Amide hydrolysis. VII. Proton inventories of transition states for solvation catalysis and proton-transfer catalysis. Decomposition of the tetrahedral intermediate in amide methanolysis

1973 ◽  
Vol 95 (10) ◽  
pp. 3280-3283 ◽  
Author(s):  
Charles R. Hopper ◽  
Richard L. Schowen ◽  
K. S. Venkatasubban ◽  
H. Jayaraman
Keyword(s):  
Proton Transfer ◽  
Transition States ◽  
Tetrahedral Intermediate ◽  
Amide Hydrolysis

A comparison of the mechanisms of hydrolysis of benzimidates, esters, and amides in sulfuric acid media

2005 ◽  
Vol 83 (9) ◽  
pp. 1391-1399 ◽  
Author(s):  
Robin A Cox
Keyword(s):  
Sulfuric Acid ◽  
Isotope Effects ◽  
Ester Hydrolysis ◽  
Water Molecules ◽  
Acid Media ◽  
Tetrahedral Intermediate ◽  
Amide Hydrolysis ◽  
Solvent Isotope ◽  
Reaction Media ◽  
Hydrolysis Of

The mechanisms given in textbooks for both ester and amide hydrolysis in acid media are in need of revision. To illustrate this, benzimidates were chosen as model compounds for oxygen protonated benzamides. In aqueous sulfuric acid media they hydrolyze either by a mechanism involving attack of two water molecules at the carbonyl carbon to give a neutral tetrahedral intermediate directly, as in ester hydrolysis, or by an SN2 attack of two water molecules at the alkyl group of the alkoxy oxygen to form the corresponding amide, or by both mechanisms, depending on the structure of the benzimidate. The major line of evidence leading to these conclusions is the behavior of the excess acidity plots resulting from the rate constants obtained for the hydrolyses as functions of acid concentration and temperature. The first of these mechanisms is in fact very similar to one found for the hydrolysis of benzamides, as inferred from: (1) similar excess acidity plot behaviour; and (2) the observed solvent isotope effects for amide hydrolysis, which are fully consistent with the involvement of two water molecules, but not with one or with three (or more). This mechanism starts out as essentially the same one as that found for ester hydrolysis under the same conditions. Differences arise because the neutral tetrahedral intermediate, formed directly as a result of the protonated substrate being attacked by two water molecules (not one), possesses an easily protonated nitrogen in the amide and benzimidate cases, explaining both the lack of 18O exchange observed for amide hydrolysis and the irreversibility of the reaction. Protonated tetrahedral intermediates are too unstable to exist in the reaction media; in fact, protonation of an sp3 hybridized oxygen to put a full positive charge on it is extremely difficult. (This means that individual protonated alcohol or ether species are unlikely to exist in these media either.) Thus, the reaction of the intermediate going to product or exchanged reactant is a general-acid-catalyzed process for esters. For amide hydrolysis, the situation is complicated by the fact that another, different, mechanism takes over in more strongly acidic media, according to the excess acidity plots. Some possibilities for this are given.Key words: esters, amides, benzimidates, hydrolysis, excess acidity, mechanism, acid media.

Understanding difficulties of irregular number-membered ring transition states for intramolecular proton transfer in excited state

Tetrahedron ◽  
2017 ◽  
Vol 73 (4) ◽  
pp. 403-410 ◽  
Author(s):  
Xiaozhuan Qin ◽  
Ge Ding ◽  
Zhenqiang Wang ◽  
Yulong Gong ◽  
Fang Gao ◽  
...  
Keyword(s):  
Excited State ◽  
Proton Transfer ◽  
Transition States ◽  
Intramolecular Proton Transfer ◽  
Membered Ring

Hydration of acetone in the gas phase and in water solvent

2010 ◽  
Vol 88 (1) ◽  
pp. 56-64 ◽  
Author(s):  
Kiyull Yang ◽  
Yih-Huang Hsieh ◽  
Chan-Kyung Kim ◽  
Hui Zhang ◽  
Saul Wolfe
Keyword(s):  
Proton Transfer ◽  
Gas Phase ◽  
Isotope Effects ◽  
Bond Formation ◽  
Transition States ◽  
Water Solvent ◽  
Cooperative Process

In water solvent, the hydration of acetone proceeds by a cyclic (cooperative) process in which concurrent C–O bond formation and proton transfer to oxygen take place through a solvent and (or) catalyst bridge. Reactivity is determined primarily by the concentration of a reactant complex and not the barrier from this complex. This situation is reversed in the gas phase; although the concentrations of reactive complexes are much higher than in solution, the barriers are also higher and dominant in determining reactivity. Calculations of isotope effects suggest that multiple hydron transfers are synchronous in the gas phase to avoid zwitterionic transition states. In solution, such transition states are stabilized by solvation and hydron transfers can be asynchronous.

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