Theo Nash
The oxidation of primary alcohols is a crucial reaction in organic chemistry, converting primary alcohols to aldehydes or carboxylic acids. This process involves breaking a C-H bond and forming a C-O bond, with the oxidizing agent undergoing a colour change. The oxidation state of the carbon increases, or oxidizes up, with aldehydes one step up from primary alcohols and carboxylic acids two steps up. The oxidation of primary alcohols to aldehydes can be achieved using weak oxidants like pyridinium chlorochromate (PCC) and Dess-Martin Periodinane (DMP), while strong oxidants such as chromic acid (H2CrO4) and KMnO4 are required to reach the carboxylic acid level. The choice of reagent is essential, as harsh conditions may not be compatible with protection groups, and the reaction conditions must be carefully controlled to prevent over-oxidation.
Characteristics and Values of Primary Alcohol Oxidation in Alkaline Solution
| Characteristics | Values |
|---|---|
| Oxidising Agents | Potassium dichromate(VI), acidified with dilute sulfuric acid, Fehling's solution, Tollens' reagent, sodium hypochlorite, tetrapropylammonium perruthenate, silver carbonate, TEMPO, chromium trioxide, chromic acid, pyridinium chlorochromate, Dess-Martin Periodinane, Swern |
| Colour Change | From orange to green |
| Reaction | 3CH3CH2OH + 2Cr2O72- + 16H+ → 3CH3COOH + 4Cr3+ + 11H2O |
| Simplified Reaction | CH3CH2OH + 2 [O] → CH3COOH + H2O |
| Product | Aldehydes, Carboxylic acids |
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What You'll Learn
Using acidified sodium or potassium dichromate(VI) solution
The oxidation of alcohols using acidified sodium or potassium dichromate(VI) solution is a reaction used to make aldehydes, ketones, and carboxylic acids. It is also a way 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. The oxidation of primary alcohols to carboxylic acids usually proceeds in two steps: the alcohol is first oxidized to an aldehyde, which is then further oxidized to the acid. However, the reaction conditions can be manipulated to produce aldehydes or carboxylic acids from primary alcohols.
To perform the reaction, a few drops of the primary alcohol are added to a test tube containing the acidified potassium dichromate(VI) solution. The tube is then warmed in a hot water bath. If the orange solution containing the dichromate(VI) ions is reduced to a green solution containing chromium(III) ions, it indicates that oxidation has occurred. The electron-half-equation for this reaction is as follows:
\[ Cr_2O_7^{2-} + 14H^+ + 6e^- \rightarrow 2Cr^{3+} + 7H_2O \]
It is important to note that tertiary alcohols do not react with acidified sodium or potassium dichromate(VI) solution. In the case of secondary alcohols, there is a distinct colour change to the acidified solution, indicating the presence of a secondary alcohol.
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Employing chromium trioxide (CrO3) as an oxidising agent
Chromium trioxide (CrO3) is a strong oxidising agent that can be used to oxidise primary alcohols in an alkaline solution. This process involves several steps and requires careful handling due to the toxic nature of chromium compounds.
Firstly, it is important to ensure that the alkaline solution is free of water and neutral in pH. This is a crucial step as the presence of water can lead to the formation of aldehyde hydrates, which can interfere with the oxidation process.
For the oxidation reaction, chromium trioxide is typically dissolved in an aqueous sulfuric acid solution, creating chromic acid (H2CrO4). This mixture is then added slowly to an alcohol in acetone, resulting in oxidation products such as carbonyl compounds and carboxylic acids. This reaction proceeds smoothly with only a small amount of chromium trioxide (1-2 mol %) and the right quantity of H5IO6 in wet MeCN, yielding excellent results in carboxylic acids.
The Ratcliffe variant of Collins reagent is another method that employs chromium trioxide for the oxidation of primary alcohols. This involves adding chromium trioxide to a solution of pyridine in methylene chloride. The resulting solution can then be used to selectively oxidise primary and secondary alcohols to carbonyl compounds.
It is important to note that chromium trioxide is a strong oxidising agent, and its use may result in the complete oxidation of primary alcohols to carboxylic acids. However, by using specific reagents such as pyridine, the oxidation can be controlled to stop at the aldehyde level. Therefore, careful selection of reagents and reaction conditions is essential to achieve the desired oxidation state.
In summary, employing chromium trioxide as an oxidising agent offers an efficient and selective method for oxidising primary alcohols in alkaline solutions. By utilising different reagents and reaction conditions, the oxidation state can be controlled, making it a versatile tool in organic chemistry. However, due to the toxicity of chromium compounds, it is crucial to handle these reactions with care.
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Pyridinium chlorochromate (PCC) as a milder option
Pyridinium chlorochromate (PCC) is a milder alternative to chromic acid for oxidising primary alcohols. It is a versatile oxidant that can be used in organic synthesis.
