Solvent Effects on Dipolar Transition State Reactions

2 Solvent Effects on Dipolar Transition State Reactions 

The simplest example of an aromatic nucleophilic substitution is the SnAt reaction between l-chloro-2,4-dinitrobenzene and piperidine, the two-step mechanism of which, given in Eq. (5-26), is now fully established [501-503], It involves formation of a Meisenheimer-type zwitterionic intermediate via a dipolar activated complex, followed 

C-C6H12 CfiHfi C6H5CI CH3COCH3 HC0N(CH3)2 CH3SOCH3 

In agreement with the separation of unlike charges during the activation process, an increase in rate by up to a factor of 50 with increasing solvent polarity has been found for reaction (5-26), carried out in thirteen non-HBD solvents [503], The absence of base-catalysis suggests that specific solvent effects are negligible in non-HBD solvents. In protic solvents, however, specific solvation of the piperidine nucleophile leads to a diminution in rate with increasing HBD ability of the hydroxylic solvents [503], 

As an example of a solvent-dependent electrophilic substitution reaction, the azo coupling (SsAr) reaction of 4-nitrobenzenediazonium tetrafluoroborate with N,N-dimethylaniline is given in Eq. (5-27) [504]. According to the two-step arenium ion mechanism, the activation process of the rate-limiting first step is connected with the dispersion of the positive charge. This should lead to a decrease in rate with increasing solvent polarity. 

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