Chapter 17

Chapter 17: Aldehydes and Ketones

Naming Aldehydes and Ketones

  • Aldehydes take precedence over ketone group in nomenclature.

  • IUPAC treats aldehydes as derivatives of alkanes:

    • Ending -e replaced by -al.

    • Example: Alkane becomes alkanal.

  • Names parallel those of 1-alkanols; aldehyde carbon is assumed as C1 (no specification needed).

Carbaldehydes

  • When an aldehyde is attached to a ring, it’s termed a carbaldehyde.

  • The carbon bearing the aldehyde is designated as C1.

  • Simplest aromatic aldehyde:

    • Benzenecarbaldehyde (common name: benzaldehyde).

Naming Ketones

  • Ketones are designated as alkanones:

    • -e in alkane replaced by -one.

  • The carbonyl carbon of a ketone is assigned the smallest number.

Structure of the Carbonyl Group

  • Carbonyl group has a short, strong, and polar bond.

  • Hybridization of C and O in carbonyl is sp2.

Differences Between Carbonyl Group and Double Bonds

  • Carbonyl vs ordinary double bond:

    • Oxygen has two lone pairs in sp2 orbitals.

    • Oxygen is more electronegative than carbon, leading to a polarized C=O bond.

    • Results in a slight positive charge on carbon and negative on oxygen.

    • Polarization affects physical constants of aldehydes and ketones.

Preparation of Aldehydes and Ketones

  • Syntheses include:

    1. Oxidation of alcohols

    2. Ozonolysis of alkenes

    3. Hydration of alkynes

    4. Friedel-Crafts alkanoylation

Oxidation of Alcohols

  • Overoxidation of primary alcohols involves water.

  • Use of PCC (CrO3 + Pyridine + HCl) to oxidize a primary alcohol to an aldehyde.

Selective Allylic Oxidations

  • Manganese dioxide can be used for selective allylic oxidations.

Ozonolysis

  • Ozonolysis involves converting alkenes to carbonyl groups using O3 and (CH3)2S.

Hydration of Alkynes

  • Follow Markovnikov Rule for the hydration of alkynes.

  • Anti-Markovnikov Hydration is done through hydroboration.

Friedel-Crafts Alkanoylation

  • Involves acylation steps to introduce carbonyl functions.

Reactivity of the Carbonyl Group

  • Regions of Reactivity:

    • Attack by electrophiles on carbonyl carbon.

    • Attack by nucleophiles on carbonyl oxygen.

Addition Reactions

  • Carbonyl group undergoes ionic additions due to its dipolar nature.

  • Reagent types:

    • Less basic nucleophiles used favorably.

Formation of Hydrates

  • Water can hydrate a carbonyl group.

    • Acid or base catalyzes the reaction, leading to geminal diols (carbonyl hydrates).

  • Hydration of aldehydes and ketones differ in endothermic characteristics relative to formaldehyde.

Addition of Alcohols to Form Hemiacetals and Acetals

  • Hemiacetals form through reaction with alcohols, reversible reactions favoring carbonyl compounds.

Intramolecular Hemiacetal Formation

  • Stability of cyclic hemiacetals upon formation.

Acetal Formation

  • Acids catalyze acetal formation; resulting compounds called acetals.

Use of Acetals as Protecting Groups

  • Cyclic acetals protect carbonyl groups from nucleophilic attacks.

Thioacetals

  • Formed from aldehydes and ketones in presence of Lewis acids.

  • Stable to acids and can be dehydrolyzed to hydrocarbons.

Nucleophilic Addition of Ammonia

  • Reaction with amines yields imines; formation involves hemiaminals, nitrogen analogs of hemiacetals.

Mechanism of Hemiaminal Dehydration

  • Hemiacetal formation leads eventually to imine formation via dehydration.

Identification through Special Imines

  • More stable imine forms due to resonance compared with simple imines.

Enamines Formation

  • Aldehydes/ketones with secondary amines yield enamines which readily hydrolyze in acid.

Wolff-Kishner Reduction

  • Converts hydrazones to hydrocarbons; useful for alkylbenzene synthesis.

HCN and Cyanohydrins

  • HCN formation allows synthesis of cyanohydrins, a useful intermediate.

The Wittig Reaction

  • A ylide reacts with aldehydes/ketones to synthesize alkenes.

Migration During Baeyer-Villiger Oxidation

  • Converts ketones to esters; the migration of substituents depends on their reactivity.

Detection of Aldehydes

  • Simple chemical tests can indicate aldehyde presence by oxidation to carboxylic acids.

Important Concepts

  1. Carbonyl Group: Planar structure and polarization; partial charges present.

  2. Reactivity Order: Aldehydes more reactive than ketones due to electrophilic nature.

  3. Reactions with Amines: Primary amines form imines, secondary amines form enamines.

  4. Alkylbenzene Synthesis: Combination of methods for effective substitutions.

  5. Wittig Reaction: Directly produces alkenes from aldehydes and ketones.

  6. Peroxycarboxylic Acids lead to ester formation from carbonyls.