PCC oxidises primary alcohols to aldehydes and secondary alcohols to ketones. It is a one-rung oxidation process, and unlike chromic acid, it will not further oxidise aldehydes to carboxylic acids. This makes it a useful option when only a mild reaction is required.
The reaction mechanism involves a transfer of two electrons from the chromium in PCC to the substrate. The by-products of the reaction are Cr(IV) and pyridinium hydrochloride. The amount of water present in the reaction must be carefully controlled. If water is present, it can add to the aldehyde to create a hydrate, which could then be further oxidised by a second equivalent of PCC.
The mildly acidic nature of PCC can be buffered with sodium acetate to protect acid-labile groups. This makes it a useful reagent for moderate to large-scale oxidations.
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Fétizon oxidation using silver carbonate on Celite
Fétizon oxidation is a process used to oxidize primary and secondary alcohols. It utilizes the compound silver(I) carbonate absorbed onto the surface of celite, also known as Fétizon's reagent. This method was first employed by Marcel Fétizon in 1968.
Fétizon's reagent is prepared by adding silver nitrate to an aqueous solution of a carbonate, such as sodium carbonate or potassium bicarbonate. This mixture is vigorously stirred in the presence of purified celite. The reaction is typically carried out in a refluxing dry non-polar organic solvent with constant stirring. The reaction time depends on the structure of the alcohol and usually takes up to three hours to complete.
The role of celite in Fétizon oxidation is crucial. Increasing the amount of celite increases the surface area available for the reaction, accelerating the rate of oxidation. However, if the amount of celite exceeds 900 grams per mole of silver(I) carbonate, the reaction slows down due to dilution effects. An excess of silver carbonate on celite is necessary for the reaction to proceed at a reasonable rate, and the recovery of silver can be achieved by dissolving the used reagent in nitric acid.
Fétizon's reagent is selective in its oxidation, primarily converting primary and secondary alcohols into aldehydes or ketones. It exhibits a slight preference for secondary alcohols and unsaturated alcohols. This selectivity makes it extremely useful in the monooxidation of [1,2] diols, where one of the alcohols is tertiary. Under different structural conditions, [1,2] diols can form diketones in the presence of Fétizon's reagent, but oxidative carbon-carbon bond cleavage may also occur.
The Fétizon reagent has proven its versatility in the total synthesis of various molecules, such as (±)-bukittinggine. It is also valuable in the preparation of aldonolactones, selectively oxidizing specific hydroxyl groups in partially protected 2-acetamido-2-deoxypyranoses. This reagent is mild, making it suitable for both acid- and base-sensitive compounds, and it can be easily separated from the reaction product by physical filtration, followed by washing with benzene.
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Oxidising agents like Fehling's and Tollens' reagents
Fehling's solution is a mixture of two solutions: Fehling solution A, which is made of aqueous copper sulfate, and Fehling solution B, which is made of Rochelle salt or alkaline sodium potassium tartrate. Equal quantities of both solutions are mixed before the test. The aldehyde is then heated with Fehling's reagent, resulting in the formation of a reddish-brown colour precipitate. The electron-half-equation for the reaction is:
> 2Cu^{2+}_{complexed} + 2OH^- + 2e^- → Cu2O + H2O
Tollens' reagent contains the diamminesilver(I) ion, [Ag(NH3)2]+, which is made from silver(I) nitrate solution. To prepare the reagent, a drop of sodium hydroxide solution is added to form a precipitate of silver(I) oxide, and then dilute ammonia solution is added to redissolve the precipitate. To carry out the test, a few drops of the aldehyde or ketone are added to the reagent, and the mixture is warmed gently in a hot water bath for a few minutes. Aldehydes reduce the diamminesilver(I) ion to metallic silver, and the solution turns a silver-mirror colour.
Both Fehling's and Tollens' reagents are mild oxidising agents that can be used to distinguish between aldehydes and ketones. Aldehydes are easily oxidised due to the presence of a hydrogen atom in the carbon-oxygen double bond, which acts as a strong reducing agent. Ketones, on the other hand, lack this hydrogen atom and are resistant to oxidation, requiring very strong oxidising agents like potassium manganate(VII) solution.
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Frequently asked questions
Primary alcohols can be oxidised to form aldehydes, which can undergo further oxidation to form carboxylic acids.
The presence of an aldehyde group (-CHO) in an unknown compound can be determined by using oxidising agents like Fehling's and Tollens' reagents.
Some examples of oxidising agents used to oxidise primary alcohols include acidified potassium dichromate, acidified potassium manganate, pyridinium chlorochromate (PCC), and Dess-Martin Periodinane (DMP).
When an alcohol is oxidised, the orange dichromate(VI) ions (Cr2O72-) are reduced to green Cr3+ ions.
For introductory organic chemistry, oxidants for alcohols can be categorised as "weak" and "strong". Weak oxidants, such as PCC and DMP, convert primary alcohols to aldehydes. Strong oxidants, such as chromic acid, oxidise primary alcohols to carboxylic acids.
