Acid-base chemistry proton transfer

Electron-driven acid-base chemistry proton transfer from hydrogen chloride to ammonia. 

S. N. Eustis, D. Radisic, K. H. Bowen, R. A. Bachorz, M. Haranczyk, G. K. Schenter, and M. Gutowski, Science, 319, 936-939 (2008). Electron-Driven Acid-Base Chemistry Proton Transfer from Hydrogen Chloride to Ammonia. 

Proton transfer is a particularly important transport process. Beyond acid-base reactions, proton transfer may be coupled to electron transfer in redox reactions and to excited-state chemistry. It is of enormous significance in biochemical processes where it is an essential step in hydrolytic enzyme processes and redox reactions spanning respiration, and photosynthesis where proton motion is responsible for sustaining redox gradients. In relatively recent times, proton transfer in the excited state has undergone significant study, primarily fueled by advances in ultrafast spectroscopy. 

Any acid that undergoes quantitative reaction with water to produce hydronium ions and the appropriate anion is called a strong acid. Table gives the structures and formulas of six common strong acids, all of which are supplied commercially as concentrated aqueous solutions. These solutions are corrosive and normally are diluted for routine use in acid-base chemistry. At the concentrations normally used in the laboratory, a solution of any strong acid in water contains H3 O and anions that result from the loss of a proton. Example shows a molecular view of the proton transfer reaction of a strong acid. 

Up to this point, we have dealt with the subject of acid-base chemistry in terms of proton transfer. If we seek to learn what it is that makes NH3 a base that can accept a proton, we find that it is because there is an unshared pair of electrons on the nitrogen atom where the proton can attach. Conversely, it is the fact that the hydrogen ion seeks a center of negative charge that makes it leave an acid such as HC1 and attach to the ammonia molecule. In other words, it is the presence of an unshared pair of electrons on the base that results in proton transfer. Sometimes known as the electronic theory of acids and bases, this shows that the essential characteristics of acids and bases do not always depend on the transfer of a proton. This approach to acid-base chemistry was first developed by G. N. Lewis in the 1920s. 

Lewis acid-base chemistry provides one of the most useful tools ever devised for systematizing an enormous number of chemical reactions. Because the behavior of a substance as an acid or a base has nothing to do with proton transfer, many other types of reactions can be considered as acid-base reactions. For example,... 

This chapter provides our first detailed examination of a chemical reaction, the acid-base reaction or transfer of a proton. Although acid—base reactions are simple, they are very important in oiganic chemistry because more complicated reactions often involve one or more proton transfer steps. In addition, an important purpose of this chapter is to introduce many concepts about reactions in general. Much of what we learn about the acid-base reaction is applied to other reactions in later chapters. 

The first mechanism the stndents enconnter (Chapter 4) describes the conversion of alcohols to alkyl halides. Not only is this a nsefnl fnnctional-gronp transformation, bnt its first step proceeds by the simplest mechanism of all— proton transfer. The overall mechanism provides for an early reinforcement of acid-base chemistry and an early introduction to carbocations and nucleophilic substitution. 

The reduction of the pyrimidine to dihydropyrimidine is the reverse of the oxidation reaction carried out by DHODs. The structure of the FMN/pyrimidine-binding site is very similar to the structure of L. lactis DHODs. Three Asn residues form hydrogen bonds with the nitrogens and carbonyls of the pyrimidine analogous to DHODs. DPD has an active site cysteine proposed to act in acid/base chemistry similar to Class 1 DHODs. When mutated to alanine, only 1% of the wild-type activity was retained, indicating the importance of this residue in catalysis. Secondary tritium isotope effects using 5- H-uracil were determined in both H2O and D2O an inverse isotope effect was observed in H2O and the value became more inverse in D20. " This was taken as evidence of a stepwise mechanism in which hydride transfer to C6 is followed by protonation at C5. 

In contrast to inorganic molten salts, the fluidity of ionic hquids at room temperature permits their use as solvents for chemical reactions. Electrostatic properties and charge mobility in ionic hquids can play a distinctive role in chemical reactivity, as compared with neutral solvents. In particular, hydrogen and proton transfer reactions are likely to be sensitive to an ionic environment due to the hydrogen-bond acceptor ability of the anions. Such type of reactions are fundamental in acid-based chemistry and proton transport in solution. 

The dual entry of H2O in Fig. 3 reflects the fact that water plays more than the passive role of a solvent in acid-base chemistry. Water is in fact a direct participant in any proton transfer reaction that takes place in aqueous solution, and its conjugate acid H30+ and base OH are respectively the strongest acid and the strongest base that can exist in aqueous solution. 

