Theo Nash
The oxidation of alcohols 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, but tertiary alcohols are usually not affected by oxidation. The oxidizing agent used in these reactions is typically a solution of sodium or potassium dichromate(VI) acidified with dilute sulfuric acid. Primary alcohols can be oxidized to either aldehydes or carboxylic acids, depending on the reaction conditions. Secondary alcohols are oxidized to produce ketones. Tertiary alcohols cannot be oxidized under normal conditions because they do not have a hydrogen atom that can be removed.
| Characteristics | Values |
|---|---|
| Reaction of primary alcohols with oxidizing agents | Forms aldehydes or carboxylic acids |
| Reaction of secondary alcohols with oxidizing agents | Forms ketones |
| Reaction of tertiary alcohols with oxidizing agents | No reaction |
| Colour change of primary and secondary alcohols with potassium dichromate(VI) solution | From orange to green |
| Colour change of tertiary alcohols with potassium dichromate(VI) solution | No colour change |
| Oxidizing agent used in the reaction | Sodium or potassium dichromate(VI) acidified with dilute sulfuric acid |
| Reaction equation | Cr2O72- + 14H+ + 6e- → 2Cr3+ + 7H2O |
| Aldehyde formation from primary alcohol | CH3CH2OH + Cr2O72- + 8H+ → CH3CHO + 2Cr3+ + 7H2O |
| Common oxidizing agent for secondary alcohols | Chromium trioxide (CrO3) |
| Common oxidizing agent for primary alcohols | Potassium permanganate (KMnO4) |
| Mild oxidizing agent for primary alcohols | Pyridinium chlorochromate (PCC) |
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What You'll Learn
- Primary alcohols are oxidised to aldehydes or carboxylic acids
- Tertiary alcohols are not oxidised by acidified sodium or potassium dichromate
- Secondary alcohols are oxidised to ketones
- Pyridinium chlorochromate (PCC) is a milder version of chromic acid
- Aldehydes can be further oxidised to carboxylic acids
Primary alcohols are oxidised to aldehydes or carboxylic acids
The oxidation of primary alcohols leads to the formation of aldehydes or carboxylic acids. The oxidation of primary alcohols to aldehydes can be achieved using pyridinium chlorochromate (PCC), a milder version of chromic acid. This reaction converts primary alcohols to aldehydes without further oxidation to carboxylic acids. The Dess-Martin periodinane (DMP) is another reagent that can be used to oxidize primary alcohols to aldehydes. This reagent offers advantages such as higher yields and less stringent reaction conditions.
The oxidation of primary alcohols to carboxylic acids can be achieved using strong oxidizing agents such as potassium permanganate (KMnO4) and Cr(VI) species, which are precursors of chromic acid (H2CrO4). These strong oxidants can directly convert primary alcohols to carboxylic acids in a single flask. The oxidation of primary alcohols to carboxylic acids typically proceeds via the corresponding aldehyde as an intermediate step. This intermediate aldehyde can be isolated by using an excess of the primary alcohol, preventing further oxidation to the carboxylic acid.
The choice of reagent and reaction conditions determines whether the primary alcohol is oxidized to an aldehyde or carboxylic acid. For example, the reaction using PCC stops at the aldehyde stage, while the reaction with KMnO4 proceeds to the carboxylic acid stage. The presence of water also influences the reaction pathway, as the oxidation of primary alcohols to carboxylic acids often involves the formation of an aldehyde hydrate (gem-diol) by reaction with water. By performing the reaction in the absence of water, the oxidation of a primary alcohol can be limited to the aldehyde level without further oxidation to the carboxylic acid.
The oxidation of primary alcohols can be identified by a colour change in the reaction mixture. The oxidation of primary alcohols with acidified potassium dichromate(VI) solution turns the orange solution containing dichromate(VI) ions into a green solution containing chromium(III) ions. This colour change is indicative of the formation of aldehydes or carboxylic acids from the oxidation of primary alcohols.
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Tertiary alcohols are not oxidised by acidified sodium or potassium dichromate
Tertiary alcohols are resistant to oxidation due to their structure, and they are not oxidised by acidified sodium or potassium dichromate(VI) solution. In contrast, primary and secondary alcohols are susceptible to oxidation, and acidified sodium or potassium dichromate(VI) solution is a commonly used oxidising agent for these alcohols.
The oxidation of alcohols is an important reaction in chemistry, and it is often used to distinguish between primary, secondary, and tertiary alcohols. Primary alcohols can be oxidised to aldehydes or carboxylic acids, depending on the reaction conditions. Secondary alcohols are typically oxidised to ketones. Tertiary alcohols, however, are generally not affected by oxidation reactions.
When primary or secondary alcohols are treated with acidified sodium or potassium dichromate(VI) solution, the orange solution turns green, indicating a change in the chemical composition. Specifically, the dichromate(VI) ions in the solution are reduced to chromium(III) ions, which have a green colour. This colour change is a simple and effective way to distinguish between primary/secondary alcohols and tertiary alcohols, as tertiary alcohols do not cause this colour change.
The reason tertiary alcohols are resistant to oxidation by acidified sodium or potassium dichromate(VI) solution lies in their molecular structure. In the oxidation of primary and secondary alcohols, the oxidising agent removes a hydrogen atom from the -OH group and a hydrogen atom from the carbon atom attached to it. However, in tertiary alcohols, there is no hydrogen atom attached to the carbon atom adjacent to the -OH group. As a result, the oxidation reaction cannot occur, and the tertiary alcohol remains unchanged.
