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.