Study Notes on Carbonyl Compounds, Aldehydes, Ketones, and Carboxylic Acids

Carbonyl Compounds

General Introduction

  • Carbonyl compounds include:
    • Aldehydes
    • Ketones
    • Carboxylic acids
  • Aldehydes and ketones are first stage oxidation products of primary and secondary alcohols, respectively.
  • They contain a carbonyl group (C=O) which classifies them under carbonyl compounds.

Physical Properties

  • Physical State:

    • Formaldehyde is a gas; other lower aldehydes and ketones are colorless liquids; higher aldehydes are solids.

  • Odor:

    • Formaldehyde and acetaldehyde are pungent; higher aldehydes have pleasant smells.

  • Boiling Points:

    • The boiling points of substances of comparable molecular mass follow the order:

    carboxylic acid > alcohol > ketone > aldehyde > ether > alkane.

  • Solubility:

    • Lower carbonyl compounds (1-4 carbon atoms) are soluble in water; higher carbonyl compounds are not due to low polarity.

Chemical Properties

Nucleophilic Addition Reactions

  • Aldehydes and ketones react with nucleophiles, leading to addition products.
Examples:
  • Forming Addition Products:
    • C=O + H₂N-Z → C=N-Z + H₂O
  • Nucleophilic Attack Example:
    • Reaction of acetaldehyde with hydrogen cyanide to produce cyanohydrin:
    • CH₃CHO + HCN → CH₃CH(OH)CN

Miscellaneous Reactions

Oxidation

  • Aldehydes undergo easy oxidation to form carboxylic acids.
    Example:

    RCHO + [O] → RCOOH

  • Ketones require stronger oxidizing agents for oxidation.

Reduction

  • Aldehydes can be reduced to primary alcohols, and ketones to secondary alcohols:

    RCHO + H₂ → RCH₂OH (1° Alcohol)
    RCO + H₂ → R(OH)C (2° Alcohol)

Halogenation
  • Aldehydes and ketones with α-H atoms react with halogens to replace one or more of these H atoms with halogens.
    Example:
  • CH₃CHO + Br₂ → CH₃C(Br)O (bromoacetaldehyde)
Reactions with Alkali

  • Aldol Condensation:

    • Two aldehydes or ketones containing α-H atoms react to form β-hydroxy aldehydes or ketones.

    Example: CH₃CHO + CH₃CHO → CH₃CH(OH)CH₂CHO (3-Hydroxybutanal)

Carboxylic Acids

Properties and Identification

  • The carboxyl (-COOH) group consists of a carbonyl group and a hydroxyl group.
  • The first few carboxylic acids are liquid and colorless, while higher ones are viscous oils or solids.
  • Monocarboxylic Acid: Contains one -COOH group. E.g. CH₃COOH (acetic acid).
  • Dicarboxylic Acid: Contains two -COOH groups. E.g. HOOC-COOH (oxalic acid).
  • Tricarboxylic Acid: Contains three -COOH groups. E.g. HOOC-CH₂-COOH (citric acid).

Nomenclature

  • IUPAC names are derived by replacing the terminal 'e' of alkanes with 'oic acid.'
  • Aromatic carboxylic acids: Benzoic acids derive their names from substituted groups. E.g. m-nitrobenzoic acid.

General Methods of Preparation

  • Oxidation of Alcohols:
    • Primary alcohols yield carboxylic acids via strong oxidizing agents.
  • From Methyl Ketones: Methyl ketones react with NaOX (hypohalite) or Br₂ + NaOH resulting in carboxylic acids.
  • Cyanides Hydrolysis:
    • Alkyl cyanides hydrolyze to produce carboxylic acids. Example: CH₃CN → CH₃COOH
  • Via Grignard Reagents:
    • Passing CO₂ through Grignard reagent yields carboxylic acids.

Chemical Properties

  • Acidic Properties: Carboxylic acids are weak acids, capable of ionizing in water to release H+ ions.
  • The strength of acidity is enhanced by electron-withdrawing groups and weakened by electron-donating groups.
  • Carboxylic acids exhibit distinct reactions with alcohols, amines, and other compounds leading to the formation of esters, amides, etc.

Distinctive Tests for Identification

  1. Tollens' Test: Aldehydes yield metallic silver with Tollens' reagent, whereas ketones do not.
  2. Fehling's Test: Aldehydes reduce Fehling's solution to yield red precipitate, ketones do not.
  3. Oxalate Formation:
    • Carboxylic acids will react with NaHCO₃ to evolve CO₂ gas.
  4. Dinitrophenylhydrazine (DNP) Test: Tests for carbonyl groups in aldehydes and ketones via formation of a yellow-orange precipitate.

The provided detailed notes guide a student through the essential aspects of carbonyl compounds and carboxylic acids while ensuring clarity in structure, properties, and reactions.