Proton transfer in acid-base reactions

The above CT systems represent the case for intermolecular electron transfer. There is some analogy to proton transfer in acid-base reactions [5]. We have also examined intramolecular electron transfer systems and studied the influence of IVR and geometric changes this work is detailed elsewhere [5]. Other reactions involving ultrafast electron transfer are those of harpooning in Xe + I2) Nal, and more recently Xe/Ch (see Ref. 1). 

The Key Event Formation of Water Proton Transfer in Acid-Base Reactions Acid-Base Titrations... 

This enzyme, which is relatively stable under reaction conditions, will retain 70% of its activity after 10 days at pH 5 and 25°C. Although it is not yet commercially available, it has been overexpressed in E. coli, making large quantities easily accessible.68 The detailed mechanism of DERA has been determined based on the atomic structure (ca. 1.0 A) combined with site-directed mutagenesis, kinetic, and NMR studies136 (Scheme 5Alb). A proton relay system composed of Lys and Asp appears to activate a conserved active-site water that functions as the critical mediator for proton transfer in acid-base catalysis. 

Half-Reactions and Electrode Potentials. Reduction-oxidation (redox) reactions involve electron donors (reductants or reducing agents) and electron acceptors (oxidants or oxidizing agents), analogous to the involvement of proton donors and proton acceptors in acid-base reactions. Redox reactions are characterized by the transfer of an integral number of electrons from the reductant to the oxidant molecule. The tendency of the redox reaction to proceed is determined by the electromotive force or potential change E) for the particular reaction. Since there is always an electron donor that becomes oxidized by the transfer, and an electron acceptor that becomes reduced by the transfer, any redox reaction can be written as the sum of two half-reactions. For example, the redox reaction... 

From the Brpnsted-Lowry perspective, the only requirement for an acid-base reaction is that one species donates a proton and another species accepts it an acid-base reaction is a proton-transfer process. Acid-base reactions can occur between gases, in nonaqueous solutions, and in heterogeneous mixtures, as well as in aqueous solutions. 

Our emphasis throughout this chapter has been on water as the solvent and on the proton as the source of acidic properties. In such cases we find the Bronsted—Lowry definition of acids and bases to be the most useful. In fact, when we speak of a substance as being acidic or basic, we are usually thinking of aqueous solutions and using these terms in the Arrhenius or Bronsted—Lowry sense. The advantage of the Lewis definitions of acid and base is that they allow us to treat a wider variety of reactions, including those that do not involve proton transfer, as acid—base reactions. To avoid confusion, a substance such as BF3 is rarely called an acid unless it is clear from the context that we are using the term in the sense of the Lewis definition. Instead, substances that function as electron-pair acceptors are referred to explicitly as Lewis acids. ... 

In contrast to redox reactions, only proton transfer takes place in acid-base reactions (see also p.30). When an acid dissociates (1), water serves as a proton acceptor (i. e., as a base). Conversely, water has the function of an acid in the protonation of a carboxylate anion (2). 

The Arrhenius theory accounts for the properties of many common acids and bases, but it has important limitations. For one thing, the Arrhenius theory is restricted to aqueous solutions for another, it doesn t account for the basicity of substances like ammonia (NH3) that don t contain OH groups. In 1923, a more general theory of acids and bases was proposed independently by the Danish chemist Johannes Bronsted and the English chemist Thomas Lowry. According to the Bronsted-Lowry theory, an acid is any substance (molecule or ion) that can transfer a proton (H + ion) to another substance, and a base is any substance that can accept a proton. In short, acids are proton donors, bases are proton acceptors, and acid-base reactions are proton-transfer reactions ... 

All spectroscopic lines have a natural line width, and this can be of great use to the kineticist. This natural line width is determined by the lifetime of the excited state of the molecule. If this is short the line width is broad, while longer lifetimes give more sharply defined lines. If reaction occurs, this can alter the lifetime of the excited state and so change the natural line width of the transition. A detailed spectroscopic analysis gives relations between the width of the line, the lifetime of the reacting species and the rate constant for the reaction. This has proved a very important tool, especially for reactions in solution such as proton transfers and acid/base ionization processes. 

O. F. Mohammed, D. Pines, J. Dreyer, E. Pines and E. T. J. Nibbering, Sequential proton transfer through water bridges in acid-base reactions, Science, 310 (2005) 83-86. 

Redox reactions, i.e., when electron exchange occurs, are considered charge transfer reactions in solutions. It is frequently accompanied by the transfer of heavier species (ions). Processes in which charged species react in solutions but no electron exchange takes place, e.g., proton exchange in protonation-deprotonation or acid-base reactions are not called charge transfer reactions. 

