Oxidation of Aldehydes and Ketones

A carbonyl group (-C=O) is found at the end of a carbon chain, which distinguishes aldehydes from other organic molecules. They are often present in nature and have significant uses across several sectors. In this assignment, the structure, characteristics, and reactivity of aldehydes will be covered.

Aldehyde

RCHO is the general formula for aldehydes, where R stands for an alkyl group or a hydrogen atom. The carbonyl group is joined to either an alkyl group or a hydrogen atom at the end of the carbon chain. The carbon atom in the carbonyl group has a trigonal planar shape and is sp² hybridized.

Structure of Aldehyde

The structural formula of the Aldehyde is shown in the image below,

Aldehyde Properties

Aldehydes have unique chemical and physical characteristics. They are polar solvents like water-soluble, have a distinctively pungent smell, and have relatively low boiling temperatures in comparison to other organic compounds. Moreover, aldehydes are capable of a wide range of chemical processes, including nucleophilic addition, oxidation, and reduction.

Reactions of Aldehydes

Many chemical processes, such as nucleophilic addition, oxidation, and reduction, are possible with aldehydes.

Nucleophilic Addition

Aldehydes can engage in nucleophilic addition processes and are very reactive to nucleophiles. A nucleophile attacks the carbonyl group, which causes the creation of a new carbon-carbon bond. For instance, formaldehyde and ammonia can combine to create a primary amine.

Oxidation

Aldehydes can be oxidised to produce either pure carboxylic acids or a combination of alcohols and carboxylic acids. Aldehydes are often oxidised using the reagents potassium permanganate (KMnO₄), chromic acid (H₂CrO₄), and silver nitrate (AgNO₃).

Reduction

With reducing agents such as sodium borohydride (NaBH₄) or lithium aluminium hydride, aldehydes can be converted to primary alcohols (LiAlH₄). A new carbon-hydrogen bond is created when the carbonyl group is reduced to a hydroxyl group (-OH).

Applications of Aldehydes

Aldehydes have important applications in various industries such as,

• Production of plastics, dyes, and perfumes. 
• Formaldehyde, the simplest aldehyde, is widely used as a disinfectant and preservative.

Ketones

A carbonyl group (-C=O) is found in the centre of a carbon chain, which distinguishes ketones from other organic molecules. Due to the carbonyl group’s attachment to two additional carbon atoms, ketones have unique chemical and physical characteristics.

Ketones are soluble in polar solvents like water and have relatively high boiling temperatures. Many chemical processes, including nucleophilic addition, oxidation, and reduction, are possible for them. Ketones are often present in nature and play a significant role in a number of fields, including the manufacture of medicines, polymers, and solvents.

Structure of Ketones

The structural formula of the Ketones is shown in the image below,

Types of Ketones

Ketones are of various types some of the important Ketones are,

• Propanone: CH₃COCH₃

• Cyclohexanone: C₆H₁₀O

• Butanone: CH₃COCH₂CH₃

Carboxylic Acids

A carboxyl group (-COOH) is joined to a carbon atom in a family of chemical molecules known as carboxylic acids. The carbonyl group (-C=O) and the hydroxyl group (-OH) are both connected to the same carbon atom in the carboxyl group, which is what gives carboxylic acids their distinguishing characteristics.

Comparatively to other organic compounds, carboxylic acids have high melting and boiling temperatures, and they often dissolve in polar solvents like water. Many chemical processes, including esterification, amidation, and reduction, are among those they are capable of. Carboxylic acids are frequently present in nature and play a significant role in a number of fields, including the manufacture of medicines, detergents, and soaps.

Due to the great variety of reactions and uses they have, carboxylic acids are significant in organic chemistry and are the subject of much research. In biochemistry, where they are engaged in the metabolism of lipids and carbohydrates, they also play a significant function.

Structure of Carboxylic Acid

The structural formula of the Carboxylic Acids is shown in the image below,

Oxidation Reaction

A chemical reaction called oxidation occurs when an atom, molecule, or ion loses electrons or experiences an increase in oxidation status. This procedure can be carried out either by removing hydrogen atoms from a molecule or by reacting with an oxidising substance like oxygen or hydrogen peroxide.

New compounds can be created during oxidation processes, or existing compounds can undergo modification. For instance, secondary alcohols can be oxidised to form carboxylic acids whereas primary alcohols can be oxidised to form aldehydes or ketones. The generation of energy and metabolism depend heavily on oxidation processes in biological systems.

Since it enables the creation of new compounds and the change of old compounds into various forms, oxidation is a crucial chemical process. It is also a significant industrial process since it is used to create a wide range of goods, such as chemicals, fuels, and medicines.

