Acid-Catalyzed Dehydration of Naphthalene-cis-1,2-dihydrodiols: Origin of Impaired Resonance Effect of 3-Substituents

Acid-catalyzed dehydrations of substituted naphthalene-cis-1,2-dihydrodiols occur with loss of the 1- or 2-OH group to form 2- and 1-naphthols, respectively. Effects of substituents MeO, Me, H, F, Br, I, and CN at 3-, 6-, and 7-positions of the naphthalene ring are consistent with rate-determining formation of β-hydroxynaphthalenium ion (carbocation) intermediates. For reaction of the 1-hydroxyl group the 3-substituents are correlated by the Yukawa–Tsuno relationship with ρ = −4.7 and r = 0.25 or by σp constants with ρ = −4.25; for reaction of the 2-hydroxyl group the 3-substituents are correlated by σm constants with ρ = −8.1. The correlations for the 1-hydroxyl imply a surprisingly weak resonance interaction of +M substituents (MeO, Me) with a carbocation reaction center but are consistent with the corresponding correlation for acid-catalyzed dehydration of 3-substituted benzene-cis-1,2-dihydrodiols for which ρ = −6.9 and r = 0.43. Substituents at the 6- and 7-positions of the naphthalene rings by contrast are correlated by σ+ with ρ = −3.2 for reaction of the 1-hydroxyl group and ρ = −2.7 for reaction of the 2-hydroxyl group. The unimpaired resonance implied by these substituent effects appears to be inconsistent with a previous explanation of the weak resonance of the 3-substituents in terms of imbalance of charge development and/or nonplanarity of the benzenium ring in the transition state. An alternative possibility is that the adjacent hydroxyl group interferes sterically with conjugation of +M substituents. “Hyperaromaticity” of the arenium ion intermediates does not appear to be a factor influencing this behavior.