The most common type of biocatalytic reactions is proton transfer (115). Nearly, every enzymatic reaction involves one or more proton-coupled steps. Transition-state proton bridging and intramolecular proton transfer (general acid-base catalysis) are important strategies to accelerate substrate conversion processes. Moreover, proton transfer also plays a fundamental role in bioenergetics (116). 

Fig. 5. Model for proton transfer in the bacterial reaction center of Rb. sphaeroides based on its known crystal structure. Some amino-acid residues are shown in the protein interior. See text for detaiis. Figure source Paddock, Rongey, McPherson, Juth, Feher and Okamura (1994) Pathway of proton transfer in bacterial reaction centers Role of aspartate-L213 in proton transfers associated with reduction of quinone to dihydroquinone. Biochemistry 33 743.

The experimental techniques for studying fast reactions provided a means of studying fundamental processes in solution that were previously considered to be instantaneous. These include electron and proton transfer reactions. Proton transfer is the elementary step involved in acid-base reactions, which are so important in classical analytical chemistry. On the other hand, electron transfer is the elementary step involved in redox reactions. The theory of electron transfer is especially well developed and is discussed in detail below. 

Many chemical reactions involve a catalyst. A very general definition of a catalyst is a substance that makes a reaction path available with a lower energy of activation. Strictly speaking, a catalyst is not consumed by the reaction, but organic chemists frequently speak of acid-catalyzed or base-catalyzed mechanisms that do lead to overall consumption of the acid or base. Better phrases under these circumstances would be acid promoted or base promoted. Catalysts can also be described as electrophilic or nucleophilic, depending on the catalyst s electronic nature. Catalysis by Lewis acids and Lewis bases can be classified as electrophilic and nucleophilic, respectively. In free-radical reactions, the initiator often plays a key role. An initiator is a substance that can easily generate radical intermediates. Radical reactions often occur by chain mechanisms, and the role of the initiator is to provide the free radicals that start the chain reaction. In this section we discuss some fundamental examples of catalysis with emphasis on proton transfer (Brpnsted acid/base) and Lewis acid catalysis. 

Basic Idea Obviously, the protons in acid-base reactions are simply transferred from one base B to another B ... 

Many organic reactions involve the transfer of a proton by an acid-base reaction. An important consideration, therefore, is the relative strengths of compounds that could potentially act as Bronsted-Lowry acids or bases in a reaction. 

Another class of chemical reactions also covered here is that of proton-transfer reactions. These processes play a key role in solution chemistry, and more specifically in acid—base reactions. In this class of reactions the cmcial step involves the motion of the hydrogen atom, which typically occurs on the picosecond or femtosecond time-scale. By investigating the time dynamics of these processes in size-selected clusters, for a given system, information is gained at which specific cluster size the onset of the proton transfer reaction occurs. 

Quantitative Calculations In acid-base titrimetry the quantitative relationship between the analyte and the titrant is determined by the stoichiometry of the relevant reactions. As outlined in Section 2C, stoichiometric calculations may be simplified by focusing on appropriate conservation principles. In an acid-base reaction the number of protons transferred between the acid and base is conserved thus... 

A catalyst is defined as a substance that influences the rate or the direction of a chemical reaction without being consumed. Homogeneous catalytic processes are where the catalyst is dissolved in a liquid reaction medium. The varieties of chemical species that may act as homogeneous catalysts include anions, cations, neutral species, enzymes, and association complexes. In acid-base catalysis, one step in the reaction mechanism consists of a proton transfer between the catalyst and the substrate. The protonated reactant species or intermediate further reacts with either another species in the solution or by a decomposition process. Table 1-1 shows typical reactions of an acid-base catalysis. An example of an acid-base catalysis in solution is hydrolysis of esters by acids. 

Whenever possible, the chemical reactions involved in the fonnation of diastereomers and their- conversion to separate enantiomers are simple acid-base reactions. For example, naturally occurring (5)-(—)-malic acid is often used to resolve fflnines. One such amine that has been resolved in this way is 1-phenylethylarnine. Amines are bases, and malic acid is an acid. Proton transfer from (5)-(—)-malic acid to a racemic mixture of (/ )- and (5)-1-phenylethylarnine gives a mixture of diastereorneric salts. 

As pointed out in Chapter 4, the first step in the reaction is proton transfer to the alcohol from the hydrogen halide to yield an alkyloxonium ion. This is an acid-base reaction. 

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