Halogeenal

Halogeenal, or halogenoaldehyde, is a class of organic compounds that contain both a halogen substituent and an aldehyde functional group. In most examples the halogen is on the carbon adjacent to the carbonyl group (the alpha position), giving haloacetaldehydes such as 2-chloroacetaldehyde (Cl−CH2−CHO), 2,2-dichloroacetaldehyde (CHCl2−CHO), or the trihalogenated chloral (trichloroacetaldehyde, CCl3−CHO). Mono-, di-, and polyhalogenated variants with fluorine, chlorine, bromine, or iodine are known.

Synthesis and occurrence: Halogenoacetaldehydes are typically prepared by enolization of aldehydes followed by halogenation at the alpha position, or by oxidation of corresponding haloalcohols. They can also arise from direct halogenation of aldehydes under appropriate conditions. Many halogeenal compounds are reactive and some are short-lived or oxidize readily, so they are often generated in situ for use as intermediates.

Properties and reactivity: The presence of halogen atoms increases the electrophilicity of the carbonyl carbon and influences the acidity of the alpha hydrogen. Alpha-halogenated aldehydes readily participate in nucleophilic additions and subsequent transformations, such as condensations or substitutions. Their reactivity is strongly influenced by the number and type of halogen substituents. Many halogeenal compounds are volatile and can be toxic or irritating, requiring careful handling.

Uses: Halogeenal compounds are important intermediates in organic synthesis, serving as building blocks for pharmaceuticals, agrochemicals, and dyes. Historically some halogenated aldehydes, such as chloral hydrate, have been used in medicinal or sedative contexts, but such applications are tightly regulated due to safety concerns.

See also: haloalkyl aldehydes, haloform chemistry, enolization and halogenation.