A tetrahedral intermediate is the unstable, sp³-hybridised species formed when a nucleophile adds to a carbonyl carbon, converting the flat sp² carbon of the carbon–oxygen double bond (C=O) into a four-coordinate carbon. It matters because it is the pivotal intermediate of nucleophilic acyl substitution: whether it expels the original leaving group or the newly added nucleophile decides the product. In a substitution it collapses to re-form a C=O, whereas in a simple addition it is instead protonated to give the final product.

四面体中间体是亲核试剂加到羰基碳上时所形成的、不稳定的 sp³ 杂化物种,它把碳氧双键(C=O)中平面的 sp² 碳转变为四配位的碳。它之所以重要,是因为它是亲核酰基取代的关键中间体:它排出原来的离去基团还是新加上的亲核试剂,决定了产物。在取代中它塌缩以重新形成 C=O,而在简单加成中它则被质子化以给出最终产物。