Mason Delacroix
Alcohol oxidation is a collection of oxidation reactions in organic chemistry that convert alcohols to aldehydes, ketones, carboxylic acids, and esters. The reaction applies to primary and secondary alcohols, which can be distinguished by testing the product of oxidation under reflux with 2,4-DNPH. Tertiary alcohols, on the other hand, cannot be oxidized under normal conditions as they do not have a hydrogen atom that can be removed. The oxidation of primary alcohols to aldehydes can be achieved using mild oxidizing agents, while stronger oxidizing agents are needed to convert them to carboxylic acids. A variety of oxidants can be used, with oxygen or air being the most common on an industrial scale.
| Characteristics | Values |
|---|---|
| Oxidation of primary alcohols | Aldehydes, Carboxylic acids |
| Oxidation of secondary alcohols | Ketones |
| Tertiary alcohols | No oxidation under normal conditions |
| Oxidizing agents | Potassium dichromate(VI) solution, Schiff's reagent, Jones reagent, Dess-Martin periodinane, Ley oxidation, Fétizon oxidation, Oxoammonium-catalysed oxidation, Potassium permanganate, Pyridinium chlorochromate, Sodium dichromate |
| Oxidation evidence | Change in colour from orange to green, formation of precipitate, change in fumes, change in boiling point, change in pH, formation of carbonyl group |
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What You'll Learn
Primary alcohols are oxidised to form aldehydes
Alcohol oxidation is a collection of oxidation reactions in organic chemistry that convert alcohols to aldehydes, ketones, carboxylic acids, and esters. The oxidation of primary alcohols to aldehydes can be facilitated by the addition of an organic co-solvent such as dioxane, pyridine, acetone, or t-BuOH. The oxidizing agent used in these reactions is typically a solution of sodium or potassium dichromate(VI) acidified with dilute sulfuric acid.
The oxidation of primary alcohols to aldehydes can be observed through a colour change in the solution. The orange solution containing the dichromate(VI) ions is reduced to a green solution containing chromium(III) ions. This colour change is evidence of the oxidation of the primary alcohol to an aldehyde.
The presence of an aldehyde group (-CHO) in an unknown compound can be further confirmed using oxidizing agents such as Fehling's and Tollens' reagents. When warmed with an aldehyde, the aldehyde is oxidized to a carboxylic acid. This reaction can be identified by the formation of a precipitate or a colour change. For example, the clear blue solution may turn opaque red due to the formation of a copper(I) oxide precipitate.
The oxidation of primary alcohols to aldehydes can also be achieved using other reagents and methods. For instance, the Dess-Martin periodinane is a mild oxidant for converting alcohols to aldehydes. The reaction is performed under standard conditions and takes approximately 30 minutes to two hours to complete. Another example is the use of chlorites as terminal oxidants in conjunction with hypochlorites and TEMPO, which results in the formation of carboxylic acids without chlorination side products.
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Secondary alcohols are oxidised to form ketones
Alcohol oxidation is a collection of oxidation reactions in organic chemistry that convert alcohols to aldehydes, ketones, carboxylic acids, and esters. The oxidation of alcohols can be used to distinguish between primary, secondary, and tertiary alcohols. The oxidizing agent used in these reactions is typically a solution of sodium or potassium dichromate(VI) acidified with dilute sulfuric acid. If oxidation occurs, the orange solution containing the dichromate(VI) ions is reduced to a green solution containing chromium(III) ions.
Chromium trioxide (CrO3) is a common oxidizing agent used by organic chemists to oxidize a secondary alcohol to a ketone. 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. Chromic acid, also known as Jones reagent, is prepared by adding chromium trioxide (CrO3) to aqueous sulfuric acid.
The oxidation of primary alcohols to carboxylic acids can be carried out using a variety of reagents, but O2/air and nitric acid dominate as the oxidants on a commercial scale. Large-scale oxidations of this type are used for the conversion of cyclohexanol alone or as a mixture with cyclohexanone to adipic acid. Similarly, cyclododecanol is converted to the 12-carbon dicarboxylic acid.
There are several other methods for oxidizing alcohols. Ley oxidation uses NMO as the stoichiometric oxidant with tetrapropylammonium perruthenate as a catalyst. Fétizon oxidation, a seldom-used method, uses silver carbonate supported on Celite. Another method is the oxoammonium-catalyzed oxidation. TEMPO exhibits a strong, pH-dependent selectivity for either primary or secondary alcohols, but the effect is primarily steric, and other N-oxides behave differently.
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Tertiary alcohols cannot be oxidised
Alcohol oxidation is a collection of oxidation reactions in organic chemistry that convert alcohols to aldehydes, ketones, carboxylic acids, and esters. The reaction mainly applies to primary and secondary alcohols. Primary alcohols form aldehydes or carboxylic acids, while secondary alcohols form ketones.
