones is a well-known process in organic chemistry, with several methods available for conversion. One common method involves using chromium trioxide (CrO3) as an oxidizing agent, which is reduced to form H2CrO3 during the reaction. Other oxidizing agents include potassium permanganate (KMnO4) and sodium dichromate (Na2Cr2O7). In addition to traditional methods, alternative approaches such as the use of hydrogen peroxide or bleach have been explored, although these may require additional catalysts and can be corrosive or explosive. Furthermore, advancements in transition metal catalysis have enabled the development of a dual photo/cobalt-catalytic method for ketone synthesis from primary alcohols and alkenes, offering a streamlined and cost-effective approach.
| Characteristics | Values |
|---|---|
| Procedure | Oxidation of secondary alcohols |
| Reagents | Jones reagent (CrO3, H2SO4, H2O), pyridinium chlorochromate (PCC), Dess-Martin periodinane, sodium hypochlorite, hydrogen peroxide, bleach, potassium permanganate (KMnO4), sodium dichromate (Na2Cr2O7), molecular iodine, palladium(II) acetate-pyridine complex, ferric nitrate, sodium nitrate, 3-methylimidazolinium hydrogensulfate, Burgess reagent, diisopropyl azodicarboxylate (DIAD) |
| Conditions | High reaction temperatures, mild reaction conditions, room temperature |
| Yield | High, low |
| Advantages | High yields, short reaction times, mild conditions, efficient, simple, cost-effective, environmentally benign, safe, easily accessible reagents |
| Disadvantages | Toxic, corrosive, explosive, dangerous, low yield, requires rigorous reaction conditions |
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What You'll Learn
Using hydrogen peroxide
While it is possible to use hydrogen peroxide to turn a secondary alcohol into a ketone, it is not a common method. The traditional methods of oxidizing secondary alcohols to ketones are well-established and work just as well, so there is little incentive to develop new methods.
Chromium trioxide (CrO3) is a common oxidizing agent used by organic chemists to oxidize secondary alcohols to ketones. It is quite dangerous and can ignite alcohols on contact. Another common method for oxidizing secondary alcohols to ketones uses chromic acid (H2CrO4) as the oxidizing agent. This is also known as Jones reagent and is prepared by adding CrO3 to aqueous sulfuric acid. Other commonly used oxidizing agents include potassium permanganate (KMnO4) and sodium dichromate (Na2Cr2O7).
However, if you wish to use hydrogen peroxide, it is important to note that it is corrosive and explosive. It is also likely that you will need to use it in combination with other compounds. For example, an efficient bismuth tribromide-catalyzed oxidation of various alcohols with aqueous hydrogen peroxide provides carbonyl compounds in good yields. This reaction can be performed under visible light irradiation at room temperature in high yield without producing side products (except H2 gas). Urea-hydrogen peroxide in the presence of a catalytic amount of magnesium bromide efficiently oxidizes primary and secondary benzylic alcohols into the corresponding aromatic aldehydes and ketones.
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Using chromium trioxide
Chromium trioxide (CrO3) is a common oxidizing agent used by organic chemists to oxidize secondary alcohols to ketones. This reaction involves the conversion of the alcohol group to a carbonyl group, which has a carbon atom double-bonded to an oxygen atom.
To begin the reaction, chromium trioxide is dissolved in aqueous sulfuric acid, forming chromic acid (H2CrO4). This process is known as Jones oxidation, named after the chromic acid's alternative name, Jones reagent. The acetone used in Jones oxidation acts as a co-solvent in the reaction, preventing the over-oxidation of the organic product.
During the oxidation process, the secondary alcohol undergoes a loss of hydrogen atoms (dehydrogenation) from the hydroxyl group and its adjacent carbon. This loss is facilitated by the oxidizing agent, resulting in the formation of a ketone and a reduced form of chromium in the by-products.
The balanced chemical equation for the oxidation of a secondary alcohol to a ketone using chromium trioxide is:
> R2CHOH + CrO3 + H2O → R2CO + Cr(OH)3 + H2O
In this equation, R2CHOH represents the secondary alcohol, R2CO is the resulting ketone, and Cr(OH)3 is chromium hydroxide, a byproduct formed during the reaction.
It is important to note that chromium trioxide is a toxic compound and can ignite alcohols on contact. Therefore, caution must be exercised when handling this reagent.
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Using bleach
Bleach, or sodium hypochlorite, can be used in the oxidation of secondary alcohols to produce ketones. This process is a convenient, inexpensive, and safe method for undergraduate laboratories.
