semialdehyde

Semialdehyde is a term used in organic chemistry and biochemistry to describe a molecule that contains an aldehyde functional group (-CHO) together with at least one other functional group within the same molecule. The aldehyde group endows the molecule with high reactivity toward nucleophiles and oxidants, while the additional substituents influence its stability and reactivity. In many contexts semialdehydes are considered intermediates that lie between an alcohol or polyol and a carboxylic acid on an oxidation pathway, or between open-chain and cyclic forms in carbohydrate chemistry.

Semialdehydes are often formed by partial oxidation of primary alcohols or by selective oxidation of polyols, and they can arise transiently in metabolic reactions. In solution they may exist as hydrates or as cyclic hemiacetals, depending on the structure and conditions. The aldehyde functionality makes semialdehydes prone to further oxidation to carboxylic acids and to condensation or nucleophilic addition reactions such as aldol condensations or reductive aminations.

The term "semialdehyde" does not have a single strict definition across all fields. Some authors apply it to any hydroxyaldehyde or aldehyde-containing intermediate, while others reserve it for open-chain species that possess an aldehyde group along with another reactive functionality. As such, the precise interpretation may vary with context, but the general idea is that semialdehydes are aldehyde-bearing intermediates distinguished by additional functional groups.