While tertiary alcohols are not oxidised by acidified sodium or potassium dichromate(VI) solution, they can undergo oxidation under certain conditions. For example, tertiary alcohols can be oxidised by combustion. However, compared to primary and secondary alcohols, the resistance of tertiary alcohols to oxidation by commonly used oxidising agents is a notable characteristic that can be utilised for identification and separation purposes.
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Secondary alcohols are oxidised to ketones
The oxidation of alcohols is a crucial process in chemistry, and understanding how oxidizing agents react with primary, secondary, and tertiary alcohols is essential. In this discussion, we will focus on how secondary alcohols are oxidized to form ketones.
Secondary alcohols undergo oxidation to yield ketones as the final product. This reaction is distinct from the oxidation of primary alcohols, which can produce either aldehydes or carboxylic acids, depending on the reaction conditions. The oxidation of tertiary alcohols, on the other hand, typically leads to no observable changes.
When secondary alcohols are treated with oxidizing agents, the hydrogen atom from the -OH group is removed. Additionally, a hydrogen atom attached to the carbon atom adjacent to the -OH group is also removed. This process results in the formation of a carbonyl group (=O) and the conversion of the secondary alcohol into a ketone.
One commonly used oxidizing agent for this transformation is acidified sodium or potassium dichromate(VI) solution. When heated, this solution changes from orange to green, indicating the presence of chromium(III) ions. The oxidation of secondary alcohols with this reagent consistently yields ketones, and further reaction does not occur.
Another reagent employed for this purpose is pyridinium chlorochromate (PCC), which is considered a milder version of chromic acid. PCC effectively oxidizes secondary alcohols to ketones without progressing to the formation of carboxylic acids, as seen with chromic acid.
In summary, secondary alcohols are selectively oxidized to ketones using various oxidizing agents. This reaction is a fundamental concept in organic chemistry and provides a means to distinguish between different types of alcohols based on their oxidation behavior.
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Pyridinium chlorochromate (PCC) is a milder version of chromic acid
Pyridinium chlorochromate (PCC) is a readily available, stable, and efficient reagent that is a milder version of chromic acid. It is used to oxidize primary alcohols into aldehydes and secondary alcohols into ketones. Unlike chromic acid, it does not oxidize aldehydes into carboxylic acids.
The oxidation process involves a transfer of two electrons from chromium to the substrate. The first step is the attack of oxygen on the chromium atom to form the Cr-O bond. This is followed by the transfer of a proton from the (now positive) OH group to one of the oxygens of the chromium, possibly through the intermediacy of the pyridinium salt. Subsequently, a chloride ion is displaced in a reaction reminiscent of a 1,2 elimination reaction, resulting in the formation of a chromate ester. The C-O double bond is then formed when a base removes the proton on the carbon adjacent to the oxygen. This breaks the O-Cr bond, and Cr(VI) becomes Cr(IV).
The use of PCC in laboratories is being replaced by Dess-Martin periodinane (DMP), which offers advantages such as higher yields and less stringent reaction conditions. However, PCC remains a valuable reagent for oxidizing primary and secondary alcohols to carbonyl compounds.
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Aldehydes can be further oxidised to carboxylic acids
The oxidation of primary alcohols to form aldehydes has been discussed previously. Aldehydes can be further oxidised to form carboxylic acids. This is a common process, especially when starting from a primary alcohol.
The oxidation of aldehydes to carboxylic acids can be achieved using various methods. One method involves the use of permanganate as the oxidant, which has been shown to be efficient and easily scalable. Another method involves the use of 1-hydroxycyclohexyl phenyl ketone as a cheap and metal-free oxidant, which has been shown to have high isolated yields and excellent functional group tolerance. A third method involves the use of VO(acac)2 as a catalyst in the presence of hydrogen peroxide as an oxidant, which offers functional-group compatibility, an easy workup procedure, and a short reaction time.
Pyridinium chlorochromate (PCC) is a milder oxidising agent that can be used to convert primary alcohols into aldehydes without oxidising them further into carboxylic acids. However, PCC is being replaced by Dess-Martin periodinane (DMP) in laboratories due to its higher yields and less rigorous reaction conditions.
The complete equation for the conversion of a primary alcohol to a carboxylic acid is:
\3RCH_2OH + 2Cr_2O_7^{2-} + 16H^+ \rightarrow 3RCOOH + 4Cr^{3+} + 11H_2O\>
In this equation, "R" represents a hydrogen atom or a hydrocarbon group such as an alkyl group. The first stage involves the oxidation of the primary alcohol to an aldehyde, and the second stage involves the further oxidation of the aldehyde to the carboxylic acid.
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Frequently asked questions
Primary alcohols are the easiest type of alcohol to oxidize. During oxidation, the alcohol functional group (-OH) loses hydrogen atoms and electrons, resulting in the formation of an aldehyde (-CHO) or a carboxylic acid (-COOH) functional group. The oxidation of a primary alcohol to an aldehyde is an example of partial oxidation. Further oxidation of the aldehyde to a carboxylic acid is an example of complete oxidation.
Tertiary alcohols cannot undergo oxidation under normal conditions because they do not have a hydrogen atom attached to the carbon atom that is attached to the hydroxyl group (-OH).
If a potassium dichromate(VI) solution is mixed with primary or secondary alcohols, the solution will change colour from orange to green. With tertiary alcohols, there is no colour change.
