Further Organic Chemistry II - Aldehyde, Ketone and Carboxylic Acid

Learning Objectives

  • Name aldehydes, ketones, and carboxylic acids using the IUPAC system.

  • Draw structures for given IUPAC names of aldehydes, ketones, and carboxylic acids.

  • Propose synthesis methods for aldehydes, ketones, and carboxylic acids.

  • Predict the products of reactions involving aldehydes, ketones, and carboxylic acids.

Carbonyl Compounds

Aldehydes

  • General Formula: R-CHO (where R is an alkyl group)

  • IUPAC Naming: Replace -e with -al.

  • The carbon in the -CHO group is numbered as 1 in the chain.

  • If part of a ring: use -carbaldehyde as suffix.

Examples of Aldehydes

  • Methanal (formaldehyde) - Structure: HCHO

  • Ethanal (acetaldehyde) - Structure: CH₃CHO

  • Pentanal - Structure: CH₃(CH₂)₃CHO

  • 2-Chloropentanal - Structure: ClCH₂(CH₂)₂CHO

Naming Ketones

  • General Formula: R-CO-R’ (where R and R’ are alkyl groups)

  • IUPAC Naming: Replace -e with -one and indicate the position of the carbonyl.

  • The carbonyl carbon gets the lowest number.

  • For cyclic ketones: the number 1 is assigned to the carbonyl carbon.

Examples of Ketones

  • 3-Methyl-2-butanone - Structure: CH₃C(=O)CH(CH₃)CH₃

  • 3-Bromocyclohexanone - Structure: BrC₆H₁₁C=O

Physical Properties

Boiling Points

  • Aldehydes and ketones have higher boiling points than alkanes due to greater polarity.

  • However, they cannot form hydrogen bonds with each other, resulting in lower boiling points than comparable alcohols.

Solubility

  • Aldehydes and ketones are good solvents for alcohols.

  • The carbonyl's lone pair can accept hydrogen bonds from alcohols, aiding solubility.

  • Examples: Acetone and acetaldehyde are miscible with water.

Synthesis of Aldehydes and Ketones

  • Aldehydes: Synthesized from primary alcohols using milder oxidizing agents like pyridinium chlorochromate (PCC).

  • Ketones: Synthesized from secondary alcohols using strong oxidizing agents like CrO₃ or Na₂Cr₂O₇.

Chemical Reactions & Mechanisms

Nucleophilic Addition

  • A common reaction type; involves the addition of nucleophiles (e.g., water, alcohols) to the carbonyl carbon.

  • Types of products include: diols (hydration), cyanohydrins (with HCN), and imines (with primary amines).

Alcohol Reactions

  • Formation of Hemiacetals (one -OR group) and Acetals (two -OR groups) from aldehydes and ketones.

Oxidation of Carbonyl Compounds

  • Aldehydes can be oxidized to carboxylic acids; ketones are resistant to oxidation.

  • Common oxidizing agents include:

    • Acidified Dichromate: Orange to green

    • Tollens’ Reagent: Forms a silver mirror

    • Fehling's Solution: Blue to red precipitate

Carboxylic Acids

Introduction

  • Carboxylic reaction resulting from the addition of an -OH group to a carbonyl, forming a -COOH group.

  • Aliphatic acids are those with an alkyl group attached to -COOH.

Nomenclature

  • IUPAC Naming: Remove -e from alkane name, and add -oic acid.

  • Aromatic acids: Named as benzoic acids.

Chemical Features

  • The carboxyl carbon is sp² hybridized with bond angles close to 120°.

  • Higher boiling points due to dimer formation and strong intermolecular hydrogen bonding.

Synthesis of Carboxylic Acids

  • Oxidation of primary alcohols and aldehydes to form acids.

  • Alkenes can be oxidized to acids using KMnO₄ or K₂Cr₂O₇.

Key Reactions

  • Reactions with Bases: Form salts (e.g., RCOO⁻Na⁺).

  • Ester Formation: Reaction with alcohols.

  • Decarboxylation: Removal of -COOH and replacement with hydrogen.

  • Reduction: Strong reducing agents (e.g., LiAlH₄) reduce carboxylic acids to primary alcohols.