Anti addition is an addition in which the two new groups add to opposite faces of the former carbon–carbon double bond (C=C). It matters because it fixes the relative stereochemistry of the product, and observing it provides evidence for a bridged intermediate. The bromination of an alkene, which proceeds through a bromonium ion, is anti addition. It is the opposite of syn addition, in which both groups add to the same face.

反式加成是两个新基团加到原碳碳双键(C=C)相反两面的加成方式。它之所以重要,是因为它确定了产物的相对立体化学,而观察到反式加成可为存在桥连中间体提供证据。烯烃经溴鎓离子进行的溴化反应即为反式加成。它与顺式加成相反,后者两个基团都加到同一面。