Aldehydes and Ketones: Structure, Properties, Synthesis, and Reactions (Chapter 18 Study Guide)

18.1 Carbonyl Compounds

Bonding and Structure of the Carbonyl Group

• Carbonyl Group: A functional group composed of a carbon atom double-bonded to an oxygen atom (C=O).

• Atoms Involved: The carbonyl carbon is sp2 hybridized, leading to a trigonal planar geometry.

• Aldehydes: The carbonyl carbon is bonded to at least one hydrogen atom.

• Ketones: The carbonyl carbon is bonded to two other carbons.

18.2 Structure of the Carbonyl Group

Geometry, Resonance, and Polarity

• Bond Angles: Approximately 120°, consistent with sp2 hybridization.

• Bond Length: The C=O bond is shorter and stronger than a C=C bond due to the greater electronegativity of oxygen.

• Resonance: The carbonyl group can be represented by resonance structures, with the major contributor being the neutral form and a minor contributor with a positive charge on carbon and negative on oxygen.

• Dipole Moment: The C=O bond is highly polar, with a partial positive charge on carbon and partial negative on oxygen.

18.3 Nomenclature of Ketones and Aldehydes

IUPAC and Common Naming

• IUPAC Naming: Aldehydes are named by replacing the -e of the parent alkane with -al; ketones use the suffix -one.

• Common Names: Some ketones and aldehydes have widely used common names (e.g., acetone, formaldehyde).

• Examples:

  • Acetone: (propanone)

  • Acetophenone: (methyl phenyl ketone)

  • Benzophenone: (diphenyl ketone)

  • Formaldehyde: (methanal)

  • Acetaldehyde: (ethanal)

  • Benzaldehyde: (benzenecarbaldehyde)

18.4 Physical Properties of Ketones and Aldehydes

Boiling Points and Hydrogen Bonding

• Boiling Points: Higher than alkanes and ethers due to dipole-dipole interactions, but lower than alcohols (which can hydrogen bond).

• Hydrogen Bonding: Aldehydes and ketones cannot hydrogen bond with themselves but can act as hydrogen bond acceptors with water and alcohols.

18.5 Spectroscopy of Ketones and Aldehydes

IR, NMR, Mass, and UV Spectroscopy

• IR Spectroscopy:

  • C=O stretch: $1700\ \text{cm}^{-1}$ (strong, sharp)

  • Aldehyde C-H stretch: $2720-2820\ \text{cm}^{-1}$

  • Conjugation lowers the C=O stretching frequency.

• NMR Spectroscopy:

  • Protons on carbons adjacent to C=O appear downfield (2-3 ppm).

  • Aldehyde proton: 9-10 ppm.

  • 13C NMR: Carbonyl carbon appears at 190-220 ppm.

• Mass Spectrometry:

  • Common cleavage: alpha cleavage next to the carbonyl.

  • McLafferty rearrangement: characteristic for certain ketones and aldehydes with gamma hydrogens.

• UV Spectroscopy: n→π* and π→π* transitions are observed; conjugation shifts absorption to longer wavelengths.

18.7 Reviewing Syntheses of Ketones and Aldehydes

Key Synthetic Methods

• Grignard Reaction: Addition to esters or nitriles followed by hydrolysis yields ketones or aldehydes.

• Controlled Oxidation: Use of PCC or DMP oxidizes primary alcohols to aldehydes without further oxidation to acids.

• Ozonolysis: Cleavage of alkenes with ozone produces aldehydes and/or ketones.

• Friedel-Crafts Acylation: Synthesis of aryl ketones from aromatic rings and acid chlorides.

• Gatterman-Koch Reaction: Synthesis of benzaldehyde derivatives from benzene, CO, and HCl.

• Oxymercuration/Hydroboration-Oxidation of Alkynes: Terminal alkynes yield aldehydes; internal alkynes yield ketones.

18.8 Synthesis of Ketones from Carboxylic Acids

Conversion Sequence

• Carboxylic acids can be converted to ketones via formation of acid chlorides, followed by reaction with organometallic reagents and hydrolysis.

18.9 Synthesis from Nitriles

Grignard and DIBAL-H Reactions

• Grignard with Nitriles: Forms ketones after hydrolysis.

• DIBAL-H Reduction: Converts nitriles to aldehydes under controlled conditions.

18.10 Synthesis from Acid Chlorides and Esters

Reduction and Organometallic Reactions

• Reduction: Use of weak reducing agents (e.g., DIBAL-H, LiAlH(OtBu)3) stops at the aldehyde stage.

• Grignard with Acid Chlorides: Forms tertiary alcohols unless controlled.

