18.1 Carbonyl Compounds

Definition and Classes of Carbonyl Compounds

Carbonyl compounds are organic molecules containing the carbonyl group, characterized by a carbon atom double-bonded to an oxygen atom (C=O). The two main classes discussed here are aldehydes and ketones:

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

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

Other important classes of carbonyl compounds include carboxylic acids, esters, acid chlorides, and amides, each with distinct functional groups attached to the carbonyl carbon.

Condensed structural formulas are often used to represent these compounds for clarity and brevity.

18.2 Structure of the Carbonyl Group

Bonding and Polarity

The carbonyl group consists of a sigma (σ) bond and a pi (π) bond between carbon and oxygen. The C=O bond is shorter, stronger, and more polar than the C=C bond in alkenes due to the higher electronegativity of oxygen.

Key properties:

• Bond length: C=O bond is approximately 1.23 Å, shorter than the C=C bond (1.34 Å).

• Bond energy: C=O bond is stronger (745 kJ/mol) than C=C (611 kJ/mol).

The carbonyl carbon carries a partial positive charge, making it an electrophile, while the oxygen is partially negative. This polarity is reflected in the dipole moments of carbonyl compounds.

Resonance structures show that the major contributor has all atoms with complete octets and no formal charges, while the minor contributor has charge separation.

18.3 Nomenclature of Ketones and Aldehydes

Keton and Aldehyde Naming Rules

Naming of ketones and aldehydes follows IUPAC conventions, with some common names still in use:

• Ketones (IUPAC): Name the longest chain containing the carbonyl, drop the "-e" from the alkane and add "-one". The carbonyl carbon gets the lowest possible number. For cyclic ketones, the carbonyl is always position 1.

• Ketones (Common): Named as alkyl groups attached to the C=O, using Greek letters (α, β, γ) to indicate positions.

Aldehydes are named by replacing the "-e" of the parent alkane with "-al". The carbonyl carbon is always position 1. If attached to a ring, the suffix "-carbaldehyde" is used.

When other functional groups are present, ketones are named as "oxo" and aldehydes as "formyl" substituents. Aldehydes have higher priority than ketones in nomenclature.

Priority order: acids > esters > aldehydes > ketones > alcohols > amines > alkenes/alkynes > ethers > halides.

Some carboxylic acids and aldehydes have common names derived from their natural sources or historical context.

Examples of aromatic and aliphatic ketones are also commonly encountered.

Common and IUPAC names for substituted aldehydes are shown below.

18.4 Physical Properties of Ketones and Aldehydes

Boiling Points and Solubility

Most simple aldehydes and ketones are liquids at room temperature. Their boiling points are lower than those of corresponding alcohols due to the absence of hydrogen-bond donors, but higher than alkanes and ethers due to the polar C=O bond.

Ketones and aldehydes are good solvents for alcohols and can accept hydrogen bonds from O–H or N–H groups. Acetone and acetaldehyde are miscible with water.

18.5 Spectroscopy of Ketones and Aldehydes

Infrared (IR) Spectroscopy

Ketones and aldehydes show a strong C=O stretch in the IR spectrum:

• Ketones: ~1710 cm-1

• Aldehydes: ~1725 cm-1

• Aldehydes also show C–H stretches near 2710 and 2810 cm-1

Nuclear Magnetic Resonance (NMR) Spectroscopy

In 1H NMR spectroscopy:

• Aldehyde protons appear at δ 9–10 ppm (highly deshielded).

• Alpha protons (adjacent to C=O) appear at δ 2.1–2.4 ppm.

In 13C NMR spectroscopy:

• C=O carbons: δ 180–220 ppm

• Alpha carbons: δ 30–50 ppm

Mass Spectrometry

Characteristic fragmentation patterns include inductive cleavage and alpha cleavage. The McLafferty rearrangement is a notable feature for compounds with a γ-hydrogen.

UV/Vis Spectroscopy

Conjugated carbonyl systems show characteristic π → π* absorption in the UV spectrum. Additional conjugation or alkyl groups shift λmax to longer wavelengths.

18.7 Review of Syntheses of Ketones and Aldehydes

Common Synthetic Methods

• Oxidation of alcohols (primary to aldehydes, secondary to ketones)

• Friedel-Crafts acylation (aromatic ketones)

• Hydration and hydroboration-oxidation of alkynes

• Ozonolysis of alkenes

• Periodate cleavage of glycols

18.8–18.10 Synthesis from Carboxylic Acids, Nitriles, Acid Chlorides, and Esters

Key Reactions

• Organolithium reagents attack carboxylates to give ketones after protonation and dehydration.

• Grignard or organolithium reagents attack nitriles to form imines, which hydrolyze to ketones.

• Aluminum hydrides (e.g., DIBAL-H) reduce nitriles to aldehydes.

• Acid chlorides can be reduced to aldehydes or converted to ketones using lithium dialkyl cuprates (Gilman reagents).

18.11 Reactions of Ketones and Aldehydes: Nucleophilic Addition

General Mechanism

Nucleophilic addition to the carbonyl group is a central reaction for aldehydes and ketones. A nucleophile attacks the electrophilic carbonyl carbon, forming a tetrahedral alkoxide intermediate, which is then protonated.

Aldehydes are generally more reactive than ketones due to less steric hindrance and greater electrophilicity.

Grignard and Organolithium Addition

Grignard reagents (RMgX) and organolithium reagents add to carbonyls to form alcohols after hydrolysis. This is a key method for forming new C–C bonds.

Hydride Addition (Reduction)

Hydride donors such as sodium borohydride (NaBH4) and lithium aluminum hydride (LiAlH4) reduce aldehydes and ketones to primary and secondary alcohols, respectively.

18.20 Reductions of Ketones and Aldehydes

Reducing Agents and Selectivity

• NaBH4: Reduces aldehydes and ketones, but not esters, acids, or amides. More selective and less reactive.

• LiAlH4: Reduces a broader range of carbonyl compounds, including acids and derivatives. Very reactive and requires anhydrous conditions.

• Catalytic hydrogenation: Reduces C=O and C=C bonds using hydrogen gas and a metal catalyst (e.g., Raney nickel).

Complete reduction to a methylene group (-CH2-) can be achieved by Wolff-Kishner (basic conditions) or Clemmensen (acidic conditions) reductions.

18.12–18.17 Additional Reactions

Hydration, Cyanohydrin Formation, and Imine Formation

• Hydration: Aldehydes and ketones react with water to form geminal diols (hydrates), but equilibrium favors the carbonyl form for ketones.

• Cyanohydrin formation: Nucleophilic addition of cyanide ion to the carbonyl, followed by protonation.

• Imine formation: Reaction of a primary amine with a carbonyl forms an imine (Schiff base) via nucleophilic addition and dehydration.

Acetal Formation and Protecting Groups

• Acetals: Formed by reaction of aldehydes/ketones with excess alcohol and acid. Used as protecting groups for carbonyls in synthesis.

• Cyclic acetals: Formed with diols; stable in base/neutral, removable in acid.

18.18 The Wittig Reaction

Alkene Synthesis from Carbonyls

The Wittig reaction converts aldehydes or ketones to alkenes using a phosphorus ylide. The reaction is highly useful for constructing carbon–carbon double bonds with defined geometry.

18.19 Oxidation of Aldehydes

Oxidation and Tollens Test

Aldehydes are easily oxidized to carboxylic acids. The Tollens test uses a silver-ammonia complex to detect aldehydes, which reduce Ag+ to metallic silver upon oxidation.