A reactive intermediate is a short-lived, high-energy species that forms during a multi-step reaction and is consumed in a later step. Because it typically survives for less than a millisecond, it is rarely detected directly and is instead inferred from the products and kinetics. Reactive intermediates matter because they explain how a reaction proceeds step by step. Common examples include the carbenium ion, the carbanion and the free radical.

活性中间体是在多步反应过程中生成、并在随后某一步被消耗的短寿命高能物种。由于其寿命通常不足一毫秒,很少能被直接检测到,而是通过产物和动力学加以推断。活性中间体之所以重要,是因为它们解释了反应如何逐步进行。常见的例子包括碳正离子、碳负离子和自由基。