Theo Nash
Oxidation reactions in organic chemistry convert alcohols to aldehydes, ketones, carboxylic acids, and esters. The oxidation of primary alcohols to aldehydes or carboxylic acids and the oxidation of secondary alcohols to ketones are important reactions in the preparation of synthetic intermediates in organic chemistry. The oxidation of alcohols to ketones can be achieved using a variety of oxidizing agents, including chromium trioxide (CrO3), potassium permanganate (KMnO4), sodium dichromate (Na2Cr2O7), pyridinium chlorochromate (PCC), and Dess-Martin periodinane. The choice of oxidizing agent and the structure of the alcohol influence the outcome of the oxidation reaction.
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What You'll Learn
Oxidizing agents
The oxidation of alcohols to ketones involves converting the OH group into a good leaving group and eliminating the adjacent hydrogen to form a C=O double bond. This reaction typically applies to primary and secondary alcohols. Secondary alcohols form ketones, while primary alcohols form aldehydes or carboxylic acids.
There are a variety of oxidizing agents that can be used for this process. The most common mild oxidizing agents are:
- Pyridinium chlorochromate (PCC)
- Pyridinium dichromate (PDC)
- Swern oxidation using DMSO, (COCl)2 and Et3N
- Dess-Martin (DMP) oxidation
Strong oxidizing agents such as Jones reagent (a solution of chromium trioxide in aqueous sulfuric acid) can also be used, but these tend to convert primary alcohols to carboxylic acids.
Other oxidizing agents include:
- Potassium dichromate(VI) solution acidified with dilute sulfuric acid
- Chromium(VI) reagents, such as CrO3(pyridine)2
- Sodium hypochlorite (household bleach) in acetone
- TEMPO/Bu4NHSO4
- CeBr3/H2O2
- Bismuth tribromide
- Tetrapropylammonium perruthenate
- Silver carbonate supported on Celite
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Primary vs secondary alcohols
The oxidation of alcohols is a collection of oxidation reactions in organic chemistry that convert alcohols to aldehydes, ketones, carboxylic acids, and esters. The oxidation reaction mainly applies to primary and secondary alcohols.
Primary Alcohols
Primary alcohols, depending on the reagent used, produce aldehydes or carboxylic acids during oxidation. The oxidation of primary alcohols to aldehydes requires mild oxidizing agents such as pyridinium chlorochromate (PCC), pyridinium dichromate (PDC), Swern oxidation using DMSO, (COCl)2 and Et3N, and the Dess-Martin (DMP) oxidation. PCC is a milder version of chromic acid that oxidizes primary alcohols to aldehydes without further oxidation to carboxylic acids.
Potassium permanganate (KMnO4) is another reagent that oxidizes primary alcohols to carboxylic acids efficiently. For the reaction to proceed efficiently, the primary alcohol must be at least partially dissolved in the aqueous solution, which can be facilitated by adding an organic co-solvent.
Secondary Alcohols
Secondary alcohols are oxidized to produce ketones. Chromium trioxide (CrO3) is a common oxidizing agent used by organic chemists to oxidize secondary alcohols to ketones. During this reaction, CrO3 is reduced to form H2CrO3. Another common method for oxidizing secondary alcohols to ketones uses chromic acid (H2CrO4) as the oxidizing agent.
CeBr3/H2O2 is a very efficient system for the green oxidation of secondary alcohols to carbonyls. This mechanism involves the generation of a reactive brominating species (RBS) with high oxidation selectivity of secondary alcohols over primary alcohols.
Tertiary Alcohols
Tertiary alcohols are usually not affected by oxidation and cannot be oxidized to any carbonyl compound due to their structure.
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Reaction conditions
The oxidation of alcohols to ketones can be achieved using a variety of reaction conditions, each with its own advantages and considerations. Here are some common methods and their corresponding reaction conditions:
Chromium Trioxide (CrO3)
Chromium trioxide, also known as chromic acid (H2CrO4), is a common oxidizing agent used for the conversion of secondary alcohols to ketones. The reaction typically involves the reduction of CrO3 to form H2CrO3. This method is often chosen due to its effectiveness in oxidizing secondary alcohols specifically to ketones.
