Ketones and Aldehydes

Overview

Ketones and aldehydes are important classes of carbonyl compounds in organic chemistry, characterized by the presence of a carbonyl group (C=O). Their structure, nomenclature, physical properties, methods of synthesis, and chemical reactivity are foundational topics for understanding organic reactions and mechanisms.

General Structure and Hybridization

Structure of the Carbonyl Group

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

• Ketones: Two alkyl (or aryl) groups bonded to the carbonyl carbon (general formula: R2C=O).

• Aldehydes: One alkyl (or aryl) group and one hydrogen bonded to the carbonyl carbon (general formula: RCHO).

• Hybridization: The carbonyl carbon is sp2 hybridized, resulting in a planar structure with bond angles of approximately 120°.

• Bond Properties: The C=O bond is shorter (1.23 Å), stronger (745 kJ/mol), and more polar than the C=C bond in alkenes (1.34 Å, 611 kJ/mol).

Resonance in Carbonyl Compounds

• The carbonyl group exhibits resonance, with the major contributor being the neutral form where all atoms have complete octets.

• The carbonyl carbon has a partial positive charge, making it an electrophile susceptible to nucleophilic attack.

Classes of Carbonyl Compounds

Nomenclature

IUPAC Naming of Ketones

• Number the carbon chain so the carbonyl carbon has the lowest possible number.

• Replace the alkane suffix -e with -one (e.g., butanone).

• For cyclic ketones, the carbonyl carbon is always position 1.

• If both a carbonyl and a double bond are present, the carbonyl takes precedence in numbering.

IUPAC Naming of Aldehydes

• The aldehyde carbon is always number 1.

• Replace the alkane suffix -e with -al (e.g., ethanal).

• If the aldehyde is attached to a ring, use the suffix -carbaldehyde.

Carbonyl as a Substituent

• When a higher-priority functional group is present, a ketone is named as an oxo group and an aldehyde as a formyl group.

• Aldehydes have higher priority than ketones in nomenclature.

Functional Group Priority (Selected)

Common Names

• Ketones: Named as alkyl groups attached to the carbonyl (e.g., methyl ethyl ketone).

• Greek letters (α, β, γ, etc.) are used to indicate positions relative to the carbonyl group.

• Aldehydes and acids often have historical common names (e.g., formaldehyde, acetone).

Physical Properties

Boiling Points

• Ketones and aldehydes are more polar than alkanes and ethers, resulting in higher boiling points.

• They cannot hydrogen-bond to each other, so their boiling points are lower than comparable alcohols.

Solubility

• Ketones and aldehydes are good solvents for alcohols.

• The lone pair on the carbonyl oxygen can accept hydrogen bonds from O–H or N–H groups.

• Lower molecular weight ketones and aldehydes (e.g., acetone, acetaldehyde) are miscible with water.

Spectroscopic Properties

Infrared (IR) Spectroscopy

• Strong C=O stretch: Ketones at ~1710 cm-1, Aldehydes at ~1725 cm-1.

• Aldehydes show additional C–H stretches at 2710 cm-1 and 2810 cm-1.

• Conjugation lowers the C=O stretching frequency (to ~1685 cm-1).

• Ring strain increases the C=O stretching frequency.

NMR Spectroscopy

• 1H NMR: Aldehyde protons appear at δ 9–10 ppm. α-Carbon protons appear at δ 2.1–2.4 ppm.

• Protons closer to the carbonyl are more deshielded (higher δ values).

• 13C NMR: Carbonyl carbons in ketones appear at ~208 ppm; α-carbons at 30–44 ppm.

Mass Spectrometry (MS)

• Characteristic fragmentation patterns, including the formation of acylium ions and McLafferty rearrangement (especially in aldehydes).

• McLafferty rearrangement involves breaking the α,β bond and transferring a proton to the oxygen, forming an alkene and an enol.

Industrial and Household Importance

• Acetone and methyl ethyl ketone are widely used as industrial solvents.

• Formaldehyde is used in the production of polymers (e.g., Bakelite) and as a preservative.

