“nucleophilic substitution”的概念、定义、翻译、参考文献-科学参考

nucleophilic substitution

Chemistry

A type of substitution reaction in which a nucleophile displaces another group or atom from a compound. For example, in
${CR}_{3} {Cl + OH}^{-} \to {CR}_{3} {OH + Cl}^{-}$
the nucleophile is the OH⁻ ion. There are two possible mechanisms of nucleophilic substitution. In S_N1 reactions, a positive carbonium ion is first formed:
${CR}_{3} Cl \to {CR}_{3}^{+} {+ Cl}^{-}$
This then reacts with the nucleophile
${CR}_{3}^{+} {+ OH}^{-} \to {CR}_{3} OH$
The CR₃⁺ ion is planar and the OH⁻ ion can attack from either side. Consequently, if the original molecule is optically active (the three R groups are different) then a racemic mixture of products results.
The alternative mechanism, the S_N2 reaction, is a concerted reaction in which the nucleophile approaches from the side of the R groups as the other group (Cl in the example) leaves. In this case the configuration of the molecule is inverted. If the original molecule is optically active, the product has the opposite activity, an effect known as Walden inversion. The notations S_N1 and S_N2 refer to the kinetics of the reactions. In the S_N1 mechanism, the slow step is the first one, which is unimolecular (and first order in CR₃Cl). In the S_N2 reaction, the process is bimolecular (and second order overall).

单词	nucleophilic substitution
释义	nucleophilic substitution Chemistry A type of substitution reaction in which a nucleophile displaces another group or atom from a compound. For example, in ${CR}_{3} {Cl + OH}^{-} \to {CR}_{3} {OH + Cl}^{-}$ the nucleophile is the OH⁻ ion. There are two possible mechanisms of nucleophilic substitution. In S_N1 reactions, a positive carbonium ion is first formed: ${CR}_{3} Cl \to {CR}_{3}^{+} {+ Cl}^{-}$ This then reacts with the nucleophile ${CR}_{3}^{+} {+ OH}^{-} \to {CR}_{3} OH$ The CR₃⁺ ion is planar and the OH⁻ ion can attack from either side. Consequently, if the original molecule is optically active (the three R groups are different) then a racemic mixture of products results. The alternative mechanism, the S_N2 reaction, is a concerted reaction in which the nucleophile approaches from the side of the R groups as the other group (Cl in the example) leaves. In this case the configuration of the molecule is inverted. If the original molecule is optically active, the product has the opposite activity, an effect known as Walden inversion. The notations S_N1 and S_N2 refer to the kinetics of the reactions. In the S_N1 mechanism, the slow step is the first one, which is unimolecular (and first order in CR₃Cl). In the S_N2 reaction, the process is bimolecular (and second order overall).
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