Acid-base reactions proton transfer

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. 

Another solvent that may be used for acid-base reactions (proton transfer) is concentrated sulfuric acid. This is both an acid and a solvent for the nitration of benzene. 

Every acid-base reaction involves transfer of a proton from an acid to a base. 

In biochemical systems, acid-base and redox reactions are essential. Electron transfer plays an obvious, crucial role in photosynthesis, and redox reactions are central to the response to oxidative stress, and to the innate immune system and inflammatory response. Acid-base and proton transfer reactions are a part of most enzyme mechanisms, and are also closely linked to protein folding and stability. Proton and electron transfer are often coupled, as in almost all the steps of the mitochondrial respiratory chain. 

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 donor-acceptor principle is an important basic concept in modern chemical education acid-base reactions, redox reactions and complex reactions explain a huge number of chemical changes. One important group of donor-acceptor reactions are the acid-base reactions protons (H+ ions) transfer from one species to another species. One example, in the neutralization of sulfuric acid with sodium hydroxide a proton is moving from one hydronium ion H30 + (aq) of the acid solution to one hydroxide ion OH (aq) ion of the hydroxide solution. Broensted s key concept will be considered in this chapter. 

The Br0nsted—Lowry concept defines a species as an acid or a base according to its function in the acid—base, or proton-transfer, reaction. As you saw in Example 16.1, some species can act as either an add or a base. An amphiprotic species is a species that can act as either an acid or a base (it can lose or gain a proton), depending on the other reactant. For example, HC03 acts as an acid in the presence of OH but as a base in the presence of HF. Anions with ionizable hydrogens, such as HC03, and certain solvents, such as water, are amphiprotic. 

Br0nsted-Lowry acid-base theory, proton-transfer reaction, proton source, proton remover, amphoteric... 

In a Bronsted-Lowry acid-base reaction, protons are transferred from one reactant (the acid) to another (the base). Figure 2.1 shows the reaction between the Bronsted-Lowry acid HCl and the Bronsted-Lowry base NH3. 

Reactions involving charge transfer form one of the broadest classes of chemical transformations. They include such important processes as redox reactions (in the simplest case, electron transfer), acid-base transformations (proton transfer), electrolytic dissociation, and many others. A special class of heterogeneous processes of charge transfer includes electrode reactions associated with a charge transfer through the interface between electronic and ionic conductors. 

A more general theory of acids and bases was devised independently by Johannes Br0n sted (Denmark) and Thomas M Lowry (England) m 1923 In the Brpnsted-Lowry approach an acid is a proton donor, and a base is a proton acceptor The reaction that occurs between an acid and a base is proton transfer... 

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... 

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... 

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. 

Table 4-1 lists some rate constants for acid-base reactions. A very simple yet powerful generalization can be made For normal acids, proton transfer in the thermodynamically favored direction is diffusion controlled. Normal acids are predominantly oxygen and nitrogen acids carbon acids do not fit this pattern. The thermodynamicEilly favored direction is that in which the conventionally written equilibrium constant is greater than unity this is readily established from the pK of the conjugate acid. Approximate values of rate constants in both directions can thus be estimated by assuming a typical diffusion-limited value in the favored direction (most reasonably by inspection of experimental results for closely related... 

We can use this more general view to discuss the strengths of acids. In our generalized acid-base reaction (52), the proton transfer implies the chemical bond in HB, must be broken and the chemical bond in HB2 must be formed. If the HB, bond is easily broken, then HB, will be a strong acid. Then equilibrium will tend to favor a proton transfer from HB, to some other base, B2. If, on the other hand, the HB, bond is extremely stable, then this substance will be a weak acid. Equilibrium will tend to favor a proton transfer from some other acid, HB2, to base B, forming the stable HB, bond. 

In the proton transfer view of acid-base reactions, an acid and a base react to form another acid and another base. Let us see how this theory encompasses the elementary reaction between U+(aq) and OH (aq) and the reaction of disso-... 

Now the dissociation of acetic acid can be regarded as an acid-base reaction. The acid CHjCOOH transfers a proton to the base H20 forming the acid H30+ and the base CH3COO-. The reaction (54) now takes the form... 

