In organic chemistry, a semialdehyde is a compound containing an aldehyde and a carboxylic acid functional groups. Semialdehydes are common in biochemistry. The simplest semialdehydes have the formula . As illustrated by the behavior of the smallest member, glyoxylic acid, semialdehydes often exist as hydrates (geminal diols) .
Some of semialdehydes and their parent dicarboxylic acids are listed below.
Although structurally related to semialdehydes, dicarboxylic acids are rarely biochemical precursors to them.