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Aldehydes and Ketones Overview

Sep 29, 2025

Overview

This lecture covers the properties, naming, reactions, and distinguishing tests for aldehydes and ketones in organic chemistry, focusing on A2-level concepts.

Naming and Structure of Aldehydes and Ketones

  • Aldehydes: Names end in "-al" (e.g., methanal, ethanal, propanal, butanal). The functional group (–CHO) is always at the end of the carbon chain.
  • Ketones: Names end in "-one" (e.g., propanone, butanone, pentan-2-one, pentan-3-one). The functional group (C=O) is within the carbon chain, bonded to two alkyl groups.
  • Structural isomers: Isomers differ by the position and branching of the carbonyl group.

Physical Properties

  • Both aldehydes and ketones contain a polar carbonyl (C=O) group, resulting in higher boiling points than alkanes (which only have Van der Waals forces).
  • They do not form intermolecular hydrogen bonds with themselves, so their boiling points are lower than those of alcohols (which do form hydrogen bonds).
  • Both are soluble in water because the carbonyl oxygen can form hydrogen bonds with water molecules.

Chemical Tests and Distinguishing Reactions

  • 2,4-Dinitrophenylhydrazine (2,4-DNPH): Reacts with both aldehydes and ketones to give an orange-yellow precipitate.
  • Tollens' reagent (ammoniacal silver nitrate): Reacts only with aldehydes, producing a silver mirror; ketones do not react.
  • Fehling's and Benedict's solutions: React with aldehydes (not ketones), turning from blue to brick red due to reduction of copper(II) to copper(I).
  • Iodoform test (alkaline iodine): Gives a yellow precipitate (CHI₃) for methyl ketones and certain alcohols, indicating the presence of the CH₃CO– group.

Preparation Methods

Oxidation of Alcohols

  • Aldehyde Formation
    • Conditions: Distillation
    • Reagent: Acidified potassium dichromate (K₂Cr₂O₇/H₂SO₄) or acidified potassium manganate
    • Reaction: Primary alcohol → Aldehyde
      (Further oxidation forms carboxylic acid if not distilled)
  • Ketone Formation
    • Conditions: Reflux
    • Reagent: Acidified potassium dichromate or manganate
    • Reaction: Secondary alcohol → Ketone

Reduction of Carboxylic Acids

  • Aldehyde Formation
    • Conditions: Anhydrous (dry ether)
    • Reagent: Lithium aluminum hydride (LiAlH₄)
    • Reaction: Carboxylic acid → Aldehyde
      (Further reduction forms alcohol)

Reactions of Aldehydes and Ketones

Reduction to Alcohols

  • Conditions: Anhydrous (dry ether)
  • Reagent: Lithium aluminum hydride (LiAlH₄)
  • Reaction: Aldehyde or ketone → Alcohol

Nucleophilic Addition of Hydrogen Cyanide (HCN)

  • Conditions: Acidic (HCN generated from NaCN and acid)
  • Reagent: Hydrogen cyanide (HCN)
  • Reaction: Aldehyde or ketone + HCN → Cyanohydrin (adds –CN and –OH to the carbonyl carbon)
  • Hydrolysis of Cyanohydrin
    • Conditions: Acidic hydrolysis
    • Reagent: Dilute acid
    • Reaction: Cyanohydrin → Carboxylic acid (with one extra carbon)

Iodoform Test (Reaction with Alkaline Iodine)

  • Conditions: Warm, alkaline (aqueous NaOH)
  • Reagent: Iodine (I₂) in sodium hydroxide
  • Reaction: Methyl ketone (or alcohol with CH₃CH(OH)–) + I₂/NaOH → Yellow precipitate (CHI₃, iodoform)

Oxidation of Aldehydes

  • Conditions: Warm, aqueous
  • Reagent: Tollens' reagent (ammoniacal silver nitrate) or Fehling’s/Benedict’s solution (copper(II) sulfate)
  • Reaction: Aldehyde + oxidizing agent → Carboxylic acid
    • Tollens': Silver mirror forms (Ag)
    • Fehling’s/Benedict’s: Brick red precipitate (Cu₂O)

Mechanism: Nucleophilic Addition to Carbonyl

  • Step 1: Nucleophile (e.g., CN⁻) attacks the carbonyl carbon; electrons move to oxygen, forming O⁻.
  • Step 2: O⁻ is protonated by acid to form the final product (e.g., cyanohydrin).

Key Terms & Definitions

  • Aldehyde: Compound with a terminal –CHO group.
  • Ketone: Compound with an internal C=O group bonded to two alkyls.
  • 2,4-DNPH: Reagent forming orange/yellow precipitate with carbonyls.
  • Tollens' Reagent: Silver-ammonia complex for detecting aldehydes.
  • Fehling’s/Benedict’s Solution: Copper-based reagents for aldehyde detection.
  • Iodoform Test: Identifies methyl ketones with yellow CHI₃ precipitate.
  • Cyanohydrin: Compound formed by adding HCN to a carbonyl.

Action Items / Next Steps

  • Review structural isomers of aldehydes and ketones.
  • Practice drawing nucleophilic addition mechanisms.
  • Complete textbook questions on chemical tests for carbonyl compounds.

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