An important aspect of the chemical ionization technique is the fact that with different kinds of reactant ions different kinds of reactions will be involved in the production of the chemical ionization mass spectra. Thus, for example, if is used as a reactant, the reaction occurring is proton transfer, and the chemistry involved may be looked upon as an even-electron, gaseous acid-base chemistry. On the other hand, if N2 is used as a reactant ion, electron transfer occurs, and one then has an odd-electron, oxidation-reduction chemistry. It is self-evident that by using different reactant ions the intensity of the chemical ionization reaction can be varied that is. 

The acid-base chemistry of amino acids is more complicated than shown in Equations 16.48 and 16.49, however. Because the COOH group can act as an acid and the NH2 group can act as a base, amino acids undergo a self-contained Bronsted-Lowry acid-base reaction in which the proton of the carboxyl group is transferred to the basic nitrogen atom ... 

Proton transfer from one molecule to another is one of the most studied phenomena in chemistry. It is the essence of acid-base chemistry. This process is related to hydrogen bonding, as a hydrogen-bonded complex can be an intermediate step ... 

The pJCj values are now available for many hydride complexes. Extensive tables have been compiled recently by Bullock and by Tilset. The rate of proton transfer to and from transition metals is rather slow (see below), so it is often possible to detect separate NMR signals for M-H and M , and tiius to determine the position of proton transfer equilibria between hydride complexes (M-H) and bases (B), or metal bases (M") and organic acids (HA). The pX values in Table 3.1 have been obtained in acetonitrile, an excellent solvent for acid-base chemistry because it solvates cations well enough to minimize ion pair formation it is both a weak acid and a weak base, with a very low autoprotolysis constant (ion product). ... 

In Chapter 4, we introduced reaction mechanisms and reaction coordinate diagrams. These concepts were explained in the context of acid-base chemistry, which involves the transfer of a proton from one chemical entity to another occurring in a single step. Although many chemical transformations take place in one step, many others involve... 

Depending upon your definition of an acid and base, chemists can characterize almost all chemical reactions as some form of an acid-base reaction. Because proton transfers are ubiquitous in chemistry and biochemistry, we start this chapter focusing upon this single kind of acid-base reaction. However, a later portion of this chapter concentrates on filled and empty orbitals as our base and acid equivalents (Lewis definitions), thereby broadening our acid-base reactions considerably. Hence, the reason for discussing acid-base chemistry thoroughly, and even devoting an entire chapter to this one reaction type in Part I of this book, is that it plays a central role in all chemical disciplines. 

One of the most important insights we can gain about acid-base chemistry is the ability to predict what is the stronger acid or base when confronted with a comparison. Here, this will be a completely thermodynamic analysis, and we leave it until Chapter 9 to discuss the kinetics of proton transfers, In order to be able to make a sound prediction, we wilt cover correlations between gas phase and solution acidities. Then, numerous factors that control acidity will be covered—namely, solvation, resonance, electronegativity, inductive effects, etc. These are all topics that we have covered in Chapters 1-4, and hence this chapter serves as a nice recap. 

In Section 1.7 (p. 41), we introduced acids and bases. Now we know quite a bit more about structure and can return to the important subject of acids and bases in greater depth. In particular, we know about carbocations and carbanions, which play an important role in acid—base chemistry in organic chemistry The Lewis definition of acids and bases is far more inclusive than the Bronsted definition, which focuses solely on proton donation (Breasted acid) and acceptance (Breasted base). The archetypal Brensted acid-base reaction is the reaction between KOH and HCl to transfer a proton from HCl to HO. This reaction is a competition between the hydroxide and the chloride for a proton. In this case, the stronger base hydroxide wins easily (Rg. 2.57). 

Heterogeneous catalytic reactions in a class of Alumino-Silicates called zeolites, is an area which has recently been shown to be amenable to a variety of Quantum Chemistry calculations. The present review is a short introduction to this field, focusing on the problem of proton transfer, the primary process central to the acid-base chemistry exhibited in the nanopores of zeolite. This problem is closely related to the controversial question of zeolite acidity that has evoked great interest both experimentally and theoretically. Recent ab initio results are compared with experiments and some of the difficulties associated with the use of Quantum Chemistry are discussed. Finally the future of such ab initio calculations are questioned in view of the growing developments in the application of First-Principles molecular dynamics to the study of such systems. 

A class of porous alumino-sihcate materials called zeolites has been known for almost 300 years. They are best known for their role as catalysts. In chapter four, Marcel AUavena and David White present a review of applications of computational chemistry to the proton transfer, primary process for acid-base chemistry on zeolites. Recent ab initio results are compared to experimental studies and critically reviewed. Future directions of the field are given, and the importance of the Car-Parrinello method in exploring the dynamic aspects of reactivity in zeolites is discussed. 

Our principal interest in acid-base chemistry is in aqueous solutions, where the Br0nsted-Lowry theory prevails. The balance of this chapter is limited to the proton-transfer concept of acids and bases. 

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