Duality of the nucleophilic displacement mechanism

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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.