Ketones

Definition and Structure

Functional Group

Ketones: organic compounds containing a carbonyl group (C=O) bonded to two alkyl or aryl groups. General formula: R–CO–R'.

Structural Features

Planar sp2 hybridized carbonyl carbon. Bond angles approximately 120°. Polar C=O bond with partial positive charge on carbon, partial negative on oxygen.

Classification

Simple ketones: alkyl substituents. Cyclic ketones: carbonyl within ring. α, β-unsaturated ketones: conjugated double bonds adjacent to C=O.

Examples

Acetone (propanone), cyclohexanone, benzophenone.

R–CO–R'Examples:CH3–CO–CH3 (acetone)C6H10O (cyclohexanone)(C6H5)2CO (benzophenone)

Nomenclature

IUPAC Naming

Suffix: -one for ketone functional group. Parent chain: longest containing C=O. Numbering: carbonyl carbon given lowest possible number.

Common Names

Based on alkyl groups attached + "ketone". Example: diethyl ketone (butan-2-one).

Substituents and Complex Structures

Multiple ketones: use prefixes (di-, tri-) or oxo- for ketone substituents. Cyclic ketones: prefix "cyclo" + suffix -one.

Nomenclature Table

Physical Properties

Polarity and Boiling Points

Polar C=O bond: dipole moment ~2.7 D. Boiling points higher than alkanes, lower than alcohols of similar molar mass.

Solubility

Moderate solubility in water due to hydrogen bonding acceptor ability of oxygen. Soluble in organic solvents like ethers, alcohols, chloroform.

Color and Odor

Most ketones are colorless liquids or solids. Characteristic sweet or fruity odor.

Density and State

Density varies; generally liquid at room temp for low molecular weight ketones. Higher ketones solid or viscous liquids.

Synthesis Methods

Oxidation of Secondary Alcohols

Common route: secondary alcohols oxidized to ketones. Reagents: PCC, KMnO4 (mild), CrO3, DMP. Selective oxidation critical.

Friedel-Crafts Acylation

Aromatic ketone synthesis: acyl chloride + aromatic ring catalyzed by AlCl3. Electrophilic aromatic substitution mechanism.

Hydration of Alkynes

Terminal alkynes hydrated under acidic conditions produce methyl ketones. Catalysts: HgSO4, H2SO4. Markovnikov addition.

Other Methods

Ketones via organometallic reagents + nitriles, or from acid chlorides + organocopper reagents. Biocatalytic routes emerging.

Oxidation:R2CHOH + [O] → R2C=O + H2OFriedel-Crafts:Ar–H + RCOCl + AlCl3 → Ar–COR + HClAlkyne Hydration:RC≡CH + H2O + H2SO4/HgSO4 → R–CO–CH3

Reactivity and Mechanisms

Nucleophilic Addition

Carbonyl carbon electrophilic: nucleophiles attack forming tetrahedral intermediates. Reactions: cyanohydrin formation, alcohol addition.

Aldol Condensation

Enolate ion formation under base catalysis. Enolate attacks ketone carbonyl yielding β-hydroxy ketones, dehydration forms α,β-unsaturated ketones.

Reduction

Ketones reduced to secondary alcohols. Reagents: NaBH4, LiAlH4. Selectivity: NaBH4 mild, LiAlH4 strong reducing agent.

Other Reactions

Grignard reagents add to ketones forming tertiary alcohols. Baeyer-Villiger oxidation converts ketones to esters via peracids.

Nucleophilic Addition:R2C=O + Nu⁻ → R2C–ONu⁻ → R2C–Nu + H2O (after workup)Aldol:R2CH–CO–R' + base → enolate → attack on ketone → β-hydroxyketone → dehydrationReduction:R2C=O + [H] → R2CHOH

Spectroscopic Identification

Infrared (IR) Spectroscopy

Strong, sharp C=O stretch typically 1705–1725 cm⁻¹. Conjugation lowers frequency. Absence of O–H distinguishes ketones from carboxylic acids.

