Aldehyde and Ketones

In organic chemistry, aldehydes and ketones have carbonyl functional groups (C=O) with structures -CHO for aldehydes and R(C=O)R’ for ketones. A substitute for carbon is R and R’ and They are also referred to as methyl groups or formal groups. In this group, alkyl or aryl groups or they have  their substituents replace the remaining carbon bonds.

Normally, a ketone is an organic compound if its substituents in the two remaining bonds are not hydrogens. A compound that contains hydrogen as a substituent, however, is an aldehyde. Aldehyde and ketones are used for many different reasons depending on their properties.

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Aldehydes and ketone in nature are widely distributed with other functional groups. There are several compounds that are mainly found in microorganisms and plants, such as cinnamaldehyde (cinnamon bark), carvone (spearmint), citral (lemongrass), vanillin (vanilla bean), helminthosporal (a fungus toxin), and camphor (camphor trees). While compounds such as muscone (musk deer), progesterone (female sex hormone), cortisone (adrenal hormone), testosterone (male sex hormone) are found in both animal and human sources.

Method of Preparation of Aldehydes and Ketones

1. Formation by Oxidation of Alcohols

A primary or secondary alcohol is oxidized to form aldehydes and ketones. In addition to KMnO4, the most common oxidizing agents are K2Cr2O7, and CrO3. The primary alcohol is oxidized into aldehyde and then into carboxylic acid by strong oxidizing agents.

2. Formation by Dehydrogenation of Alcohols

When volatile alcohols are converted into aldehydes, this preparation method is used. Aldehyde and ketones formed by this method have many common Industrial applications. In this process, alcohol vapors pass through catalysts such as Cu or Ag. Secondary alcohols produce ketones, while primary alcohols produce aldehydes.

3. Ozonolysis of Alkenes

The process of ozonolysis results in the formation of ozonide by the addition of ozone molecules or O3 to an alkene compound. In this case, zinc dust and water are used to reduce the ozonide compound, which produces smaller molecules, such as aldehydes and ketones.Based on the substitution arrangement of the alkene compounds, the reaction produces aldehydes, ketones and sometimes both Aldehyde and ketones.

4.Hydration of Alkynes

Alkynes produce ketones according to Markovnikov’s rule, provided that they are accompanied by a proper catalyst. Water reacts with alkynes to form ketones when combined with HgSO4 and H2SO4. In contrast, ethyne and water react together in the presence of the catalyst (HgSO4 and H2SO4) to produce acetaldehyde. Alkyne produces acetaldehyde only in this one case. All the remaining alkyne is converted to ketones during hydration.

Uses of Aldehydes and ketones

Uses of Aldehydes

• A 40% solution of formaldehyde (gaseous form) in water produces formalin. The substance is helpful for preserving biological specimens.

• Bakelite is formed when formaldehyde reacts with phenol. Bakelite is a material used in coatings, adhesives, and plastics.

• Among the many industrial processes that formaldehyde performs are embalming, glue formulation, tanning, and the production of polymeric products.

• Aldehydes acts as Germicidal, insecticidal, and fungicidal.

• Drugs can be tested with formaldehyde. Additionally, it is used in photography.

• Acetaldehyde can be used to make acetic acid and derivatives of pyridine.

• Perfumes, cosmetics, dyes, and cosmetic products are made with benzaldehyde (aldehyde). Food products incorporate almond flavor by adding this compound. In addition, this compound repels bees.

Uses of Ketones

• Plastics and synthetic fibers respond well to ketone as a solvent.

• Acetone found its application as  nail paint remover and paint thinner.

• Medically a chemical peeling procedure and an acne treatment can also be conducted with it.

• Textiles, varnishes, paint removers, paraffin wax, plastics, and other products all use butanone as a solvent.

• Cyclohexanone, another important ketone, is essential for the production of nylon.

Reaction of Aldehyde and Ketone

1. Nucleophilic addition reactions –

(i) Mechanism of nucleophilic addition reactions- 

In order for nucleophiles to attack the electrophilic carbon atom of the polar carbonyl group, they must attack it perpendicularly to the plane of the sp2 hybridised orbitals on carbonyl carbon. This reaction produces a tetrahedral alkoxide intermediate by changing the carbon hybridisation from sp2 to sp3. To produce the electrically neutral product, the intermediate captures a proton from the reaction medium. Following this, there will be an addition of H+ and Nu- across the carbon oxygen double bond, as shown below

2. Reduction

 (i) Reduction to alcohols: 

Sodium borohydride (NaBH4) and lithium aluminum hydride (LiAlH4) are used to reduce aldehydes and ketones to primary and secondary alcohols, respectively, as well as by catalytic hydrogenation. 

(ii) Reduction to hydrocarbons: 

Aldehydes and ketones, whose carbonyl groups are reduced to CH2 by zinc amalgam or concentrated hydrochloric acid treatment. This process is followed by heating it with sodium or potassium hydroxide in high boiling solvents such as ethylene glycol (Wolff-Kishner reduction). 

3. Oxidation 

In their oxidation reactions, aldehydes are different from ketones. Common oxidizing agents such as nitric acid, potassium permanganate, potassium dichromate, etc. expedite the oxidation of aldehydes into carboxylic acids. Tollens’ reagent and Fehlings’ reagent are mild oxidizing agents that also oxidize aldehydes. Generally, ketone oxidation occurs under strict conditions, which means strong oxidising agents and high temperatures. Oxidation of their carbon-carbon bonds results in the formation of carboxylic acids that have a lower carbon atom count than the parent ketone. 

4. Reactions due to a-hydrogen Acidity of α-hydrogens of aldehydes and ketones: 

The acidic nature of 𝞪-hydrogen causes the aldehydes and ketones to undergo a variety of reactions. It is the electron withdrawing effect of the carbonyl group and the resonance stabilization of the conjugate base that determines the acidity of 𝞪-hydrogen atoms in carbonyl compounds. 

(i) Aldol condensation: 

When dilute alkali is added to aldehydes and ketones containing at least one 𝞪-hydrogen, the aldehydes and ketones are converted into 𝛃-hydroxy aldehydes (aldols) or 𝛃-hydroxy ketones (ketols). A reaction like this is known as the Aldol reaction. ‘Aldol’ refers to both functional groups found in the product, aldehyde and alcohol. When the aldol and ketol lose water, they give off 𝞪,𝛃-unsaturated carbonyl compounds that are the products of aldol condensation. Ketones, when combined with alcohols, give off ketols (compounds containing both a keto and an alcohol group). Nonetheless, aldol condensation is the general name for ketones because they are similar to aldehydes.

(ii) Cross aldol condensation: 

In cross aldol condensation, two different aldehydes and / or ketones are combined in a condensation reaction. This results in a mixture of four products if both contain 𝞪-hydrogen atoms. One of the components of cross aldol reactions can also be ketones.

In the following example, ethanal and propanal mixtures undergo aldol reactions.

Conclusion 

To summarize, aldehydes are molecules whose carbonyl groups are bonded to one or more hydrogen atoms. The carbonyl group in a ketone is linked to two carbon atoms. Nucleophilic addition reactions typically involve aldehydes more than ketones owing to steric and electronic factors. As a consequence, there is a greater difficulty in getting an nucleophile near a carbonyl carbon in ketones than in aldehydes with a single large substituent. 

Acetone is the smallest ketone while formaldehyde is the simplest aldehyde. Because of their chemical properties, aldehyde and ketones find applications. We will continue our exploration of aldehydes and ketones by looking at the chemistry of enolates, which are nucleophilic carbonyl-containing compounds.