Acid-base reactions, fast

A. (The gas phase estimate is about 100 picoseconds for A at 1 atm pressure.) This suggests tliat tire great majority of fast bimolecular processes, e.g., ionic associations, acid-base reactions, metal complexations and ligand-enzyme binding reactions, as well as many slower reactions that are rate limited by a transition state barrier can be conveniently studied with fast transient metliods. 

The second stage is a Brpnsted acid-base reaction and is fast... 

Step 3 This step is a fast acid base reaction that follows the nucleophilic substitution Water acts as a base to remove a proton from the alkyloxonium ion to give the observed product of the reaction tert butyl alcohol... 

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 ... 

In contrast, for cases where the protein is more rigid, the standard continuum approach can give excellent results. A striking example is the case of photosystems and redox proteins, where a low reorganization is needed to maintain fast charge-transfer kinetics. For these systems, carefully parameterized continumm models can give an accurate picture of redox potentials and their coupling to acid/base reactions [126-128],... 

The acid-base reaction is a simple example of using the mixture fraction to express the reactant concentrations in the limit where the chemistry is much faster than the mixing time scales. This idea can be easily generalized to the case of multiple fast reactions, which is known as the equilibrium-chemistry limit. If we denote the vector of reactant concentrations by and assume that it obeys a transport equation of the form... 

For non-linear chemical reactions that are fast compared with the local micromixing time, the species concentrations in fluid elements located in the same zone cannot be assumed to be identical (Toor 1962 Toor 1969 Toor and Singh 1973 Amerja etal. 1976). The canonical example is a non-premixed acid-base reaction for which the reaction rate constant is essentially infinite. As a result of the infinitely fast reaction, a fluid element can contain either acid or base, but not both. Due to the chemical reaction, the local fluid-element concentrations will therefore be different depending on their stoichiometric excess of acid or base. Micromixing will then determine the rate at which acid and base are transferred between fluid elements, and thus will determine the mean rate of the chemical reaction. 

Note that A is called the conjugate base of HA and BH+ the conjugate acid of B. Proton transfer reactions as described by Eq. 8-1 are usually very fast and reversible. It makes sense then that we treat such reactions as equilibrium processes, and that we are interested in the equilibrium distribution of the species involved in the reaction. In this chapter we confine our discussion to proton transfer reactions in aqueous solution, although in some cases, such reactions may also be important in nonaqueous media. Our major concern will be the speciation of an organic acid or base (neutral versus ionic species) in water under given conditions. Before we get to that, however, we have to recall some basic thermodynamic aspects that we need to describe acid-base reactions in aqueous solution. 

Experimentally it has proven very difficult to investigate bimolecular reaction kinetics. Although optical techniques have been developed with femtosecond time resolution, the bimolecular nature of the reactions precludes standard femtosecond pump-probe experiments, as a common starting time for the reaction is not readily established. Here we initiate the fast bimolecular acid-base reaction by converting a very weak base, NO3 to a weak base ONOO" and follow the reaction ONOO + H+ as a function of [ft]. 

From these experiments, some general conclusions can be drawn concerning the behavior of small alkanes in the strongest HF-SbF5 system. (1) The reversible protonation of the alkanes (i) is very fast in comparison with the ionization step (ii) it takes place on all cr-bonds independently of the subsequent reactivity of the alkane (iii) it involves carbonium ions (transition states), which do not undergo molecular rearrangements. (2) Protonation of an alkane is atypical acid-base reaction and carbon monoxide has no effect on this step. 

The neutralisation of acids with bases provides many valuable volumetric methods of chemical analysis and redox titrations are useful as well. But here we encounter an important difference between acid/base and redox reactions in solution. Acid/base reactions which involve the transfer of protons are very fast indeed they are usually instantaneous for all practical purposes. In protonic solvents, polar H-X bonds are very labile and undergo rapid proton exchange. For example, if B(OH)3 - a very weak acid - is recrystallised from D20, we obtain a fully-deuterated product. Redox reactions, on the other hand, are often very slow under ordinary conditions. To return to the analogy between acid/base and redox titrations, many readers will be familiar with the reaction between permanganate and oxalic acid the reaction is very slow at room temperature and, for titrimetric purposes, should be carried out at about 60 °C. The mechanism whereby a redox reaction takes place tends to be... 

The first step in this mechanism is a relatively slow reaction. (The activation energy for this step is roughly 80 kJ/mol.) If this reaction is done in water, the next step is extremely fast. The (CH3)3C+ ion is a Lewis acid because it has an empty orbital that can be used to accept a pair of electrons. Water, on the other hand, is a reasonably good Lewis base. A Lewis acid-base reaction therefore rapidly occurs in which a pair of nonbonding electrons on a water molecule are donated to the carbocation to form a covalent C—O bond. 

For the reaction experiment, a fast acid-base reaction was applied which affects the fluorescence quantum yield of a dye by changing the pH. Fluorescein again was used [25]. 

Oil burners are jet tube reactors. Jet washers are used for fast reactions such as acid-base reactions. An example is the absorption of hydrochloric acid in sodium hydroxide-sodium sulfite solutions. 

