Carbon protonation of some phenylynamines in concentrated aqueous perchloric acid solution. Lowering of the upper limit for the pKa of phenylynammonium ions

Abstract

Rates of carbon protonation of five phenylynamines (PhC≡CNH2, PhC≡CNHiPr, PhC=CNHC6F5, PhC≡CN(CH2CH2CN)2, and PhC≡CNMeC6F5) were determined in concentrated aqueous perchloric acid solution and the data were analyzed by the Cox–Yates method using the X0 acidity function. The extrapolated hydronium-ion catalytic coefficients so obtained are consistent with values measured directly in dilute acid solution, and the slopes of the Cox-Yates plots are similar to predictions made with the aid of Marcus rate theory for reactions originating from free ynamine initial states but unlike those predicted for reactions starting from nitrogen-protonated ynammonium ion initial states. This shows that none of these phenylynamines are protonated in even the most acidic solutions used (4 M) and sets new upper limits as low as pKa ≤ −3.1 for the conjugate acids of these ynamines. Comparison of the pKa limit for PhC≡CNH3+ with a literature value for the corresponding saturated compound, PhCH2CH2NH3+, gives a base-weakening effect for the phenylethynyl group of at least 12.5 pK units. Key words: acetylenic amines, concentrated acids, X0 excess acidity scale, Cox-Yates method, Marcus rate theory.