a nodal position and unable to serve as a sink for +M groups (in contrast to a carbocation at the benzylic position). Thus for reactions involving carbocations at the α-position, the σp values for electron-donating groups will appear insufficiently negative. Based on similar considerations, a set of σp+ constants give better fit for reactions involving electron-donating groups at the para position and the formation of a carbocation at the benzylic site. The σp+ are based on the rate constants of the SN1 reaction of cumyl chlorides in 90% acetone/water: for , we define . Note that the scaling factor is negative, since an electron-donating group speeds up the reaction. For a reaction whose Hammett plot is being constructed, these alternative Hammett constants may need to be tested to see if a better linearity could be obtained. Rho value With knowledge of substituent constants it is now possible to obtain reaction constants for a wide range of organic reactions. The archetypal reaction is the alkaline hydrolysis of ethyl benzoate (R=R'=H) in a water/ethanol mixture at 30 °C. Measurement of the reaction rate k0 combined with that of many substituted ethyl benzoates ultimately result in a reaction constant of +2.498. Reaction constants are known for many other reactions and equilibria. Here is a selection of those provided by Hammett himself (with their values in parenthesis): the hydrolysis of substituted cinnamic acid ester in ethanol/water (+1.267) the ionization of substituted phenols in water (+2.008) the acid catalyzed esterification of substituted benzoic esters in ethanol (-0.085) the acid catalyzed bromination of substituted acetophenones (Ketone halogenation) in an acetic acid/water/hydrochloric acid (+0.417) the hydrolysis of substituted benzyl chlorides in acetone-water at 69.8 °C (-1.875). The reaction constant, or sensitivity constant, ρ, describes the susceptibility of the reaction to substituents, compared to the ionization of benzoic acid. It is equivalent to the slope of the Hammett plot. Information on the reaction and the associated mechanism can be obtained based on the value obtained for ρ. If the value of: ρ>1, the reaction is more sensitive to substituents than benzoic acid and negative charge is built during the reaction (or positive charge is lost). 0<ρ<1, the reaction is less sensitive to substituents than benzoic acid and negative charge is built (or positive charge is lost). ρ=0, no sensitivity to substituents, and no charge is built or lost. ρ<0, the reaction builds positive charge (or loses negative charge).