• Organocuprates (Gilman Reagents): React with acid chlorides to give ketones selectively.

18.11 Reactions: Nucleophilic Addition

General Mechanism

• Nucleophiles attack the electrophilic carbonyl carbon, forming a tetrahedral intermediate.

• Basic nucleophiles (e.g., Grignard, hydride) add directly; acidic conditions involve protonation of the carbonyl oxygen first.

18.12 Hydration of Ketones and Aldehydes

Formation and Mechanism

• Hydrate: A geminal diol formed by addition of water to the carbonyl group.

• Mechanism: Nucleophilic addition of water, catalyzed by acid or base.

• Reactivity: Formaldehyde > aldehydes > ketones (due to steric and electronic effects).

18.13 Formation of Cyanohydrins

Mechanism and Products

• Cyanohydrin: Compound with a hydroxyl and a cyano group on the same carbon.

• Mechanism: Nucleophilic addition of cyanide ion to the carbonyl carbon.

• Hydrolysis: Cyanohydrins can be hydrolyzed to carboxylic acids.

18.14 Formation of Imines

Mechanism, Reagents, and Reversibility

• Imine: Compound with a C=N double bond (Schiff base if derived from a primary amine).

• Condensation Reaction: Water is eliminated during imine formation.

• Reagents: Primary amines and acid catalyst.

• Reversibility: Hydrolysis regenerates the carbonyl compound.

18.15 Condensations with Hydroxylamine and Hydrazine

Oximes, Hydrazones, and Semicarbazones

• Oxime: Formed from reaction with hydroxylamine (NH2OH).

• Hydrazone: Formed from reaction with hydrazine (NH2NH2).

• Semicarbazone: Formed from reaction with semicarbazide.

18.16 Formation of Acetals

Mechanism, Reagents, and Equilibrium

• Acetal/Ketal: Acetal from aldehyde; ketal from ketone. Both have two -OR groups attached to the same carbon.

• Hemiacetal: Intermediate with one -OR and one -OH group.

• Reagents: Alcohol and acid catalyst.

• Mechanism: Acid-catalyzed addition of alcohol to carbonyl; not favored in base.

• Equilibrium: Acetal formation is reversible; removal of water drives reaction forward.

• Cyclic Acetals: Formed when diols react with carbonyls.

• Carbohydrates: Many exist as cyclic hemiacetals or acetals.

18.17 Use of Acetals as Protecting Groups

Protection and Deprotection in Synthesis

• Acetals are stable to base and nucleophiles, making them useful for protecting carbonyl groups during multi-step synthesis.

• Deprotection is achieved by acid-catalyzed hydrolysis.

18.18 The Wittig Reaction

Alkene Synthesis via Phosphorus Ylides

• Wittig Reaction: Converts aldehydes or ketones to alkenes using phosphorus ylides.

• Ylide Synthesis: Prepared from triphenylphosphine and alkyl halide, followed by deprotonation.

• Mechanism: [2+2] cycloaddition forms a four-membered ring intermediate, which decomposes to alkene and triphenylphosphine oxide.

18.19 Oxidation of Aldehydes

Reagents and Selectivity

• Oxidizing Agents: KMnO4, CrO3, Ag2O (Tollens' reagent) oxidize aldehydes to carboxylic acids.

• Tollens Test: Silver mirror test for aldehydes; ketones do not react under these conditions.

• Ketones: Resistant to oxidation under normal conditions.

18.20 Reduction of Ketones and Aldehydes

Hydride Reagents and Catalytic Hydrogenation

• Hydride Reagents: NaBH4 (mild), LiAlH4 (strong) reduce aldehydes to primary alcohols and ketones to secondary alcohols.

• Raney Nickel: Catalytic hydrogenation reduces C=O to CH2.

• Deoxygenation: Removal of oxygen from carbonyl (e.g., Clemmensen and Wolff-Kishner reductions).

• Clemmensen Reduction: Zn(Hg)/HCl reduces carbonyl to alkane.

• Wolff-Kishner Reduction: NH2NH2/KOH, heat; forms hydrazone intermediate, then eliminates N2 to yield alkane.

Reaction	Reagents	Product
Oxidation of Aldehyde	KMnO4, CrO3, Tollens	Carboxylic Acid
Reduction (Hydride)	NaBH4, LiAlH4	Alcohol
Clemmensen Reduction	Zn(Hg)/HCl	Alkane
Wolff-Kishner Reduction	NH2NH2/KOH	Alkane
Wittig Reaction	Phosphorus ylide	Alkene

Additional info: This guide expands on the learning objectives by providing definitions, mechanisms, and examples for each reaction and property. For detailed mechanisms and more examples, refer to your textbook or lecture notes.