SN2反应SN2 reaction

亲核试剂的孤对电子进入 C–X 键的 σ* 反键轨道，因此只能从背面接近。过渡态中碳为五配位、三角双锥构型，三个取代基处于同一平面；离去后取代基像被风吹翻的雨伞一样翻转。速率方程 v = k[RX][Nu⁻]，为二级反应。

The nucleophile's lone pair feeds into the σ* orbital of the C–X bond, so it can only approach from the back. The transition state has a five-coordinate, trigonal-bipyramidal carbon with three substituents in one plane; as the leaving group departs they flip over like an umbrella in the wind. The rate law v = k[RX][Nu⁻] is second order.

1896 年瓦尔登发现，通过一系列取代反应可把 (−)-苹果酸转变为其对映体，即“瓦尔登翻转”。1935 年休斯与英戈尔德通过动力学与立体化学研究，提出 SN1 / SN2 命名，并把翻转归因于背面进攻。

In 1896 Walden found that a sequence of substitutions could turn (−)-malic acid into its enantiomer, the Walden inversion. In 1935 Hughes and Ingold, from kinetic and stereochemical studies, introduced the SN1 / SN2 labels and attributed inversion to backside attack.

位阻顺序为甲基 > 伯 > 仲 ≫ 叔，叔卤代烃基本不发生 SN2。好的离去基团（I⁻ > Br⁻ > Cl⁻ ≫ F⁻）和强亲核试剂有利于反应。极性非质子溶剂（DMSO、DMF、丙酮）不溶剂化阴离子，可使速率提高几个数量级。

The steric order is methyl > primary > secondary ≫ tertiary; tertiary halides essentially do not react by SN2. Good leaving groups (I⁻ > Br⁻ > Cl⁻ ≫ F⁻) and strong nucleophiles favour it. Polar aprotic solvents (DMSO, DMF, acetone) leave anions unsolvated and can raise the rate by orders of magnitude.

仲碳底物常在 SN2、SN1、E2 之间竞争：强碱、高温和位阻大的碱倾向 E2；质子性溶剂和能生成稳定碳正离子的底物倾向 SN1。合成中常用伯卤代烃配合弱碱性强亲核试剂（如 N₃⁻、CN⁻、I⁻）获得高选择性的 SN2。

Secondary substrates often compete among SN2, SN1 and E2: strong, bulky bases and heat favour E2, while protic solvents and substrates that form stable carbocations favour SN1. Synthetic chemists get clean SN2 by pairing primary halides with nucleophiles that are strong but weakly basic, such as N₃⁻, CN⁻ or I⁻.