Oxidation of Aldehyde

Aldehydes can be oxidised to produce either pure carboxylic acids or a combination of alcohols and carboxylic acids. Aldehydes are often oxidised using the reagents potassium permanganate (KMnO₄), chromic acid (H₂CrO₄), and silver nitrate (AgNO₃). These substances will react with the aldehyde group and produce carboxylic acid under the proper circumstances. Propanoic acid, for instance, is produced when potassium permanganate and propanal are combined.

Mechanism for Oxidation of Aaldehyde Using K₂Cr₂O₇

In the presence of an acidic solution, the aldehyde is protonated to form a more electrophilic carbonyl group.

RCHO + H⁺ → RCH(OH)₂⁺

Chromate Ester is formed by the attack of the carbonyl oxygen on the chromium in the K₂Cr₂O₇

RCH(OH)₂+ + Cr₂O₇^2- → RCH(OH)(OCrO)^2-

Chromate Ester rearranges to form a more stable intermediate.

RCH(OH)(OCrO)^2- → RCO(OH)(OCrO)^2-

Water is eliminated from the intermediate to form a carboxylic acid.

RCO(OH)(OCrO)^2- → RCO(O)CrO₃ + H₂O

Chromium in the product is reduced from the +6 oxidation state to the +3 oxidation state, forming Cr(OH)₃

2CrO₃ + 3H₂O → Cr(OH)₃ + 3H₂CrO₄

Overall Reaction

RCHO + [O] → RCOOH

Oxidation of Ketone 

As ketones are oxidised, carboxylic acids can be produced on their own or in combination with other compounds. Nitric acid and potassium permanganate (KMnO₄) are the two typically utilised chemicals for oxidising ketones (HNO₃). These substances will react with the ketone group to produce carboxylic acid under the proper circumstances. Acetic acid, for instance, is produced when potassium permanganate and acetone are combined.

Mechanism for the oxidation of a Ketone using KMnO₄

In the presence of an acidic solution, the ketone is protonated to form a more electrophilic carbonyl group.

R₂C=O + H⁺ → R₂C=OH₂⁺

Carbonyl Oxygen of the protonated ketone attacks a hydrogen atom on the adjacent carbon to form a gem-diol intermediate.

R₂C=OH₂⁺ + H-CR₂ → R₂C(OH)(CR₂)(OH₂⁺)

The potassium permanganate oxidizes the gem-diol intermediate to form a diketone intermediate.

R₂C(OH)(CR₂)(OH₂⁺) + 2KMnO₄ → R₂C=O-CR₂=O + 2MnO₂ + 2KOH + 2H₂O

Water is eliminated from the diketone intermediate to form the final product.

R₂C=O-CR₂=O → R₂C=O + CR₂=O

Overall Reaction

R₂C=O + [O] → R₂C=O

Oxidation of Carboxylic Acid

Carbon dioxide and water can be produced when carboxylic acids are oxidised, as well as a combination of carbon dioxide and other organic molecules. Potassium permanganate is a reagent frequently utilised for the oxidation of carboxylic acids (KMnO₄). This reagent will react with the carboxylic acid group under the correct circumstances to produce carbon dioxide. As an illustration, the oxidation of ethanol with potassium permanganate results in the production of carbon dioxide and water.

Mechanism for the oxidation of carboxylic acid using acidic Potassium Permanganate (KMnO₄)

In the presence of an acidic solution, the carboxylic acid is protonated to form a more electrophilic carbonyl group.

RCOOH + H⁺ → RCOOH₂⁺

The carbonyl oxygen of the protonated carboxylic acid attacks a manganese atom in the KMnO4, forming a tetrahedral intermediate.

RCOOH₂⁺ + MnO₄^– → RCOOMnO₃(OH) + H₂O

The tetrahedral intermediate rearranges to form a more stable intermediate.

RCOOMnO₃(OH) → RCOO-MnO₂(OH)₂^–

Water is eliminated from the intermediate to form a carboxylate ion and manganese dioxide.

RCOO-MnO₂(OH)₂^– → RCOO^– + MnO₂ + H₂O

The manganese dioxide is reduced to form manganese ions in a basic solution.

MnO₂ + 4OH^– → MnO₄^2- + 2H₂O + 2e^–

Overall Reaction

RCOOH + [O] → CO₂ + H₂O

Applications of Oxidation Reaction

Aldehydes, ketones, and carboxylic acids may be oxidised, and this process has several industrial uses. For instance, the creation of carboxylic acids, which are extensively employed in the pharmaceutical and food sectors, is accomplished via the oxidation of aldehydes and ketones. Carbon dioxide, a crucial gas utilised in several industrial processes, is created by the oxidation of carboxylic acids.

Tollen’s Reagent

Together with various alpha-hydroxy ketones that can tautomerize into aldehydes, Tollens’ reagent is a chemical reagent used to differentiate between aldehydes and ketones. The reagent is made up of silver nitrate, ammonia, and sodium hydroxide solution.

