Aldehydes

Definition and Structure

Functional Group

Aldehydes contain the carbonyl functional group (C=O) bonded to at least one hydrogen atom. General formula: R–CHO, where R is hydrogen or an alkyl/aryl group.

Molecular Geometry

Planar trigonal geometry around the carbonyl carbon. Bond angles approximately 120°. Polar carbonyl bond due to electronegativity difference between carbon and oxygen.

Electronic Structure

Carbonyl carbon electrophilic center. Oxygen has two lone pairs, creating dipole moment. Reactivity driven by polarized double bond.

Representative Examples

Formaldehyde (HCHO), acetaldehyde (CH3CHO), benzaldehyde (C6H5CHO) are common aldehydes with varying R groups.

Nomenclature

IUPAC Naming

Parent alkane suffix changes from -e to -al. Aldehyde carbon is always carbon 1. Example: ethanal for CH3CHO.

Common Names

Trivial names based on source or historical naming: formaldehyde, acetaldehyde, propionaldehyde.

Substituted Aldehydes

Number substituents from aldehyde carbon. Example: 3-chloropropanal.

Multiple Functional Groups

Aldehyde group prioritized in numbering over alcohols, ethers; suffix -al used accordingly.

Physical Properties

Boiling and Melting Points

Boiling points higher than hydrocarbons but lower than alcohols of similar mass. Polarity and hydrogen bonding influence.

Solubility

Low molecular weight aldehydes miscible with water due to hydrogen bonding acceptor ability. Solubility decreases with increasing chain length.

Odor

Characteristic pungent or fruity odors. Formaldehyde: sharp, irritating; benzaldehyde: almond-like scent.

Density and State

Most low-molecular aldehydes are liquids at room temperature; formaldehyde is a gas or aqueous solution (formalin).

Synthesis Methods

Oxidation of Primary Alcohols

Primary alcohols oxidized to aldehydes using PCC, CrO3, or Swern oxidation. Avoid overoxidation to acids.

Ozonolysis of Alkenes

Ozone cleaves double bonds; reductive workup yields aldehydes from terminal alkenes.

Hydroformylation

Alkenes react with CO and H2 under catalyst to form aldehydes with one extra carbon atom.

Other Methods

Partial reduction of acid chlorides or esters; organometallic formylation reactions.

RCH2OH + [O] → RCHO + H2ORCH=CH2 + CO + H2 → RCH2CH2CHO (Hydroformylation)

Chemical Reactions

Nucleophilic Addition

Key reaction type. Nucleophiles attack electrophilic carbonyl carbon, forming tetrahedral intermediates.

Oxidation

Aldehydes oxidize to carboxylic acids using strong oxidizers: KMnO4, Ag2O (Tollens’ test).

Reduction

Reduced to primary alcohols by NaBH4, LiAlH4, catalytic hydrogenation.

Condensation Reactions

Aldol condensation with enolizable aldehydes/ketones under base catalysis forms α,β-unsaturated aldehydes.

Reaction Mechanisms

Nucleophilic Addition to Carbonyl

Step 1: Nucleophile attacks carbonyl carbon. Step 2: Tetrahedral intermediate forms. Step 3: Protonation yields addition product.

Oxidation to Acid

Initial formation of gem-diol intermediate. Further oxidation cleaves C-H bond to form acid.

Aldol Condensation

Base abstracts α-hydrogen forming enolate. Enolate attacks aldehyde carbonyl. β-H elimination forms double bond.

1) Nu⁻ + RCHO → RCH(–Nu)O⁻2) Protonation → RCH(–Nu)OHBase:RCH2CHO → RCH=CH(O⁻) → condensation → α,β-unsaturated aldehyde

Biological Role

Metabolites

Aldehydes are intermediates in carbohydrate and amino acid metabolism, e.g., glyceraldehyde-3-phosphate.

Enzymatic Reactions

Aldehyde dehydrogenases catalyze oxidation of aldehydes to acids, detoxifying reactive species.

Toxicity

Reactive aldehydes cause protein/DNA cross-linking, oxidative stress, implicated in diseases.

Signaling Molecules

Some aldehydes serve as pheromones or hormone precursors in plants and animals.

Industrial Applications

Formaldehyde Production

Used for resins, plastics, disinfectants, and textile finishing.

Flavor and Fragrance Industry

Benzaldehyde and other aromatic aldehydes used as flavoring agents and perfumes.

Pharmaceutical Synthesis

Key intermediates in drug manufacture, intermediates in complex molecule assembly.

Polymer Industry

Aldehydes polymerize or copolymerize for materials like phenolic resins.

Spectroscopic Identification

Infrared (IR) Spectroscopy

Carbonyl stretch at 1720-1740 cm⁻¹. C–H aldehyde stretch at 2720-2820 cm⁻¹ (weak peaks).

NMR Spectroscopy

¹H NMR: Aldehyde proton signal at δ 9-10 ppm. ¹³C NMR: Carbonyl carbon around δ 190-200 ppm.

Mass Spectrometry

Characteristic fragmentation patterns with M⁺, loss of CO, and McLafferty rearrangement for aldehydes with γ-hydrogens.

Safety and Handling

Toxicity

Many aldehydes are irritants, sensitizers, and carcinogenic (e.g., formaldehyde). Use with ventilation and PPE.

Storage

Store in cool, dry, well-ventilated areas. Avoid light and air exposure to prevent polymerization.

Disposal

Neutralize via oxidation or reduction before disposal. Follow regulatory guidelines for hazardous waste.

Environmental Impact

Volatile Organic Compounds (VOCs)

Aldehydes contribute to photochemical smog formation and ground-level ozone.

Biodegradation

Many aldehydes biodegrade rapidly by bacteria and fungi, minimizing long-term persistence.

Pollutant Sources

Industrial emissions, combustion processes, and cigarette smoke are major aldehyde sources.

References

• Smith, M. B.; March, J. March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 6th ed.; Wiley: New York, 2007; pp 123-156.
• Clayden, J.; Greeves, N.; Warren, S.; Wothers, P. Organic Chemistry, Oxford University Press, 2012, pp 432-460.
• Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry Part A: Structure and Mechanisms, 5th ed.; Springer, 2007; pp 215-245.
• March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 4th ed.; Wiley, 1992; pp 78-104.
• Finar, I. L. Organic Chemistry, Vol. 1, 6th ed.; Pearson Education, 2002; pp 350-380.