An amino acid protected only at the N atom and a different amino acid in which only the C02H group is protected do not react with each other to form a peptide bond. On the contrary, they form an ammonium carboxylate in a fast acid/base reaction (Figure 6.29). Ammonium car-boxylates can in principle be converted into amides by strong heating. Thus, for example, in the industrial synthesis of nylon-6,6, the diammonium dicarboxylate obtained from glutamic acid and hexamethylenediamine is converted to the polyamide at 300 °C. However, this method is not suitable for peptide synthesis because there would be too many undesired side reactions. 

As you should recall from general chemistry, a favorable equilibrium constant is not sufficient to ensure that a reaction will occur. In addition, the rate of the reaction must be fast enough that the reaction occurs in a reasonable period of time. The reaction rate depends on a number of factors. First, the reactants, in this case the acid and the base, must collide. In this collision the molecules must be oriented properly so that the orbitals that will form the new bond can begin to overlap. The orientation required for the orbitals of the reactants is called the stereoelectronic requirement of the reaction. (,Stereo means dealing with the three dimensions of space.) In the acid-base reaction, the collision must occur so that the atomic orbital of the base that is occupied by the unshared pair of electrons can begin to overlap with the is orbital of the acidic hydrogen. 

Because the activation energies are small and the stereoelectronic requirements are not difficult to meet, most acid-base reactions are very fast in comparison to other types of organic reactions. Therefore, it is usually not necessary to be concerned with the rates of acid-base reactions. In organic reactions that have mechanisms involving several steps, including an acid-base step, one of the other steps in the mechanism usually controls the rate. 

Although these additions to CO double bonds have some superficial similarities to the electrophilic additions to CC double bonds that were presented in Chapter 11, there are many differences. The acidic conditions mechanism here resembles the mechanism for addition to carbon-carbon double bonds in that the electrophile (the proton) adds first, followed by addition of the nucleophile. However, in this case the first step is fast because it is a proton transfer involving oxygen, a simple acid-base reaction. The second step, the attack of the nucleophile, is the rate-determining step. (Recall that it is the first step, the addition of the electrophile, that is slow in the additions to CC double bonds.) Furthermore, in the case of additions to simple alkenes there is no mechanism comparable to the one that operates here under basic conditions, in which the nucleophile adds first. Because the nucleophile adds in the slow step, the reactions presented in this chapter are termed nucleophilic additions, even if the protonation occurs first. In... 

If the equilibrium constant of the chemical reaction (such as complex stability constant, hydration-dehydration equilibrium constant, or the piCa of the investigated acid-base reaction) is known, limiting currents can be used to calculate the rate constant of the chemical reaction, generating the electroactive species. Such rate constants are of the order from 104 to 1010 Lmols-1. The use of kinetic currents for the determination of rate constants of fast chemical reactions preceded even the use of relaxation methods. In numerous instances a good agreement was found for data obtained by these two independent techniques. 

The effect of some of the factors mentioned above can be excluded by a proper choice of the type of substance studied. For example, for iodo-phenols and iodoanilines effects of hydration and tautomeric change seem improbable. The effects of adsorption and surface reactions is more difficult to exclude. This type of complication is more frequently encountered with acid-base reactions than with other types of reaction because the former are usually very fast reactions. For such a type of reaction the layer around the electrode in which the chemical reaction takes place is very thin, and the reaction therefore occurs mainly inside the electrical double-layer where the effects of the electrical field are strongest. The slower the chemical reaction the less probable the surface reaction is. 

A few points should be noted. (1) Bases may be anionic or neutral, and acids may be neutral or cationic. (2) The acid-base reaction is an equilibrium. The equilibrium may lie far to one side or the other, but it is still an equilibrium. (3) There is both an acid and a base on both sides of the equilibrium. (4) This equilibrium is not to be confused with resonance. (5) Proton transfer reactions are usually very fast, especially when the proton is transferred from one heteroatom to another. 

Many organic compounds can act as weak Bronsted-Lowry acids or bases. Their reactions involve the transfer of H+ ions, or protons (Section 10-4). Like similar reactions of inorganic compounds, these acid-base reactions of organic acids and bases are usually fast and reversible. Consequently, we can discuss the acidic or basic properties of organic compounds in terms of equilibrium constants (Section 18-4). 

The stabilities of species in the system S-Oj-HjO have been studied by numerous researchers. Much of this work has been summarized or critiqued by Garrels and Naeser (1958), Boulegue and Michard (1979), Morse et al. (1987), Schoonen and Barnes (1988), and Williamson and Rimstidt (1992). Thermodynamic data for some substances in the system S-O2-H2O are given in Table A 12.3 in the chapter appendix. The many aqueous sulfur species that are either thermodynamically stable or are important metastably are shown in Fig. 12.14. Acid-base reactions among the sulfur species are generally rapid and reversible. The redox reactions, however, may be fast and reversible (e.g., H2S/SJ see Boulegue and Michard 1979) or more often irreversible in the absence of bacterial activity (e.g., SO4 reduction to H2S). 

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