Syn addition is an addition in which the two new groups add to the same face of the former carbon–carbon double bond (C=C). It matters because it fixes the relative stereochemistry of the product and points to a mechanism in which both groups are delivered together from one side. The catalytic hydrogenation of an alkene, in which both hydrogens add from the catalyst surface, is syn addition. It is the opposite of anti addition.

顺式加成是两个新基团加到原碳碳双键(C=C)同一面的加成方式。它之所以重要,是因为它确定了产物的相对立体化学,并指向一种两个基团从同一侧一起加上的机理。烯烃的催化氢化(两个氢都从催化剂表面加上)即为顺式加成。它与反式加成相反。