Pyridinium Chlorochromate (PCC)
PCC is a mild oxidizing agent that is particularly useful when oxidizing primary alcohols to aldehydes without over-oxidation to carboxylic acids. It is important to note that PCC will not oxidize aldehydes to carboxylic acids, making it a controlled option for the synthesis of aldehydes.
Ruthenium Tetroxide
Ruthenium tetroxide is a potent but infrequently used agent that enables mild reaction conditions. It is often used in a two-step process, where partial oxidation is achieved with TEMPO and hypochlorite, followed by the addition of chlorite to complete the oxidation. This method is selective for primary alcohol oxidation.
TEMPO-Based Catalysis
A highly efficient method employs 2,2,6,6-tetramethylpiperidin-1-yloxy (TEMPO) as a catalyst along with a terminal oxidant such as 1-chloro-1,2-benziodoxol-3(1H)-one. This system allows for the conversion of various alcohols to their corresponding carbonyl compounds, including ketones, at room temperature in ethyl acetate, which is an environmentally friendly solvent.
Choline Peroxydisulfate (ChPS)
ChPS is a task-specific ionic liquid that enables the selective oxidation of alcohols to aldehydes or ketones under mild and solvent-free reaction conditions. This reagent prevents over-oxidation to acid and offers short reaction times.
I2-KI-K2CO3-H2O System
This system employs a combination of iodine, potassium iodide, potassium carbonate, and water to selectively oxidize alcohols to aldehydes and ketones under anaerobic conditions at 90 °C. It provides excellent yields and is considered a green, mild, and inexpensive approach.
Sodium or Potassium Dichromate(VI)
Heating a secondary alcohol, such as propan-2-ol, with a sodium or potassium dichromate(VI) solution acidified with dilute sulfuric acid, results in the formation of a ketone. This reaction specifically oxidizes secondary alcohols to ketones.
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Chromium trioxide
Chromic acid (H2CrO4), also known as Jones reagent, is prepared by adding chromium trioxide (CrO3) to aqueous sulfuric acid. It is a strong acid and a reagent for oxidizing alcohols to ketones and carboxylic acids. It is a common method for oxidizing secondary alcohols to ketones.
Jones oxidation uses acetone as a co-solvent in the reaction to prevent over-oxidation of the organic product. Ketones are not oxidized by chromic acid, so the reaction stops at the ketone stage. In contrast, primary alcohols are oxidized by chromic acid first to aldehydes, and then straight on to carboxylic acids.
It is also possible to stop the oxidation of primary alcohols at the aldehyde stage by rigorously excluding water. This can be done by using PCC instead of combining a chromium source with sulfuric acid.
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Jones reagent
The Jones oxidation reaction can be monitored using various spectroscopic techniques, including infrared spectroscopy. This reaction was discovered by Sir Ewart Jones and was one of the earliest methods for the oxidation of alcohols.
Other oxidizing agents used to convert alcohols to ketones include potassium permanganate (KMnO4), sodium dichromate (Na2Cr2O7), pyridinium chlorochromate (PCC), and Dess-Martin periodinane.
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Frequently asked questions
The principle behind oxidizing an alcohol to a ketone involves converting the OH group into a good leaving group and eliminating the adjacent hydrogen to form a C=O double bond.
Common oxidizing agents used to convert alcohols to ketones include chromium trioxide (CrO3), potassium dichromate(VI), pyridinium chlorochromate (PCC), and sodium hypochlorite.
Secondary alcohols, which are bonded to two carbon atoms, can be oxidized to form ketones. On the other hand, primary alcohols form aldehydes or carboxylic acids, and tertiary alcohols cannot be oxidized to ketones due to their structure.