 Tollens test uses a chemical reagent called Tollens reagent, which has a mild oxidising effect. Silver ions are coupled to ammonia in the form of the diamine-silver (I) complex [Ag(NH₃)₂]⁺ in this colourless, basic, and aqueous solution. A two-step process is used to make Tollens’ reagent. Two things happen during the process. Initially, carbon dioxide is created from the aldehyde. Second, the Ag+ ions are converted to Ag metal, leaving a shiny look inside the test tube to signify a successful reaction.

Tollen’s Test

A qualitative laboratory test called the Tollens’ test also referred to as the silver-mirror test, is used to distinguish between an aldehyde and a ketone. It makes use of the fact that aldehydes can oxidise more quickly than ketones. Aldehydes and -hydroxy ketones, such as hydroxy acetone, cannot be distinguished by this test.

R-CHO +   2[Ag(NH₃)₂]⁺OH^–  +  H₂O →  R-COOH + 2Ag   +   4NH₃   +  2H₂O

Fehling’s Test

• One of the most popular tests for determining whether a substance is a reducing or non-reducing sugar is the Fehling test.
• To identify the different types of carbohydrates present in a solution or to determine their presence.

Fehling’s solution: The Fehling Solution is created by combining two different solutions. The first is Rochelle salt produced strongly with sodium hydroxide, which is a colourless solution and is known as Fehling A solution. The second is copper sulphate, which is a deep blue aqueous solution and is known as Fehling B solution. The Fehling Solution is created by combining the Rochelle salt and copper sulphate from the A and B solutions. Also, each of the solutions A and B is prepared independently and stored during the evaluation. The active chemical in this reaction is the tartrate complex, which acts as an oxidising agent.

Reactions

Fehling’s solution, the reaction between copper(II) ions and an aldehyde is stated as,

RCHO+2Cu²⁺+5OH⁻→RCOO⁻+Cu₂O+3H₂O

After Tartrate is introduced,

RCHO+2Cu(C₄H₄O₆)₂⁻² + 5OH⁻→ RCOO⁻+ Cu₂O + 4C₄H₄O₂⁻⁶ + 3H₂O

Haloform Reaction

Carboxylates and trihalomethane, often known as haloforms, are the end products. Up until the 3 H has been substituted, the reaction continues at the alpha position through a series of quicker halogenations. As a chemical test, this reaction is also carried out to identify methyl ketones using iodine.

In the process of halogenating a methyl ketone in a basic solution, the halogen takes the place of all three -hydrogen atoms. When this trihaloketone reacts further, a carbon-carbon bond is broken. A carboxylic acid and a trihalomethane compound known as haloform are the byproducts of acidification.

3 R-C(CH₃)₂(C=O)R’ + X₂ + 4NaOH → R-CX₃ + R’COO-Na+ + 2 CH₃COONa + 3H₂O

The reaction of ethyl methyl ketone with chlorine and sodium hydroxide yields chloroform

3CH₃COC₂H₅ + 3Cl₂ + 4NaOH → CHCl₃ + C₂H₅COONa + 5NaCl + 3H₂O

Baeyer-Villiger Oxidation

Using the Baeyer-Villiger The process of oxidation, which changes ketones into esters and cyclic ketones into lactones, involves the oxidative cleavage of a carbon-carbon bond next to a carbonyl. You can perform the Baeyer-Villiger with peracids like MCBPA or with hydrogen peroxide and a Lewis acid.

Ketone + Peroxy Acid  = Ester

R₂C=O + RCO₃H → R₂C(OH)OR’ (ester) + HO₂CR’

Cycloketone + Peroxy Acid  =  Lactone

R₂C=O + RCO₃H → R₂C(OH)OR’ (lactone) + HO₂CR’

FAQs on Aldehydes and Ketones

Q1: What are the chemical structures of Aldehydes, Ketones and Carboxylic Acids?

Answer: 

The chemical structures of Aldehydes, Ketones and Carboxylic Acids is,

• Aldehydes -> -CHO
• Ketones -> -C=O
• Carboxylic Acids -> -COOH

Q2: What are Examples of Aldehydes?

Answer:

Examples of Aldehydes

• Formaldehyde (Methanal)
• Acetaldehyde (Ethanal)
• Propionaldehyde (Propanal)
• Butyraldehyde (Butanal)

Q3: What is the method through which ketones are oxidised?

Answer: 

Ketones are typically resistant to oxidation, but under some circumstances, they can go through an oxidative cleavage that results in carboxylic acids or other molecules.

Q4: What are the three types of oxidation?

Answer:

Oxidation can be categorized into three types:

• Spontaneous Oxidation
• Rapid Oxidation
• Slow Oxidation