Tertiary alcohols, on the other hand, cannot be oxidized under mild conditions. This is because the oxidation of alcohols involves the formation of a carbon-to-oxygen double bond. To accommodate this new double bond, the carbon atom bearing the OH group must be able to release one of its attached atoms. In primary and secondary alcohols, the reagents and conditions are able to break the C-H bond, but this is not the case for tertiary alcohols. Tertiary alcohols do not have a C-H bond on the carbon with the OH group; instead, the carbon is bonded to other carbon atoms. Breaking a C-C bond requires much more energy than breaking a C-H bond, and the formation of a C=O bond does not provide enough energy to compensate. Therefore, the oxidation of tertiary alcohols is not viable under mild conditions.
However, it is important to note that tertiary alcohols can still be oxidized under certain conditions. For example, they can be burned, which is a form of oxidation. Additionally, some sources suggest that tertiary alcohols can be oxidized in the presence of K2Cr2O7 and H2SO4 to form alkenes.
In summary, while tertiary alcohols cannot be oxidized under mild conditions due to the stability of the C-C bond, they can undergo oxidation under specific conditions or through processes such as combustion.
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Oxidation reaction involves the loss of electrons
Alcohol oxidation involves the conversion of alcohols to aldehydes, ketones, carboxylic acids, and esters. The reaction primarily applies to primary and secondary alcohols. Secondary alcohols form ketones, while primary alcohols form aldehydes or carboxylic acids.
The oxidation of primary alcohols to carboxylic acids can be carried out using a variety of reagents, but O2/air and nitric acid dominate as the oxidants on a commercial scale. Large-scale oxidations of this type are used for the conversion of cyclohexanol alone or as a mixture with cyclohexanone to adipic acid. The oxidation of primary alcohols to aldehydes can be verified by adding a few drops of the alcohol to a test tube containing an acidified potassium dichromate(VI) solution. The orange solution turns green, indicating a positive result.
There are several methods for oxidizing alcohols to aldehydes or ketones. The Dess-Martin periodinane is a mild oxidant for this conversion, and the reaction is performed at room temperature, most often in dichloromethane. The Swern oxidation uses oxalyl chloride, dimethylsulfoxide, and an organic base, and the by-products are dimethyl sulfide, carbon monoxide, carbon dioxide, and triethylammonium chloride when triethylamine is used as the base.
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Alcohol oxidation using acidified sodium or potassium dichromate(VI) solution
Alcohol oxidation is a collection of oxidation reactions in organic chemistry that convert alcohols to aldehydes, ketones, carboxylic acids, and esters. The reaction mainly applies to primary and secondary alcohols. Secondary alcohols form ketones, while primary alcohols form aldehydes or carboxylic acids.
One of the methods to oxidize alcohols is to use acidified sodium or potassium dichromate(VI) solution. This reaction is used to make aldehydes, ketones, and carboxylic acids, and as a way of distinguishing between primary, secondary, and tertiary alcohols. The oxidizing agent used in these reactions is typically a solution of sodium or potassium dichromate(VI) acidified with dilute sulfuric acid. If oxidation occurs, the orange solution containing the dichromate(VI) ions is reduced to a green solution containing chromium(III) ions.
The electron-half-equation for this reaction is as follows:
\[ Cr_2O_7^{2-} + 14H^+ + 6e^- \rightarrow 2Cr^{3+} + 7H_2O\]
Primary alcohols can be oxidized to either aldehydes or carboxylic acids, depending on the reaction conditions. In the case of the formation of carboxylic acids, the alcohol is first oxidized to an aldehyde, which is then oxidized further to the acid. An aldehyde is obtained if an excess amount of the alcohol is used, and the aldehyde is distilled off as soon as it forms.
A practical application of this method is a microscale experiment where students add acidified dichromate(VI) to primary, secondary, and tertiary alcohols to observe the difference in their oxidation reactions. The experiment can be done by students in 20 minutes. The colour change of the dichromate(VI) indicates where the reaction is occurring. Primary, secondary, and tertiary alcohols can be distinguished by the rate of reaction, though no attempt is made to identify the products.
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Frequently asked questions
When an alcohol is oxidised, the solution changes from orange to green due to the reduction of C2O7-2 ions to Cr3+ ions.
The easiest way to oxidise an alcohol is by combustion, or burning the alcohol in plenty of oxygen, which produces a pale blue flame.
The Jones oxidation uses chromic acid (CrO3 in H2SO4) as the oxidising agent.
Acidified sodium or potassium dichromate(VI) solution is used to distinguish between primary, secondary and tertiary alcohols.
The evidence for the oxidation of a primary alcohol is the formation of an aldehyde, which can be identified by a colour change in Schiff's reagent.
