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.