Acid mechanism

Most peptic ulcers occur in the presence of acid and pepsin when HP, NSAIDs, or other factors disrupt normal mucosal defense and healing mechanisms. Acid is an independent factor that contributes to disruption of mucosal integrity. Increased acid secretion has been observed in patients with DU and may result from HP infection. Patients with GU usually have normal or reduced rates of acid secretion. 

Barthes-Labrousse MG, Joud JC (2000) Acid-Base Characterization of metallic materials and the use of model molecules in the study of adhesion mechanism. Acid-Base Interactions Relevance to Adhesion Science and Technology, Mittal KL (ed) VSP Utrecht, 453... 

General Mechanism—Acid-Catalyzed Nucleophilic Addition 787... 

Ab initio methods have been used to compare enzyme-catalysed enolization mechanisms. Acid- and base-catalysed stepwise mechanisms have been compared with the concerted reaction the latter is favoured by several hydrogen-bonding interactions. 

The reaction of a tertiary alcohol with HX takes place by an SnI mechanism. Acid protonates the hydroxyl oxygen atom, water is expelled to generate a carbocation, and the cation reacts with nucleophilic halide ion to give the alkyl halide product (Figure 17.7). 

Figure 3.6 Reasonable basic problem space for the example proton transfer mechanism. Acid routes have been dropped. Only proton transfer routes tire shown. Routes deprotonating carbons 3, 4, and 5 have been dropped because the proton transfer ATeq is less than 10 , and thus not useful.

Buoyant force (compensation of forces) B Local, at the top electrolyte gassing acceleration forces inclination friction of mechanism acid stratification No High... 

There is another type of reaction of hemoglobin which can readily be explained by a similar radical mechanism. Acid or pyridine denature Hb4(02)4 and liberate two of the four 02 molecules, a third one being evidently required for the oxidation of four iron equivalents, while the... 

When the carbanion is formed in a rapid pre-equilibrium, isotopic exchange between the substrate and the solvent should be a faster process than formation of the olefin. The elimination reaction of 2-(p-nitrophenyl)ethyltrimethyl-ammonium ion in aqueous solution is accelerated by hydroxide ion but retarded by acid. This observation led to the postulation of the carbanion mechanism acid depressing the rate by reversing the equilibrium step (22). 

Both mechanisms, acid-catalyzed and base-catalyzed, consist of two proton-transfer steps proton abstraction from carbon and proton transfer to oxygen. The difference between the two is that the sequence of steps is reversed. Proton abstraction from the a carbon is the first step in the base-catalyzed mechanism it is the second step in the acid-catalyzed one. In each mechanism, proton abstraction from the a carbon is rate-determining. 

MECHANISM Acid-Catalyzed Dehydration of 2-Butanol—An E1 Reaction... 

MECHANISM Acid-Catalyzed Addition of an Alcohol to an Alkene ... 

MECHANISM Acid-Catalyzed Cleavage of a Dialkyl Ether... 

MECHANISM Acid-Catalyzed Equilibration of Keto and Enol Tautomers... 

Notice the difference between these two mechanisms (acidic vs. basic conditions). The first mechanism (basic conditions) has a negatively charged intermediate, and the second mechanism (acidic conditions) has a positively charged intermediate. Other than that, the difference between these two mechanisms is pretty small. Each mechanism has only two steps. And both steps are just proton transfers. The only qnestion is the seqnence of events. Is it deprotonate, then protonate Or is it protonate, then deprotonate ... 

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