Proton (1H) NMR

α-hydrogens appear at 2.0–2.5 ppm. No aldehyde proton (~9–10 ppm). Methyl or methylene groups adjacent to carbonyl deshielded.

Carbon-13 (13C) NMR

Carbonyl carbon resonance at 190–220 ppm. Adjacent carbons shifted downfield relative to alkanes.

Mass Spectrometry (MS)

Characteristic fragment ions: loss of alkyl groups, McLafferty rearrangement common in ketones with γ-hydrogens.

Biological Importance

Metabolic Role

Ketone bodies (acetoacetate, β-hydroxybutyrate, acetone) produced during fatty acid metabolism. Energy source during fasting.

Enzyme Substrates and Products

Ketones serve as intermediates in biosynthesis and degradation of steroids, vitamins, carbohydrates.

Pharmaceutical Relevance

Many drugs contain ketone groups: corticosteroids, antibiotics, analgesics. Ketone functionality modulates biological activity.

Natural Products

Ketones found in essential oils, pheromones, and pigments. Contribute to aroma and flavor in plants and animals.

Industrial and Synthetic Applications

Solvents

Acetone and methyl ethyl ketone: common industrial solvents. Properties: volatility, polarity, miscibility with water and organics.

Polymer Precursors

Ketones used in synthesis of polymers, resins (e.g., polyvinyl chloride plasticizers, epoxy curing agents).

Pharmaceutical Synthesis

Key intermediates in drug synthesis. Functional group transformations enable complex molecule construction.

Fragrance and Flavor Industry

Ketones contribute to aroma compounds. Synthetic ketones mimic natural scents and flavors.

Comparison with Aldehydes and Other Carbonyls

Structural Differences

Aldehydes: at least one hydrogen bonded to carbonyl carbon. Ketones: two carbon substituents.

Reactivity Differences

Aldehydes more reactive due to less steric hindrance and stronger electrophilicity. Ketones more stable and less reactive towards nucleophiles.

Physical Properties

Boiling points: ketones generally higher due to larger molecular weight. Odor differences: aldehydes pungent, ketones sweet/fruity.

Other Carbonyl Compounds

Esters, acids, amides differ by substituents on carbonyl carbon: O–R, OH, NH2 respectively. Different reactivity patterns.

Environmental Impact and Safety

Toxicity

Low to moderate acute toxicity. Prolonged exposure or inhalation may cause CNS effects, irritation.

Volatility and Flammability

Many ketones volatile, flammable. Proper storage, handling required to prevent fire hazards.

Environmental Persistence

Ketones biodegrade moderately fast in environment. Some aromatic ketones more persistent.

Regulatory Aspects

Subject to workplace exposure limits (OSHA, NIOSH). Disposal regulated to minimize environmental release.

Advanced Topics in Ketone Chemistry

Enantioselective Ketone Reactions

Catalysts promote asymmetric addition to ketones. Important for chiral drug synthesis. Examples: chiral auxiliaries, organocatalysts.

Photochemical Behavior

Ketones absorb UV light; undergo Norrish type I and II reactions. Used in photoinitiators, synthetic transformations.

Transition Metal-Catalyzed Reactions

Couplings, reductions, and insertions involving ketones catalyzed by metals (Pd, Rh, Ru). Key in complex molecule synthesis.

Computational Studies

Quantum calculations elucidate ketone reactivity, conformations, electronic structure. Guide experimental design.

References

• March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure. Wiley, 4th Ed., 1992, pp. 290-315.
• Clayden, J., Greeves, N., Warren, S., Wothers, P. Organic Chemistry. Oxford University Press, 2nd Ed., 2012, pp. 150-180.
• Smith, M. B. March’s Advanced Organic Chemistry: Reactions and Synthesis. Wiley, 7th Ed., 2013, pp. 600-630.
• Silverstein, R. M., Webster, F. X., Kiemle, D. J. Spectrometric Identification of Organic Compounds. Wiley, 7th Ed., 2005, pp. 120-135.
• Carey, F. A., Sundberg, R. J. Advanced Organic Chemistry Part A: Structure and Mechanisms. Springer, 5th Ed., 2007, pp. 220-255.