The α-position, or alpha carbon, is the carbon atom directly adjacent to a functional group, together with any hydrogens attached to it. It matters because the neighbouring group can make the α-carbon and its α-hydrogens unusually reactive, for example acidic next to a carbonyl group (C=O). The α-carbon is defined by position relative to the functional group, and the next carbon along is correspondingly called the β-position.

α 位(α 碳)是指与官能团直接相邻的碳原子,以及连在其上的任何氢。它之所以重要,是因为相邻基团可使该 α 碳及其 α-氢表现出异常的反应活性,例如在羰基(C=O)旁边变得具有酸性。α 碳是相对于官能团的位置来定义的,再往外的下一个碳则相应地称为 β 位。