Aldehyds

Aldehydes are organic compounds containing the carbonyl group bonded to at least one hydrogen atom. The general structure is R-CHO, where R represents a hydrogen, an alkyl, or an aryl group. The simplest aldehyde is methanal (formaldehyde). Systematic IUPAC name endings are -al (alkanals), with examples such as ethanal (acetaldehyde) and benzaldehyde.

Nomenclature and structure: Aldehydes are named by identifying the alkyl or aryl group attached to the carbonyl carbon and adding the -al suffix. When the aldehyde is part of a larger molecule, the prefix indicates the substituent or the parent chain, while the aldehyde group remains the suffix.

Physical properties: Aldehydes are polar due to the carbonyl group, leading to higher boiling points than corresponding alkanes but generally lower than alcohols of similar size. Small aldehydes are often miscible with water; solubility decreases as the hydrocarbon portion grows. The carbonyl carbon is susceptible to nucleophilic addition, and many aldehydes form hydrates in water.

Reactions: Aldehydes undergo nucleophilic addition to the carbonyl, reduction to primary alcohols, and oxidation to carboxylic acids. Aldehydes without alpha hydrogens can undergo the Cannizzaro reaction in strong base. Aldol reactions occur for aldehydes with alpha hydrogens, enabling condensation to higher-molecular-weight products. Some aldehydes are produced by hydroformylation of alkenes (oxo process), giving aldehydes such as propanal.

Occurrences and uses: Formaldehyde is used in resins, plastics, and disinfection; acetaldehyde in fragrances and flavorings; benzaldehyde in almond flavors. Industrially, aldehydes arise from oxidation of primary alcohols or from hydroformylation of alkenes.

Safety: Aldehydes can be irritants and some, notably formaldehyde, are toxic or carcinogenic. Proper ventilation and handling are important.