Acid-base reactions mechanisms

Proton transfers between oxygen and nitrogen acids and bases are usually extremely fast. In the thermodynamically favored direction, they are generally diffusion controlled. In fact, a normal acid is defined as one whose proton-transfer reactions are completely diffusion controlled, except when the conjugate acid of the base to which the proton is transferred has a pA value very close (differs by g2 pA units) to that of the acid. The normal acid-base reaction mechanism consists of three steps ... 

The determination of the metal ions affinity for the organic bti.ses is important to elucidate the acid-base reaction mechanism as well as to obtain information about fundamental biochemical processes (i.e. synthesis, replication structural integrity and cleavage of DNA and RNA) [ 108J. 

Our first three chapters established some fundamental principles concerning the structure of organic molecules and introduced the connection between structure and reactivity with a review of acid-base reactions In this chapter we explore structure and reactivity m more detail by developing two concepts functional groups and reaction mechanisms A functional group is the atom or group m a molecule most respon sible for the reaction the compound undergoes under a prescribed set of conditions How the structure of the reactant is transformed to that of the product is what we mean by the reaction mechanism... 

The first step of this new mechanism is exactly the same as that seen earlier for the reaction of tert butyl alcohol with hydrogen chloride—formation of an alkyloxonmm ion by proton transfer from the hydrogen halide to the alcohol Like the earlier exam pie this IS a rapid reversible Brpnsted acid-base reaction... 

Rainwater and snowmelt water are primary factors determining the very nature of the terrestrial carbon cycle, with photosynthesis acting as the primary exchange mechanism from the atmosphere. Bicarbonate is the most prevalent ion in natural surface waters (rivers and lakes), which are extremely important in the carbon cycle, accoxmting for 90% of the carbon flux between the land surface and oceans (Holmen, Chapter 11). In addition, bicarbonate is a major component of soil water and a contributor to its natural acid-base balance. The carbonate equilibrium controls the pH of most natural waters, and high concentrations of bicarbonate provide a pH buffer in many systems. Other acid-base reactions (discussed in Chapter 16), particularly in the atmosphere, also influence pH (in both natural and polluted systems) but are generally less important than the carbonate system on a global basis. 

Where the bond between the metal and the ring is covalent, the usual arenium ion mechanism operates. Where the bonding is essentially ionic, this is a simple acid-base reaction. For the aliphatic counterpart of this reaction, see Reaction 12-23. 

There are always two arrows. One is drawn coming from the base and grabbing the proton. The second arrow is drawn coming from the bond (between the proton and whatever atom is connected to the proton) and going to the atom currently connected to the proton. That s it. There are always two arrows. Each arrow has a head and a tail, so there are four possible mistakes you can make. You might accidentally draw either of the heads incorrectly, or you might draw either of the tails incorrectly. With a little bit of practice you will see just how easy it is, and you will realize that acid-base reactions always follow the same mechanism. 

EXERCISE 3.38 Show the mechanism for the following acid-base reaction ... 

They considered that cement formation was the result of an acid-base reaction leading to the formation of hydrates by a through-solution mechanism, by nucleation and precipitation from pore fluids. Two phases were found in the matrix, one amorphous and the other crystalline. The crystalline phase was newberyite. Finch Sharp concluded that the amorphous phase was a hydrated form of aluminium orthophosphate, AIPO4, which almost certainly contained magnesiiun. They ruled out a pure AlP04.nH20, for they considered that the reaction could not be represented by the equation... 

The hydrogenation of simple alkenes using cationic rhodium precatalysts has been studied by Osborn and Schrock [46-48]. Although kinetic analyses were not performed, their collective studies suggest that both monohydride- and dihydride-based catalytic cycles operate, and may be partitioned by virtue of an acid-base reaction involving deprotonation of a cationic rhodium(III) dihydride to furnish a neutral rhodium(I) monohydride (Eq. 1). This aspect of the mechanism finds precedent in the stoichiometric deprotonation of cationic rhodium(III) dihydrides to furnish neutral rhodium(I) monohydrides (Eq. 2). The net transformation (H2 + M - X - M - H + HX) is equivalent to a formal heterolytic activation of elemental... 

In a previous study [33] we confirmed the reaction mechanism proposed by Harris et al. [32] for 40T, which is known to be a general acid-base reaction that takes place in two steps (see Figure 3-1). In the first step, one of the hydrogens from C3... 

First there are the physical chemists, chemical engineers, and surface scientists, who study mainly nonpolar hydrocarbon reactions on clean and relatively clean metals and metal oxides. These have been the traditional studies formerly driven by the petroleum industry and now driven by environmental concerns. These workers typically treat the surface as a real entity composed of active sites (usually not identified, but believed in). These investigators typically, although not always, interpret mechanisms in terms of radical reactions on metals and in terms of acid-base reactions on metal oxides. 

In acid-base catalysis there is at least one step in the reaction mechanism that consists of a generalized acid-base reaction (a proton transfer between the catalyst and the substrate). The protonated or deprotonated reactant species or intermediate then reacts further, either with... 

The transition between the two limiting situations is a function of the parameter (k-e/kc)Cp. The ratio between the catalytic peak current, ip, and the peak current of the reversible wave obtained in the absence of substrate, Pp, is thus a function of one kinetic parameter (e.g., Xe) of the competition parameter, (k e/A c)c and of the excess ratio y = C /Cp, where and Cp are the bulk concentrations of the substrate and catalyst, respectively. In fact, as discussed in Section 2.6, the intermediate C, obtained by an acid-base reaction, is very often easier to reduce than the substrate, thus leading to the redox catalytic ECE mechanism represented by the four reactions in Scheme 2.13. Results pertaining to the EC mechanism can easily be transposed to the ECE mechanism by doubling the value of the excess factor. 

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