Oxidation of Alcohol

Oxidation of Alcohols is a fundamental reaction in organic chemistry that converts alcohols to carbonyl-containing compounds such as aldehydes, ketones, and carboxylic acids. Alcohols are organic compounds with the functional group -OH. The mechanism of alcohol oxidation typically involves the reduction of the oxidizing agent and the formation of a carbon-oxygen double bond.

In this article, we look into what alcohol is, the Types of alcohols, the mechanism of oxidation of alcohols, uses of alcohols, etc.

What is Alcohol in Chemistry?

Alcohols in chemistry are organic compounds that contain the hydroxyl functional group (-OH) bonded to a carbon atom of an alkyl or substituted alkyl. They are among the most important molecules in organic chemistry, with a wide range of applications. They can be converted into various other compounds. They can be named using common names or IUPAC nomenclature with the suffix -ol. They have the general formula ROH, where R is an alkyl group. Alcohols are known for their diverse physical and chemical properties, and they play a significant role in various industrial processes and natural occurrences. Examples of alcohol include methanol, ethanol, propanol, etc.

Characteristics of Alcohol

The characteristics of alcohol are as follows:

• Odor: Alcohols have a sweet smell, except for glycerol and a few lower alcohols.
• Colour: Most common alcohols are colourless at room temperature, but higher alcohols with more carbon atoms can be viscous or oily, and some may be solids at room temperature
• Solubility: Alcohols are soluble in water as they form hydrogen bonds with water molecules. The solubility decreases with increase in the size of the alkyl group.
• Acidity: Alcohols are acidic, as they can react with active metals like sodium and potassium to form alkoxides. Primary alcohols are more acidic than secondary and tertiary alcohols.
• Polarity: Alcohols are polar compounds, as they have a polar -OH bond. This polarity makes them more soluble in water than simple hydrocarbons.
• Flammability: Alcohols are highly flammable, as they evaporate quickly and form a vapour-air mixture that can ignite
• Reactivity: Alcohols undergo oxidation, dehydration, and esterification reactions.

Types of Alcohol Oxidation

Alcohol Oxidation is a process in which alcohols are converted to carbonyl-containing compounds such as aldehydes, ketones, and carboxylic acids. The oxidation of alcohols is classified based on the type of alcohol and the product formed:

• Primary alcohols: It can be oxidized to aldehydes or carboxylic acids, depending on the reaction conditions. If oxidation proceeds to the carboxylic acid, the primary alcohol is first oxidized to an aldehyde, which is then oxidized further to the acid.

RCH₂OH + [O] → RCOOH + H₂O

• Secondary alcohols: They are oxidized to ketones.

R₂CHOH + [O] → R₂C=O + H₂O

• Tertiary alcohols: It doesn't undergo oxidation, as they do not have a hydrogen atom attached to the carbon that would be oxidized.

Mechanism of Alcohol Oxidation

The oxidation of alcohols to carbonyl compounds (aldehydes, ketones, and carboxylic acids) involves a general mechanism that can be summarized as follows:

• Formation of a good leaving group on the oxygen atom, typically a chromate ion (CrO₄²⁻) or a sulfonate ion (SO₃⁻).
• Deprotonation of an adjacent carbon-hydrogen bond, forming a carbon-oxygen double bond (C=O) and breaking the C-H bond.
• Reduction of the oxidizing agent, such as chromium(VI) to chromium(IV) or dichromate(VI) to dichromate(IV).

For example, the following steps represent the oxidation of a primary alcohol to an aldehyde:

Formation of a chromate ester:

R-OH + CrO₃ → R-O-Cr(VI)O₃ + H⁺

Deprotonation and formation of the C=O bond:

R-O-Cr(VI)O₃ + H₂O → R-OH₂-Cr(VI)O₃ → R-C(=O)-H + Cr(IV)O₃ + H⁺

Reduction of chromium(VI) to chromium(IV):

Cr(VI)O₃ + 6H⁺ → Cr(IV)O₃ + 3H₂O

The oxidation of a secondary alcohol to a ketone can be represented by a similar mechanism, with the loss of hydrogen from the carbon adjacent to the hydroxyl group.

Products of Alcohol Oxidation

The products of alcohol oxidation depend on the type of alcohol and the specific oxidizing agent used.

• For primary alcohols, oxidation with acidified sodium or potassium dichromate(VI) solutions typically produces aldehydes or carboxylic acids, depending on whether the reaction proceeds through partial oxidation or complete oxidation. With full oxidation, excess alcohol is required, followed by distillation of the aldehyde intermediate to prevent further oxidation to the carboxylic acid.
• Secondary alcohols are converted into ketones regardless of the oxidizing agent, provided the reaction does not continue beyond the initial oxidation step.
• Tertiary alcohols do not undergo oxidation because they lack a hydrogen atom directly bound to the carbon bearing the hydroxyl group.

Oxidation of Alcohol into Aldehydes and Ketone

Aldehydes and ketones can be formed by oxidizing primary and secondary alcohols, respectively, using an oxidizing agent such as sodium or potassium dichromate(VI) acidified with dilute sulfuric acid. Primary alcohols can also be oxidized to aldehydes using reagents like Dess-Martin periodinane, while secondary alcohols can be oxidized to ketones using chromic acid (Jones reagent) or other reagents like pyridinium chlorochromate (PCC) or pyridinium dichromate (PDC). Tertiary alcohols cannot be oxidized.

Making of Aldehyde from Alcohol

Formation of primary alcohols in aldehydes typically involves using strong oxidizing agents such as chromium(VI) salts, like potassium dichromate (K₂CrO₄) in an acidic medium.

