Technique of Quasi-symmetrical Reactions

in a reaction of a series of nucleophiles (NU2) with A-Nuj a plot of logkoveraii vcrsus a polarity parameter of the nucleophile (for example pAa or a) will yield a break when the value of the parameter for NU2 is that for the constant nucleofuge Nuj. A similar result would occur for 

We identify the leaving group as Nu, to emphasise the symmetry of the reaction the leaving group in a nucleophilic displacement reaction, although it is a nucleophile, is usually identified as Lg. 

The absence of a break-point at the predicted value of the polarity parameter can in principle exclude an association intermediate (N-U2 .A Nuf) and thus indicate that the mechanism involves only a single transition structure. 

The parameter Ap is defined as P2 - p i. A similar equation can be written for Hammett dependencies with p replacing p and a replacing pK. If the investigator is fortunate enough to be studying a system where the reverse reaction can be followed, Equation (12) can also be fitted to the data. Only one such system is at present known to have been investigated in this way.  

The quasi-symmetrical technique relies on a relatively smooth variation of rate with substituent parameter. In practice, due attention must be paid to uncertainties in data fitting which arise from microscopic medium effects (Chapter 6). A reliable conclusion demands a large number of data which moreover cover a substantial range above 

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