Hammett susceptibility factor

As indicated below, 1-naphthylamine and quinoline exhibit very different susceptibility factors p (2.81 versus 5.90) in the corresponding Hammett equations. Try to explain this fact. 

Predictive Capability, Several empirical structure-activity relationships for oxidation by RO2 (polyani relation), HO (Hammett sigma-rho relations), and 02 have been developed. A large data base is available to estimate reliably the rate constants for new chemicals within a factor of 3-5. Precise evaluation of rate constants for highly susceptible compounds such as phenols, aromatic amines, alkyl sulfides, and electron-rich olefins and dienes is needed. Oxidation of saturated alkyl compounds including alkanes, haloalkanes, esters, and ketones is slow in water and air. Most chemicals excepting the above-mentioned compounds are readily oxidized by HO radicals. 

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