• Many ketones and aldehydes are used as flavorings, fragrances, and in household products (e.g., vanillin, camphor, acetophenone).

Synthesis of Aldehydes and Ketones

1. Grignard Reagents

• Grignard reagents react with aldehydes to form secondary alcohols, which can be oxidized to ketones.

• General reaction:

2. Oxidation of Alcohols

• Primary alcohols are oxidized to aldehydes using reagents such as PCC, Swern oxidation, or TEMPO.

• Secondary alcohols are oxidized to ketones.

3. Ozonolysis of Alkenes

• Ozone cleaves double bonds, forming aldehydes and/or ketones after reductive workup.

• General reaction:

4. Friedel–Crafts Acylation

• Acyl halides react with aromatic rings in the presence of AlCl3 to form aryl ketones.

5. Hydration of Alkynes

• Markovnikov hydration of alkynes yields ketones (via enol intermediates).

• Anti-Markovnikov hydration (hydroboration–oxidation) yields aldehydes from terminal alkynes.

6. Synthesis from Carboxylic Acids

• Organolithium reagents attack carboxylate anions to form dianions, which upon protonation and dehydration yield ketones.

7. Synthesis from Nitriles

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

• Reduction of nitriles with DIBAL-H yields aldehydes.

8. Reduction of Acid Chlorides

• Lithium tri-tert-butoxyaluminum hydride selectively reduces acid chlorides to aldehydes.

9. Lithium Dialkylcuprate Reagents

• Lithium dialkylcuprates (Gilman reagents) react with acid chlorides to form ketones.

Reactions of Aldehydes and Ketones

Nucleophilic Addition

• Strong nucleophiles attack the electrophilic carbonyl carbon, forming an alkoxide intermediate that is then protonated.

• Aldehydes are generally more reactive than ketones due to less steric hindrance and greater partial positive charge on the carbonyl carbon.

Hydration

• In aqueous solution, aldehydes and ketones are in equilibrium with their hydrates (geminal diols).

• Equilibrium favors the unhydrated form for ketones.

• Hydration can be acid- or base-catalyzed.

Cyanohydrin Formation

• Cyanide ion adds to the carbonyl carbon, followed by protonation to form a cyanohydrin.

• HCN is highly toxic and must be handled with care.

Imine Formation

• Reaction of aldehydes or ketones with ammonia or primary amines forms imines (Schiff bases).

• Optimal pH for imine formation is around 4.5.

• Mechanism involves acid-catalyzed addition of the amine, followed by dehydration.

Acetal Formation

• Alcohols add to aldehydes or ketones (acid-catalyzed) to form hemiacetals, which can react further to form acetals.

• Acetals are stable in neutral/basic conditions and can be hydrolyzed back to carbonyls in acid.

• Cyclic acetals can be formed using diols and are used as protecting groups for carbonyls in synthesis.

The Wittig Reaction

• Converts carbonyl groups to alkenes using phosphorus ylides.

• Mechanism involves betaine and oxaphosphetane intermediates, ultimately yielding an alkene and triphenylphosphine oxide.

Oxidation and Reduction

• Oxidation: Aldehydes are easily oxidized to carboxylic acids (e.g., Tollens test).

• Reduction: Sodium borohydride (NaBH4) reduces aldehydes to primary alcohols and ketones to secondary alcohols. Lithium aluminum hydride (LiAlH4) is a stronger reducing agent and can reduce carboxylic acids and derivatives.

• Catalytic hydrogenation (e.g., Raney nickel) reduces both carbonyls and C=C bonds.

• Deoxygenation: Clemmensen (Zn(Hg)/HCl) and Wolff–Kishner (hydrazine/KOH) reductions remove the carbonyl oxygen, converting ketones/aldehydes to alkanes.

Summary Table: Common Reactions of Aldehydes and Ketones

Additional info: This guide covers the essential aspects of ketones and aldehydes as presented in a typical college-level organic chemistry course, including structure, nomenclature, physical and spectroscopic properties, synthesis, and reactivity.