SN1 SN2 E1 E2

簡單來說有四種分類: 取代反應 SN1 SN2 (nucleophilic substitution) 及 脫去反應 E1 E2 (nucleophilic elimination)

SN1 SN2 :
顧名思義,就是利用親核性(nucleophile)強的原子上的電子對去攻打帶有離去基(leaving group)的碳,而也就是說離去基脫離了,那個親核性的原子也就接到碳上面.
那 麼1跟2的差別就在於離去基(leaving group)的問題,如果離去基先脫離,形成一個碳正離子(carbocation),然後親核原子(nucleophile)在接上去就是 SN1 ; 若是親核原子(nucleophile)直接就接上碳原子上而離去基還沒脫去,形成一個中間體的錯合物,之後再藉由電子的轉移,使得讓離去基再脫去,這就 是SN2

E1 E2 :
這種反應跟前者是類似的,也就是親核原子去攻打帶有離去基(leaving group)的碳之旁邊那個碳上的氫原子,再經由電子的轉移,讓離去基脫去,就會形成烯類的化合物(具有雙鍵).
那麼1跟2的分別,跟前者是一樣的,先脫去離去基的就為E1,反之同時進行就為E2.

http://www.bluffton.edu/~bergerd/classes/CEM221/sn-e/home.html


SN1

The S_N1 reaction

The S_N1 reaction is a two-step reaction in which

1. The leaving group leaves, forming a carbocation. This is the slow step, and so the rate is dependent only on the concentration of the substrate.
2. The nucleophile attacks the carbocation. It can do this from either side, typically in a 50/50 ratio. Therefore about half the product has retained the original configuration, and about half is inverted.
3. Protonated nucleophiles, such as methanol, then lose a proton to the solvent.

SN2

The S_N2 reaction

The S_N2 reaction is a single-step displacement of a leaving group by a nucleophile. During the transition state, the bond to the nucleophile forms at the same time that the bond to the leaving group breaks. Therefore the nucleophile is required to approach from the back, and configuration at carbon is inverted.

The E1 reaction

The E1 reaction is a two-step reaction in which

1. The leaving group leaves, forming a carbocation. This is the slow step, and so the rate is dependent only on the concentration of the substrate.
2. The base deprotonates the carbocation to form the alkene.

The E2 reaction

The E2 reaction is a single-step displacement of a leaving group by the electrons from a vicinal C-H bond. During the transition state, the C=C bond forms at the same time that the bonds to the leaving group and the a-hydrogen break.