Aldehydes, Ketones and Carboxylic Acids

Methods of Preparation of Aldehydes and Ketones

Aldehydes and ketones can be prepared using various methods. Aldehydes are commonly synthesized through the oxidation of primary alcohols, controlled oxidation of alkenes, or by using reagents like PCC. Ketones, on the other hand, can be prepared by oxidizing secondary alcohols, through the hydration of alkynes, or via Friedel-Crafts acylation. Methods of preparation of aldehydes and ketones highlight the versatility of organic reactions in synthesizing these compounds.

Reactions of Aldehydes and Ketones

Aldehydes and ketones exhibit a wide range of chemical reactions due to the presence of the carbonyl group (C=O). They undergo nucleophilic addition reactions such as the formation of cyanohydrins and hemiacetals. Aldehydes, being more reactive, also participate in oxidation reactions to form carboxylic acids and can reduce to primary alcohols. Ketones undergo similar reduction reactions to yield secondary alcohols. Special reactions like aldol condensation and Cannizzaro reaction further illustrate their reactivity.

Carboxylic Acids

Carboxylic acids are organic compounds containing the carboxyl group (−COOH). They are generally prepared through the oxidation of primary alcohols or aldehydes, and hydrolysis of nitriles or esters. Carboxylic acids exhibit unique properties such as hydrogen bonding, making them highly polar with relatively high boiling points. They participate in reactions like esterification, decarboxylation, and reduction to alcohols, showcasing their versatility in organic synthesis.

Structure of the Carbonyl Group

Aldehydes (−CHO) and ketones (C=O) contain the carbonyl functional group. In aldehydes, the carbonyl group is bonded to at least one hydrogen atom, while in ketones, it is bonded to two alkyl or aryl groups. The carbonyl carbon is sp² hybridized having a bond angle of approximately 120^o.

Preparation Methods

Aldehydes: There are various methods of preparation of aldehydes and most prominent among themselves are oxidation of primary alcohols, controlled oxidation of alkenes, or Rosenmund reduction of acyl chlorides.

Ketones: Ketones are prepared by oxidation of secondary alcohols, hydration of alkynes, or Friedel-Crafts acylation of aromatic compounds.

Carboxylic Acids: Carboxylic acids are formed by the oxidation of aldehydes or primary alcohols, hydrolysis of nitriles, or carbonation of Grignard reagents.

Reactivity of the Carbonyl Group

The polarity of the C=O bond makes aldehydes and ketones reactive towards nucleophiles. Aldehydes are more reactive than ketones due to less steric hindrance and greater electrophilicity.

Reactions of carbonyl groups include nucleophilic addition (e.g., addition of HCN to form cyanohydrins) and condensation reactions (e.g., aldol condensation).

Acidity of Carboxylic Acids

Carboxylic acids are acidic due to the resonance stabilization of their conjugate base (carboxylate ion). The electron-withdrawing effect of substituents increases acidity, while electron-donating groups decrease it.

Named Reactions

These are some of the most important named reactions form the topic Aldehyde, Ketone and Carboxylic acids. These reactions must be understood well with all the steps in their mechanisms. Named reactions are important for various exams like JEE Mains and NEET as direct questions are asked from this topic.

Rosenmund Reduction: Acyl chlorides to aldehydes using H₂ and Pd/BaSO₄.

Clemmensen Reduction: Reduction of carbonyl compounds to hydrocarbons using Zn/HCl.

Aldol Reaction: Formation of β-hydroxy aldehydes or ketones from enolate ions and carbonyl compounds.

Cannizzaro Reaction: Aldehydes without alpha hydrogens undergo disproportionation in the presence of a base.

Tests for Functional Groups

Following are the tests for various functional groups. These tests have various applications in qualitative analysis and chemical industries.

Aldehydes: Tollens' test (silver mirror), Fehling's test, and Schiff’s test.

Ketones: Iodoform test for methyl ketones.

Carboxylic Acids: Reaction with sodium bicarbonate to release CO₂.

Mechanisms of Reactions

Nucleophilic addition is the primary reaction mechanism for aldehydes and ketones and it is due to polarity of the carbonyl group (C=O). The oxygen atom in the carbonyl group is more electronegative than carbon which makes the carbonyl carbon electron-deficient and highly susceptible to nucleophilic attack.

Steps in the Nucleophilic Addition Mechanism:

1. Nucleophilic Attack: A nucleophile (Nu^-) attacks the electrophilic carbonyl carbon, breaking the π-bond between carbon and oxygen. This results in the formation of a tetrahedral alkoxide intermediate.
2. Protonation: The negatively charged oxygen atom of the intermediate is then protonated by a suitable proton donor (e.g., water, acid) to form the final addition product.

Example Reactions:

• Addition of Hydrogen Cyanide (HCN): The carbonyl group reacts with HCN which forms cyanohydrin. This reaction is significant in the synthesis of α-hydroxy acids.
• Addition of Alcohols: Reaction with alcohols produces hemiacetals and acetals in the presence of an acid catalyst.

2. Acid-Base Mechanism

Carboxylic acids predominantly undergo acid-base reactions due to the acidic nature of the −COOH group. The acidity arises from the resonance stabilization of the carboxylate ion formed after deprotonation.

Steps in Acid-Base Mechanism:

1. Deprotonation: In the presence of a base, the hydroxyl proton of the carboxyl group is removed, forming a carboxylate ion (RCOO⁻). The carboxylate ion is stabilized by resonance, where the negative charge is delocalized between the two oxygen atoms.
2. Protonation: In acidic conditions, the carboxylate ion can accept a proton to revert back to the carboxylic acid.

Example Reactions:

• Reaction with sodium bicarbonate (NaHCO₃): Carboxylic acids react with NaHCO₃ to form salt, water, and carbon dioxide (CO₂). This is often used as a qualitative test for carboxylic acids.
• Formation of esters (Esterification): Carboxylic acids react with alcohols in the presence of an acid catalyst to form esters and water.