A concerted reaction is one in which bond breaking and bond forming occur simultaneously in a single step, with no intermediate being formed. It matters because such a reaction passes directly through one transition state, which has consequences for its rate law and its stereochemistry. The E2 and SN2 mechanisms are concerted, in contrast to the stepwise E1 and SN1 mechanisms, which pass through a carbenium-ion intermediate.

协同反应是指断键与成键在单一步骤中同时发生、不生成任何中间体的反应。它之所以重要,是因为这样的反应直接经过一个过渡态,这对其速率定律和立体化学都有影响。E2 和 SN2 机理是协同的,与分步进行、经由碳正离子中间体的 E1 和 SN1 机理形成对照。