A specific example of this process involves the use of Bleach and Catalytic Tetrabutylammonium Bromide. This method is outlined in the Organic Process Research & Development journal from 2012.
Another method involves the use of sodium hypochlorite pentahydrate crystals with very low NaOH and NaCl contents. This process can oxidize primary and secondary alcohols to the corresponding aldehydes and ketones in the presence of TEMPO/Bu4NHSO4 without pH adjustment. This method was published in Synlett in 2014.
Additionally, a 2018 publication in the Russian Journal of General Chemistry discusses the use of ruthenium carbonyl complexes with pyridylalkanol ligands. This method involves the synthesis, characterization, and catalytic properties for the aerobic oxidation of secondary alcohols.
It is important to note that while bleach is a viable option for oxidizing secondary alcohols to ketones, there are various other methods available, such as those utilizing potassium or sodium dichromate with sulfuric acid.
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Using potassium permanganate
Potassium permanganate (KMnO4) is a powerful oxidizing agent that is commonly used in organic chemistry. It can be used to oxidize a wide range of organic molecules, including alcohols. When used with primary alcohols, KMnO4 oxidizes them to form carboxylic acids. Meanwhile, with secondary alcohols, KMnO4 can be used to produce ketones.
It's important to note that KMnO4 is not considered ideal for converting alcohols to ketones due to the risk of overoxidation. This can lead to the cleavage of carbon-carbon bonds if the reaction conditions, such as temperature and concentration, are not carefully controlled. However, under mild conditions, KMnO4 can be effective in converting alkenes to glycols.
To use KMnO4 for oxidizing a secondary alcohol to a ketone, the reaction typically occurs under basic conditions. The KMnO4 solution is prepared in an aqueous environment, often with the presence of nitric acid. The secondary alcohol is then added to the KMnO4 solution, initiating the oxidation process.
During the reaction, the secondary alcohol (-OH group) is oxidized, resulting in the formation of a ketone. The specific conditions and reagents used can vary, but careful control of the reaction parameters is crucial to prevent overoxidation and unwanted side reactions.
It's worth mentioning that there are alternative oxidizing agents available, such as chromium trioxide (CrO3), which is commonly used to convert secondary alcohols to ketones. Other options include pyridinium chlorochromate (PCC), a milder oxidizing agent that can be used when avoiding the formation of carboxylic acids is desired. The choice of oxidizing agent depends on the specific reaction conditions and the desired outcome.
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Using sodium dichromate
The oxidation of a secondary alcohol typically leads to the formation of a ketone. This involves the loss of hydrogen atoms and electrons from the alcohol functional group (-OH), resulting in the formation of a ketone functional group (-CO-).
One standard method for this process uses an oxidizing agent like sodium dichromate (Na2Cr2O7). Sodium dichromate is a strong oxidizing agent and is more likely to cause over-oxidation, leading to the formation of carboxylic acids. To avoid this, mild oxidizing agents such as pyridinium chlorochromate (PCC) or Collins reagent can be used.
The general mechanism for the oxidation of secondary alcohols to ketones involves the following steps:
- Protonation of the secondary alcohol's hydroxyl group by the acid catalyst.
- Attack by the oxidizing agent, resulting in the formation of a cationic intermediate.
- Dehydration of the intermediate, yielding a carbocation.
- Nucleophilic attack by a water molecule, generating a hydroxyl group.
- Loss of a proton, producing the ketone.
The presence of an alcohol must first be confirmed by testing for the -OH group. The liquid must be verified as neutral, free of water, and reacted with solid phosphorus(V) chloride to produce a burst of acidic steamy hydrogen chloride fumes. A few drops of the alcohol would then be added to a test tube containing a sodium dichromate(VI) solution acidified with dilute sulfuric acid. The tube would be warmed in a hot water bath. In the case of a secondary alcohol, the orange solution turns green.
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Frequently asked questions
A simple method to turn a secondary alcohol into a ketone involves using molecular iodine as the oxidant and potassium carbonate.
Some common oxidizing agents to convert secondary alcohols into ketones include chromium trioxide (CrO3), potassium permanganate (KMnO4), and sodium dichromate (Na2Cr2O7).
Hydrogen peroxide can be used, but it may not be effective on its own. It is corrosive and explosive, and other reagents are often added to improve its performance.
Alternative methods include using benzylic alcohols with oxygen from the air in an ultrasonic bath, or a heterogeneous palladium(II) acetate-pyridine complex supported by hydrotalcite under mild conditions in toluene.
