Effect of nucleophileOne of the most fascinating problems involved in nucleophilic displacement reactions is that of trying to understand what properties of a reagent make it a good nucleophile. There have been several attempts to correlate the extrakinetic properties of the reagent with its reactivity, and some of these have been moderately successful. All of them are based on the principle of linear free-energy relationship (L.F.E.R.). One point that has become abundantly clear is that, just as there is no one scale of acid-base strengths, also there is no scale of nucleophilic reactivities. The substrate is most important and must be considered in any discussion of relative order of nucleophilic strengths of various reagents. Fortunately, a qualitative but useful generalization is possible. The existing kinetic data on many different substrates show that soft (polarizable) nucleophilic are most effective towards soft substrates.Similiarly hard nucleophiles, such as OH- are most effective towards hard substrates. Afurther rule is that polarizability in the nucleophile as always more important for rates than for equilibria.Bronsted successfully applied the L.F.E.R. orinciple to displacement reactions at hydrogen in various compounds. He found a linear relationship between the log of the rate constant for protom transfer and the log of the ordinary base strength of of the reagent. It should be noted that the base strengths are towards the acid H+, and the displacements also involve an attack on hydrogen. Swain and Scott found that the rates of displacement at carbon do not adequately parallel the base strength of the reagent and used the rates of reaction of methyl bromide to prepare a nucleophilicity scale towards carbon. Edward made the significant suggestion that in order to account for the nucleophilic properties of different reagents a two-parameter equation would be necessary. His equations are given in Chapter 3 (p.138).