Study Notes on Aldehydes and Ketones

CARBONYL COMPOUNDS: ALDEHYDES AND KETONES

INTRODUCTION

  • Carbonyl compounds encompass a variety of structures, including:

    • Aldehydes

    • Ketones

    • Carboxylic acids and derivatives

    • Various classes of cellular molecules

  • General structure represented as R-C(=O)-Y, where R = alkyl or aryl groups and Y represents functional groups.

NOMENCLATURE

Aldehydes

  • The carbonyl carbon atom in aldehydes is bonded to at least one hydrogen atom.

  • Simplest aldehyde: Formaldehyde (H₂C=O)

    • Condensed formula: RCHO or ArCHO, where Ar = aryl group.

  • Bond angles around the carbonyl carbon are approximately 120°, though structures often depicted linearly.

Common Names of Aldehydes

  • Methanal (Formaldehyde)

  • Ethanal (Acetaldehyde)

  • Propanone (Acetone)

  • Benzenecarbaldehyde (Benzaldehyde)

    • A solution of formaldehyde in water known as formaline (used as disinfectant and fixative).

    • Formaldehyde is highly toxic to all animals regardless of the intake method.

Ketones

  • In ketones, the carbonyl carbon is bonded to two alkyl (–R) or aryl (–Ar) groups.

  • Simplest ketone: Propanone (Acetone)

  • Nomenclature involves replacing the final -e of the parent hydrocarbon with the ending -one.

PHYSICAL PROPERTIES

Solubility

  • Aldehydes and ketones are capable of significant hydrogen bonding due to their carbonyl functional groups.

  • Low molecular weight carbonyl compounds like formaldehyde, acetaldehyde, and acetone dissolve in water in all proportions.

  • Solubility decreases with increasing chain length, although carbonyl oxygen atoms can still act as hydrogen bond acceptors.

OXIDATION OF ALDEHYDES AND KETONES

General Trends

  • Aldehydes are easily oxidized to yield carboxylic acids.

  • Ketones are generally unreactive to oxidation, but can be reduced to alcohols.

  • The carbonyl group of an aldehyde is positioned between the oxidation states of alcohols and carboxylic acids, thus can be oxidized or reduced.

Oxidizing Agents and Qualitative Tests

  • Common reagents for oxidizing aldehydes to carboxylic acids:

    • KMnO₄

    • CrO₃

    • Na₂Cr₂O₇

    • HNO₃

  • Provides methods to distinguish aldehydes from ketones through specific reactions such as:

    • Tollens’ reagent (produces metallic silver with aldehydes)

    • Benedict’s solution (turns red when reducing aldehydes)

    • Fehling’s solution (also oxidizes aldehydes, forming a precipitate)

REDUCTION OF ALDEHYDES AND KETONES

General Reducing Agents

  • Reduction of carbonyl compounds usually requires strong conditions using lithium aluminum hydride (LiAlH₄) or sodium borohydride (NaBH₄).

    • Example of reduction reactions:

    • Reaction of NaBH₄ with aldehyde:
      R-C(=O)-H + ext{[H]}
      ightarrow R-CH₂-OH

STRUCTURE AND CHEMISTRY OF THE CARBONYL GROUP

Chemical Structure

  • Consists of a double bond between carbonyl carbon and carbonyl oxygen.

  • Hybridization: carbonyl carbon is sp²-hybridized, forming three σ bonds with bond angles approximately 120°.

  • Carbonyl oxygen is sp²-hybridized and contributes one valence electron.

  • Electronegativity of the oxygen atom contributes to the polarity of the carbonyl bond.

NUCLEOPHILIC ADDITION REACTIONS

Mechanism Overview

  • The carbonyl carbon is electrophilic due to its positive charge attraction to nucleophiles.

  • The addition of nucleophiles to the carbonyl carbon results in a shift from sp² (planar) to sp³ (tetrahedral) geometry.

  • General reaction format:

    • Nucleophile (Nu) attacks the carbonyl carbon, forming a new bond.

Examples of Nucleophilic Additions

  • Formation of Cyanohydrins:

    • Hydrogen cyanide (HCN) adds to carbonyls, leading to cyanohydrins, with the nucleophilic CN⁻ attacking the carbonyl carbon.

  • Hydration of Carbonyl Compounds:

    • Reaction with water forms gem diols (hydrates). Formaldehyde is predominantly hydrated.

MECHANISMS OF ACID AND BASE-CATALYZED ADDITION REACTIONS

General Features

  • Carbonyl compounds react with unsymmetrical reagents where hydrogen (H⁺) acts as an electrophile, and nucleophiles (Nu⁻) add to carbonyl groups.

  • The addition reactions vary whether catalyzed by acid or base, affecting the order of steps in the reaction.

Acid-Catalyzed Nucleophilic Addition

  1. Protonation of the carbonyl oxygen leads to a carbocation.

  2. The carbocation reacts with the nucleophile.

  3. Acid-base reaction with solvent stabilizes the product.

Base-Catalyzed Nucleophilic Addition

  1. Nucleophile attacks the carbonyl carbon, forming a tetrahedral intermediate.

  2. Acid-base reaction with solvent protonates the alkoxide forming a stable product.

NUCLEOPHILIC ADDITION OF ALCOHOLS: FORMATION OF ACETALS AND KETALS

Hemiacetal and Hemiketal Formation

  • A single mole of an alcohol with an aldehyde yields hemiacetals/hemiketals.

  • Two moles of alcohol yield acetals or ketals.

ADDITION OF NITROGEN COMPOUNDS

  • Ammonia and primary amines react more rapidly with carbonyl groups than water/alcohols, forming hemiaminals, which are nitrogen analogs of hemiacetals/hemiketals.

  • The formed hemiaminal can lose water, resulting in stable imine structures (Schiff bases).

α-Carbon Chemistry

Reactivity

  • The α-carbon is a reactive site within carbonyl compounds, capable of deprotonation to create a nucleophilic α-carbon due to slightly acidic α-hydrogens.

  • Resonance stabilization of the enolate anion upon removal of an α-hydrogen increases stability.