The Swain-Scott Equation Class II

The Swain-Scott equation (Equation 42) compares the reactivity of nucleophiles in an Sn2 mechanism with the reaction of nucleophiles with methyl bromide (Equation 42) as a reference reaction. Water is taken as the standard nucleophile (n = 0) and the equation can be written where the nucleophilicity (n) of a given nucleophile is defined by setting the reaction constant (5) for the reference reaction at unity (Equation 43). 

Since the reaction of nucleophiles with methyl iodide can be measured much more easily and accurately by spectroscopic analysis of the product iodide ion, a subsequent set of n values was devised using Equation (44). The n value of methanol with methyl iodide is set at zero and the reaction constant, s, at unity. Values of n derived from the methyl iodide reaction are tabulated in Appendix 3, Table 5. 

The slope of the Swain-Scott correlation, s = 0.26, indicates that the reaction is about one quarter as sensitive to change in the nucleophile as is the standard reaction with methyl halides (where 5=1.0). This result is consistent with the small value of (0.4) for the extended Bronsted slope for reaction of the substrate with primary aliphatic amines and a 

---