Duality of the nucleophilic displacement mechanism

利用<18>~ 13 C-NMR谱，在35 ℃的乙腈中，利用~ 13 C-NMR谱中的~ 2 O/~ 2 O同位素取代引起的<13><18>烷氧基碳的分裂信号，测定了对甲苯磺酸对甲基苄酯-S_2O_2与N，N-二甲基苯胺的Menschutkin反应（典型的S_N_2置换反应<13><13><18><16>）中起始磺酸盐中~ 13 O的混乱。对于<13>起始甲苯磺酸酯的苄基碳的^ C峰，由^ O同位素效应引起的新的^<13>C峰<18>随着时间逐渐增长。在<18>不同DMA浓度下，测定了该反应的产物生成速率（k_t）和未反应甲苯磺酸盐的减少速率（k_i）。k_t和k_i值之差与DMA浓度无关，与DMA的亲核性无关，k_i-k_t值与DMA不<18>存在时对甲苯磺酸对甲基苄酯的~ 2 O-乱序速率相一致，这一结果为甲苯磺酸苄酯独立地以S_N1机理电离而不以S_N2机理电离提供了重要证据。在这个单分子电离过程中，随后的亲核攻击是如此之慢，以至于不能有效地去除离子对中间体的返回。结果表明，S_N_1和S_N_2过程是完全不同的，没有证据表明S_N_1和S_N_2之间存在单一的中间体。

The ^<18>O-scrambling within starting sulfonate during the Menschutkin reaction of pmethylbenzyl-1-^<13>C tosylate-S^<18>O_2 with N,N-dimethylanilines which is typical S_N2 displacement reaction, was determined in MeCN at 35ﾟC by ^<13>C-NMR spectroscopy utilizing the split ^<13>C-NMR signals of alkoxy carbon induced by ^<18>O/^<16>O isotopic substitution. For the ^<13>C peak of benzyl carbon of starting tosylates, a new ^<13>C peak caused by ^<18>O isotope effect gradually grew up with time. This finding of unambiguous ^<18>O-scrambling in starting tosylate demonstrated the existence of the carbocationic intermedates in the typical S_N2 reaction in MeCN.The rate of product formation (k_t) and the decreasing rate of unreacted tosylate (k_i) of this reaction are determined with various DMA concentrations. The difference between k_t and k_i values is unchanged with DMA conentrations, irrespective of the nucleophilicity of DMA.This k_i-k_t value is in accordance with the rate of ^<18>O-scrambling of p-methylbenzyl tosylate in the absence of DMA.This result gives the important evidence that benzyl tosylate ionizes independently via S_N1 mechanism irrespective of concurrent S_N2 mechansim. The subsequent nucleophilic attack in this unimolecular ionization process is so slow that the return from the ion-pair intermediate can not be removed effectively. It is concluded that the S_N1 and S_N2 processes are quite distinct that these results afford no evidence for a single mechanism intermediate between S_N1 and S_N2 in this reaction.