Unit 4: Carbonyl Compounds (Aldehydes and Ketones)

Semester 2

BP202T

Introduction to Carbonyl Compounds (Aldehydes and Ketones)

Unit 4 focuses on the most reactive of all organic functional groups – the Carbonyl group (C=O). The key reactions are all about nucleophiles attacking the electron-deficient carbon. You will master important named reactions (Aldol, Cannizzaro, Perkin, Benzoin) and qualitative tests, along with the pharmaceutical uses of compounds like Formaldehyde, Acetone, and Benzaldehyde.

Syllabus & Topics

1Nucleophilic Addition to Carbonyl compounds: Mechanism and Electromeric effect.
2Aldol Condensation: Mechanism and products.
3Crossed Aldol Condensation.
4Cannizzaro Reaction: Self-oxidation-reduction of aldehydes without α-H.
5Crossed Cannizzaro Reaction.
6Benzoin Condensation: Cyanide ion catalyzed.
7Perkin Condensation: Formation of α,β-unsaturated acids.
8Qualitative Tests: Tollens’ reagent (Silver mirror test) and Fehling’s solution.
9Structure and uses of Formaldehyde (Formalin) – antiseptic, preservative.
10Structure and uses of Paraldehyde – sedative.
11Structure and uses of Acetone – solvent.
12Structure and uses of Chloral hydrate – sedative/hypnotic.
13Structure and uses of Hexamine (Methenamine) – urinary antiseptic.
14Structure and uses of Benzaldehyde, Vanillin, and Cinnamaldehyde (flavoring agents).

Learning Objectives

Write the Mechanism: Draw the complete mechanism of nucleophilic addition of HCN to acetaldehyde.

Distinguish Aldol & Cannizzaro: Explain why Cannizzaro reaction occurs with HCHO but not CH3CHO.

Apply Qualitative Tests: Explain how Tollens’ test distinguishes aldehydes from ketones.

Explain Perkin Condensation: Write the products of Perkin condensation between benzaldehyde and acetic anhydride.

Know Pharmaceutical Uses: State the use of Hexamine, Paraldehyde, and Chloral hydrate.

Frequently Asked Questions (FAQs)

Q1. What is Nucleophilic Addition?

Nucleophilic addition is the characteristic reaction of carbonyl compounds. A nucleophile (electron-rich species) attacks the electrophilic carbonyl carbon, breaking the π bond and adding across the C=O bond.

Q2. Why does Cannizzaro reaction occur only with no α-H aldehydes?

Aldehydes containing α-hydrogen atoms undergo Aldol condensation preferentially. In the absence of α-hydrogen (e.g., formaldehyde, benzaldehyde), the aldehyde undergoes Cannizzaro reaction, where one molecule is oxidized to a carboxylic acid and another is reduced to an alcohol (disproportionation reaction).

Q3. How does Tollens’ Test work?

Tollens’ reagent contains [Ag(NH₃)₂]⁺ ions. Aldehydes reduce Ag⁺ to metallic silver, which deposits as a silver mirror on the test tube. Ketones generally do not react.

Q4. What is the use of Hexamine in pharmacy?

Hexamine (Methenamine) is used as a urinary antiseptic. In acidic urine, it decomposes to release formaldehyde, which acts as the actual antibacterial agent.

Q5. What is Perkin Condensation?

Perkin condensation is a base-catalyzed reaction between an aromatic aldehyde and an acid anhydride (in the presence of its sodium salt as base) to form an α,β-unsaturated carboxylic acid.