立体专一性Stereospecificity

SN2 反应：构型翻转（瓦尔登翻转）；E2：反式共平面消除，(1R,2R)/(1R,2S) 底物分别给出 E/Z 烯烃；溴对烯烃反式加成：顺-2-丁烯得外消旋二溴化物，反-2-丁烯得内消旋；过氧酸环氧化、西蒙斯–史密斯、狄尔斯–阿尔德反应保持烯烃顺反关系；电环化（顺旋/对旋）。

SN2 inversion (Walden); E2 anti-periplanar elimination giving E or Z from diastereomeric substrates; anti addition of Br₂ (cis-2-butene → racemic dibromide, trans → meso); peracid epoxidation, Simmons–Smith and Diels–Alder preserve alkene geometry; electrocyclic con/disrotation.

立体专一性是协同或高度有序机理的证据；若产物立体化学“混乱”（如 SN1 的部分消旋），则提示存在平面中间体（碳正离子、自由基）。

Stereospecificity is evidence for concerted or tightly ordered mechanisms; scrambled stereochemistry (partial racemisation in SN1) points to planar intermediates (cations, radicals).

IUPAC 把立体专一性定义为机理性质；酶反应的“立体专一性”有时也指只作用于某一立体异构体（底物专一性）。

IUPAC defines stereospecificity as a mechanistic property; for enzymes the word is also used for acting on only one stereoisomer (substrate specificity).