Operational scale of hydronium ion activities for strongly acidic media

Self Cite

This paper is dedicated to Prof. Raymond U . Lemieux on the occasion of his 60th birthday PETER WAN, TOMASZ A. KEITH YATES. Can. J. Chem. 58,2423 (1980). The acid-catalysed hydrolysis of lactams of ring size 4-7, and of N-ethylacetamide, has been studied at various temperatures in 0-70% aqueous sulfuric acids. The kinetics have been analysed by means of the r-hydration parameter treatment and the transition-state activity coefficient (TSAC) approach. Except for the (3-lactam, all substrates show a strongpos~tiverate dependence on water activity, indicating a tetrahedral intermediate type of mechanism (A82), similar to that found for simple (acyclic) amide hydrolyses; only the (3-lactam shows a negative rate -water activity dependence, which is interpreted in terms of a unimolecular (Aal) mechanism, involving rate-determining acylium ion formation. Similarly, all substrates except the (3-lactam show a pronounced destabilization of the transition-state as ac~dity is increased, typical of oxonium Ion specles. The behaviour of the (3-lactam resembles that of methyl mesitoate hydrolys~s. Basicity constants for lactams of ring size 4-9 show a definite dependence on ring s u e , with the 6-lactam being unusually basic. Both rate and pK dependence on ring size are discussed In terms of ring strain and steric effects. The activat~on parameters, particularly AS*, are fully consistent with the conclus~ons drawn concerning transitionstate hydration.

mentioning

confidence: 99%

The kinetics and mechanism of acid catalysed hydrolysis of lactams

Wan¹,

Modro²,

Yates³

1980

Self Cite

“…Moreover the experimental HGF values are spread over wide acidity ranges. These facts might suggest a choice of the excess acidity function of ferrocene as the reference, notwithstanding some approximations (15). W e have followed this suggestion in order to obtain plots (e) and (f) in Fig.…”

Section: Discussionmentioning

confidence: 97%

The hydration parameter and the excess acidity functions in H₂SO₄–H₂O and HClO₄–H₂O media

Traverso¹

1984

PIETRO GIUSEPPE TRAVERSO. Can. J. Chem. 62, 153 ( 1984). Excess acidities, X, for aqueous H'SO, and HCIO, arc derived under conditions of fixed pKBbI+ and 111:' : in both media. and of fixed m* only, and compared among themselves and with newly derived and previous scales obtained without constraint, or with fixed pK,,+ valucs. Analysis shows few or no real diffcrcnces among these scalcs. which means that the equation log I + log C,,+ = pK,H+ + nl:':X applies without the necessity of involving a "second-order" ni*:~"X'terni, at least for thcsc media.PIETRO GIUSEPPE TRAVERSO. Can. J . Chcm. 62, 153 (1984). On a derive les aciditis en exctts, X. dcs milieux aqucux dc HISO, ct de HClO, pour dcs conditions ou le PKRII. ct le III": sont fixes dans les deux milieux ainsi que lorsqu'uniqucnicnt m* est fixit; on compare ccs valeurs cntre clles, avcc Ics nouvellcs itchelles ainsi qu'avec les ancicnnes Cchclles obtenucs sans contraintc ou avcc dcs valcurs fixits de pK,,,. . L'analyse ne rkvhle que peu ou pas de diffkrence riellc parmi ccs Cchcllcs. ce qui signifie quc I'itquation log I + log CI.,5 = pK,,,t. + tn:%X s'applique sans la nCcessitC dc fairc intcrvcnir un terme du "second dcgrit" I~"""x', au moins pour ccs milicux.ITraduit par Ic journal]

“…1 was calculated using this equation, assuming log k/K, = -0.69 and log vH,A*+/fT+) = 0. The latter assumption is unlikely to be exactly true (9), particularly in the higher acid concentrations, and protonation of the carboxyl of HA may follow an acidity function differing slightly from HA (8),' SO the deviations of the experimental points from the theoretical curve at the higher acid concentrations are not serious.…”

Section: Co;mentioning

confidence: 98%

“…Furthermore, the decreasing activity of water as the sulfuric acid concentration is increased to 71.6% will result in a progressive loss of stabilizing hydration from both HzAf and T'. In spite of its more diffuse charge, T= is likely to receive greater stabilization from hydration than H2At , because oxonium ions are more hydrated than ammonium ions (9).…”

Section: Co;mentioning

confidence: 99%

Rate of lactamization of benzylstrychnine in acid solution

Edward¹,

Wong²,

McClelland³

1984

The rate of lactamization of benzylstrychnine in aqueous solution at 25 °C increases with acid concentration up to 5–10% sulfuric acid, and thereafter decreases. The maximum can be explained by taking account of the extent of protonation of the aromatic amino group with increasing acid concentration. This amino group is unusually weakly basic [Formula: see text], perhaps because it is locked in an extremely rigid cage structure which opposes the rehybridization of N which must take place when it is protonated.