Noah Vinter
Alcohol oxidation is a chemical reaction in which an alcohol molecule is converted into an aldehyde, ketone, carboxylic acid, or ester. This reaction typically involves the removal of a hydroxyl group from the alcohol molecule, resulting in the formation of a carbonyl functional group. The removal of alcohol is a reduction reaction, as it involves the gain of oxygen or loss of hydrogen. Oxidation and reduction reactions always occur together, with one compound being oxidized and another reduced. Common oxidizing agents used in alcohol oxidation include chromic acid, potassium dichromate, and pyridinium chlorochromate. The easiest way to oxidize alcohol is by combustion, which involves burning the alcohol in oxygen to form carbon dioxide and water.
| Characteristics | Values |
|---|---|
| Definition | Alcohol oxidation is a chemical reaction in which an alcohol molecule is converted into an aldehyde, ketone, carboxylic acid, or ester. |
| Mechanism | Alcohol oxidation typically follows a two-step mechanism, involving the formation of an intermediate alkoxide ion and the subsequent elimination of a hydroxyl group. |
| Oxidizing Agents | Common oxidizing agents include chromic acid (H2CrO4), potassium dichromate (K2Cr2O7), pyridinium chlorochromate (PCC), and chromium trioxide (CrO3). |
| Reaction Products | The products of oxidation depend on the type of alcohol: primary alcohols form aldehydes or carboxylic acids, secondary alcohols form ketones, and tertiary alcohols are typically unaffected. |
| Combustion | Alcohols can be completely oxidized by combustion in plenty of oxygen, forming carbon dioxide and water. |
| Reduction | The reverse process of oxidation is reduction, where a compound gains electrons. In organic chemistry, reduction can also be considered the gain of hydrogen or loss of oxygen. |
| Reagents | Reagents used for alcohol oxidation include O2/air, nitric acid, potassium permanganate (KMnO4), and Dess-Martin periodinane. |
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What You'll Learn
Primary alcohols are oxidised to aldehydes or carboxylic acids
The removal of alcohol is an oxidation reaction. One of the most important oxidation reactions in organic chemistry involves alcohols, which are converted to carbonyl-containing compounds such as aldehydes, ketones, and carboxylic acids.
In the case of the formation of carboxylic acids, the primary alcohol is first oxidised to an aldehyde, which is then further oxidised to the carboxylic acid. This two-step procedure is often employed by organic chemists due to the harsh conditions of direct oxidation to carboxylic acids, which are incompatible with common protection groups. The oxidation of primary alcohols to carboxylic acids can be efficiently achieved using potassium permanganate (KMnO4). This reaction, described by Fournier, involves adding KMnO4 to an alkaline aqueous solution of the alcohol. The alcohol must be at least partially dissolved in the solution, which can be facilitated by adding an organic co-solvent.
The oxidation of primary alcohols can also be carried out using a variety of reagents, with O2/air and nitric acid being the most common oxidants on a commercial scale. Chromium trioxide (CrO3) is another oxidising agent used to convert primary alcohols to carboxylic acids.
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Secondary alcohols are oxidised to ketones
The removal of alcohol is both an oxidation and a reduction process. This is because, in oxidation and reduction reactions, when one compound is oxidized, another compound must be reduced.
One of the most important reactions of alcohols is their oxidation to carbonyl-containing compounds such as aldehydes, ketones, and carboxylic acids. Secondary alcohols are oxidised to ketones. This reaction is used to distinguish between primary, secondary, and tertiary alcohols.
Pyridinium chlorochromate (PCC) is a milder version of chromic acid that is suitable for converting a primary alcohol into an aldehyde without oxidizing it completely to a carboxylic acid. The first step of the mechanism is the attack of alcohol oxygen on the chromium atom to form the Cr-O bond. Secondly, a proton on the (now positive) OH is transferred to one of the oxygens of the chromium, possibly through the intermediacy of the pyridinium salt. A chloride ion is then displaced, in a reaction reminiscent of a 1,2 elimination reaction, to form what is known as a chromate ester. The C-O double bond is formed when a base removes the proton on the carbon adjacent to the oxygen.
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Tertiary alcohols are not oxidised
Tertiary alcohols are organic compounds with a hydroxyl (-OH) group attached to a carbon atom. This carbon atom is distinct as it is connected to three other carbon atoms. This structural composition makes tertiary alcohols relatively stable and less reactive to oxidation under typical laboratory conditions.
To understand why tertiary alcohols are resistant to oxidation, it is crucial to grasp the fundamental principles of oxidation reactions. In the realm of chemistry, oxidation reactions involve the transfer of electrons between atoms or molecules. Specifically, oxidation occurs when a compound or atom loses electrons during a reaction. Conversely, reduction refers to the process by which a compound or atom gains electrons.