Mechanism of Making of Aldehyde from Alcohol

The mechanism can be presented as follows:

• Protonation of the alcohol's hydroxyl group by the acidic medium makes the hydroxyl group more susceptible to oxidation.
• Electrophilic attack by the positively charged chromium(VI) species leads to breaking the C-OH bond and forming a chromium(V) intermediate bound to the alcohol carbon.
• Deprotonation of the adjacent carbon by a base, such as a conjugate base derived from the acid (e.g., HSO₄⁻ from sulfuric acid).
• Eliminating the chromium(V) species as a reducing agent and regenerating the chromium(VI) species.
• Formation of the C=O bond between the previously attacked carbon and the newly generated oxygen atom.

The overall reaction for making of aldehyde from alcohol can be summarized as:

ROH + [Cr(VI)]²⁺ → RCHO+ [Cr(V)]³⁺

Making of Ketone from Alcohol

Ketones can be synthesized from alcohols by oxidizing secondary alcohols, which do not convert directly into aldehydes due to steric hindrance around the secondary carbon. One standard method uses oxidizing agents like potassium permanganate (KMnO₄) or sodium dichromate (Na₂Cr₂O₇) in combination with an acid catalyst, such as sulfuric acid (H₂SO₄).

Mechanism of making Ketone

The general mechanism for the oxidation of secondary alcohols to ketones involves the following steps:

• Protonation of the secondary alcohol's hydroxyl group by the acid catalyst.
• Attack by the oxidizing agent, resulting in the formation of a cationic intermediate.
• Dehydration of the intermediate, yielding a carbocation.
• Nucleophilic attack by a water molecule, generating a hydroxyl group.
• Loss of a proton, producing the ketone.

Overall reaction:

R−CH−(OH)−R {Acid} ⟶ R−CH₂⁺− OH −R → R−CH=CR₂′+ H₂O → R−C(=O)−CR₂′

Where R and R' are alkyl groups or aromatic rings.

Making Carboxylic Acid from Alcohol

Carboxylic acids can be synthesized from primary alcohols and aldehydes through oxidation processes. A general reaction for the oxidation of primary alcohols to carboxylic acids using potassium dichromate(VI) in the presence of dilute sulfuric acid is given below:

RCH₂OH + 2[O] → RCOOH + H₂O

Where R represents an organic group, and [O] symbolizes an oxidizing species, such as the dichromate(VI) ion.

Mechanism of Making Carboxylic Acid

The mechanism of converting primary alcohols to carboxylic acids through oxidation with potassium dichromate(VI) in the presence of dilute sulfuric acid can be followed by :

• Step 1: Initial protonation of the alcohol's hydroxyl group by the sulfuric acid, generating an oxonium ion.
• Step 2: Attack of the oxonium ion by the dichromate(VI) ion, resulting in the cleavage of the Cr—O bond and the formation of a chromate ester.
• Step 3: Dehydration of the chromate ester leads to forming an aldehyde intermediate.
• Step 4: Oxidation of the aldehyde to a carboxylic acid by another dichromate(VI) ion.
• Step 5: Regeneration of the dichromate(VI) ion, completing the cycle.

This sequence of events is consistent with the overall redox behavior observed during the reaction, where potassium dichromate(VI) turns from orange to green due to the reduction of Cr(VI) to Cr(III)

Production of carboxylic acid

• Oxidation of primary alcohols and aldehydes is the best way to form carboxylic acids.
• Potassium permanganate (KMnO₄) in acidic, alkaline, or neutral medium
• Potassium dichromate (K₂Cr₂O₇) in acidic medium
• Other strong oxidizing agents like PCC or PDC

Identification of Alcohols

Various chemical tests are employed to identify alcohols. An alcohol can be initially detected using phosphorus(V) chloride, which produces hydrogen chloride gas when reacting with an alcohol. Subsequent tests distinguish between primary, secondary, and tertiary alcohols.

• Chromic Acid Oxidation Test and Schiff's Test: Primary alcohols turn green with chromic acid and magenta with Schiff's reagent. Secondary alcohols turn green with chromic acid but do not react with Schiff's reagent. Tertiary alcohols do not change color with chromic acid and do not react with Schiff's reagent.
• Lucas Test: Tertiary alcohols react immediately with Lucas reagent (zinc chloride and hydrochloric acid) to form a cloudy solution. Primary and secondary alcohols do not react with Lucas reagent at room temperature.
• Acetyl Chloride Test: Primary alcohols, acetaldehyde, and methyl ketones form a white precipitate with acetyl chloride.
• Sodium Metal Test: Alcoholic groups produce hydrogen gas when reacting with sodium metal.
• Ceric Ammonium Nitrate Test: Alcoholic groups produce a red precipitate with ceric ammonium nitrate.
• Iodoform Test: Methanol, ethanol, and isopropanol react with iodine to form iodoform (CHI₃) when treated with potassium iodide and sodium iodide.
• Ferric Chloride Test: Aromatic alcohols change the color of ferric chloride solution from red-orange to purple.
• Jones Test: Primary alcohols are oxidized to carboxylic acids, while secondary alcohols are oxidized to ketones.
• Lucas Test: The reaction rate of alcohols with Lucas reagent is dependent on the stability of the carbocation formed, which helps distinguish between primary, secondary, and tertiary alcohols

Uses of Alcohol Oxidation

Alcohol oxidation has numerous applications in organic chemistry, including:

• Resolution of racemic mixtures: Oxidation of a chiral alcohol can lead to the formation of a chiral ketone or carboxylic acid, which can be separated from its enantiomer.
• Biological processes: Oxidation of alcohols is a fundamental process in biological systems, such as the metabolism of alcohols in living organisms.
• Analytical chemistry: Oxidation reactions can detect the presence of alcohols in a sample, as the orange dichromate ion is reduced to the green Cr(III) ion during oxidation.