In the context of alcohols, oxidation reactions are commonly employed to convert them into carbonyl-containing compounds, such as aldehydes, ketones, and carboxylic acids. These transformations are facilitated by oxidizing agents, which play a pivotal role in the reaction mechanism. Notable examples of oxidizing agents include sodium or potassium dichromate(VI) solution, acidified with dilute sulfuric acid.
However, when it comes to tertiary alcohols, a unique phenomenon emerges. Tertiary alcohols exhibit a distinct structural feature that sets them apart from primary and secondary alcohols. The carbon atom in tertiary alcohols, to which the hydroxyl group is attached, lacks a hydrogen atom. Consequently, during an oxidation reaction, the oxidizing agent is unable to remove the hydrogen atoms necessary to establish a carbon-oxygen double bond. This inability to remove the hydrogen atoms renders the oxidation process ineffective for tertiary alcohols.
Notably, under specific conditions, such as when tertiary alcohols are allylic, they can undergo unique reactions, including allylic shifts. Allylic shifts involve the migration of a double bond to a neighbouring carbon atom adjacent to a functional group, often an alcohol. This facilitates the oxidation of allylic alcohols and the formation of carbonyl compounds. Nonetheless, under typical circumstances, tertiary alcohols remain resistant to oxidation.
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Oxidation and reduction occur together
The removal of alcohol is a reduction reaction. In this process, an alcohol molecule is converted into an aldehyde or ketone through the removal of one or more hydroxyl groups. This involves the loss of electrons from the alcohol molecule, resulting in the formation of a carbonyl functional group.
Alcohol oxidation is a chemical reaction that typically follows a two-step mechanism. The first step involves the formation of an intermediate alkoxide ion, and the second step involves the elimination of a hydroxyl group. The oxidizing agent used in these reactions is usually 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.
Primary alcohols can be oxidized to either aldehydes or carboxylic acids, depending on the reaction conditions. Secondary alcohols are oxidized to produce ketones, while tertiary alcohols are generally unaffected by oxidation. The easiest way to oxidize an alcohol is by combustion—when alcohols are burnt in an oxygen-rich environment, they are completely oxidized to form carbon dioxide and water.
Oxidation and reduction reactions always occur together. When one compound is oxidized, another compound is reduced. For an alcohol to be oxidized, there must be a compound being reduced, known as the oxidizing agent. Chromium trioxide (CrO3), for instance, is a common oxidizing agent used to oxidize secondary alcohols to ketones. During this reaction, CrO3 is reduced to form H2CrO3.
In organic chemistry, reduction can be understood as the gain of hydrogen or loss of oxygen, while oxidation is the gain of oxygen or loss of hydrogen. For example, when hydrogen is added to ethene to reduce it to ethane, the oxidation number of the carbon atoms decreases from −II to −III. Conversely, when hydrogen is removed from 2-propanol, the oxidation number of the central carbon atom increases from 0 to +II.
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Alcohol oxidation is a one-way reaction
The removal of alcohol is an oxidation reaction. In this process, an oxidizing agent removes the hydrogen from the -OH group of primary and secondary alcohols. Tertiary alcohols, on the other hand, are unaffected by oxidation. This is because, unlike primary and secondary alcohols, tertiary alcohols do not have a hydrogen atom attached to the carbon atom.
Secondary alcohols, on the other hand, are oxidized to produce ketones. This reaction is straightforward, and changing the reaction conditions does not affect the product. A common method for oxidizing secondary alcohols to ketones involves using chromic acid (H2CrO4) as the oxidizing agent.
Tertiary alcohols, however, are not oxidized by acidified sodium or potassium dichromate(VI) solution. This is because tertiary alcohols do not have a hydrogen atom attached to the carbon atom, which is necessary for the oxidation reaction to occur.
The oxidation of alcohols plays a crucial role in 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 dichromate(VI) ions is reduced to a green solution containing chromium(III) ions.
One example of an oxidizing agent used in laboratory settings is Dess-Martin periodinane (DMP), which has replaced pyridinium chlorochromate (PCC). DMP offers advantages such as higher yields and less stringent reaction conditions.
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Frequently asked questions
Alcohol oxidation is a chemical reaction in which an alcohol molecule is converted into an aldehyde, ketone, carboxylic acid, or ester. The reaction mainly applies to primary and secondary alcohols.
An oxidizing agent is a chemical species that accepts electrons from the alcohol molecule and facilitates the removal of the hydroxyl group. Common oxidizing agents used in alcohol oxidation include chromic acid (H2CrO4), potassium dichromate (K2Cr2O7), and pyridinium chlorochromate (PCC).
Primary alcohols can be oxidized to form aldehydes, which can be further oxidized to form carboxylic acids. Secondary alcohols, on the other hand, typically form ketones rather than aldehydes or carboxylic acids.