According to this theory, an acid is defined as a proton donor and a base as a proton acceptor (a base must have a pair of electrons available to share with the proton this is usually present as an unshared pair, but sometimes is in a 7t orbital). An acid-base reaction is simply the transfer of a proton from an acid to a base. (Protons do not exist free in solution but must be attached to an electron pair). When the acid gives up a proton, the species remaining still retains the electron pair to which the proton was formerly attached. Thus the new species, in theory at least, can reacquire a proton and is therefore a base. It is referred to as the conjugate base of the acid. All acids have a conjugate base, and all bases have a conjugate acid. All acid-base reactions fit the equation... 

Acid strength may be defined as the tendency to give up a proton and base strength as the tendency to accept a proton. Acid-base reactions occur because acids are not equally strong. If an acid, say HCI, is placed in contact with the conjugate base of a weaker acid, say acetate ion, the proton will be transferred because the HCI has a greater tendency to lose its proton than acetic acid. That is, the equilibrium... 

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 ... 

Many books abbreviate the hydronium ion as H (a g) or just H. We prefer H3 O because it serves as a reminder of the molecular structure of the hydronium ion and of the proton-transfer nature of acid-base reactions. 

Any reaction in which a proton is transferred from one substance to another is an acid-base reaction. More specifically, the proton-transfer view is known as the Bronsted-Lowiy definition of acids and bases. In an acid-base reaction, an acid donates a proton, and a base accepts that proton. Any species that can give up a proton to another substance is an acid, and any substance that can accept a proton from another substance is a base. The production of two water molecules from a hydroxide anion (a base) and a hydronium ion (an acid) is just one example of an acid-base reaction acids and bases are abundant in chemistry. 

Notice that the expressions for and do not include the water molecules that act as starting materials for the proton transfer reactions. Water, as the solvent, is always present in huge excess. Thus, as described in Section 16-1. the concentration of water does not change significantly during an acid-base reaction and is omitted from Z a and. ... 

In an acid-base reaction, a proton (H ) is transferred from one chemicai species to another. A species that donates a proton is an acid, and a species that accepts a proton is a base. This identification of acids and bases is the Bronsted-Lowry definition of acid-base reactions. From this perspective, every acid-base reaction has two reactants, an acid and a base. Every acid-base reaction aiso forms two products ... 

C17-0128. Pure sulfuric acid (H2 SO4) is a viscous liquid that causes severe bums when it contacts the skin. Like water, sulfuric acid is amphiprotic, so a proton transfer equilibrium exists in pure sulfuric acid, (a) Write this proton transfer equilibrium reaction, (b) Construct the Lewis stmcture of sulfuric acid and identify the features that allow this compound to function as a base, (c) Perchloric acid (HCIO4) is a stronger acid than sulfuric acid. Write the proton transfer reaction that takes place when perchloric acid dissolves in pure sulfiaric acid. 

Thus, the relationship between acid and base is a reciprocal one and an acid-base reaction involves the transfer of a proton. This concept is not restricted to aqueous solutions and it discards Arrhenius prerequisite of ionization. 

Thus an acid-base reaction involves the transfer of an oxide ion (compared with the transfer of a proton in the Bronsted theory) and the theory is particularly applicable in considering acid-base relationships in oxide, silicate and aluminosilicate glasses. However, we shall find that it is subsumed within the Lewis definition. 

On the other hand, Arnett and his coworkers have reported both the enthalpies of the protonation (AHJ and the hydrogen bond (AHf) for acid-base reactions. They calculated Hj by measuring the association constants for the proton transfer (ionization) in a number of bases by using FSO3H as the acid and determined Ai/j by calorimetric measurements of the heat of dissolution of P-FC6H4OH in various hydrogen bond acceptors, including sulphoxides, in They have also tried to correlate and... 

Both these methods require equilibrium constants for the microscopic rate determining step, and a detailed mechanism for the reaction. The approaches can be illustrated by base and acid-catalyzed carbonyl hydration. For the base-catalyzed process, the most general mechanism is written as general base catalysis by hydroxide in the case of a relatively unreactive carbonyl compound, the proton transfer is probably complete at the transition state so that the reaction is in effect a simple addition of hydroxide. By MMT this is treated as a two-dimensional reaction proton transfer and C-0 bond formation, and requires two intrinsic barriers, for proton transfer and for C-0 bond formation. By NBT this is a three-dimensional reaction proton transfer, C-0 bond formation, and geometry change at carbon, and all three are taken as having no barrier. 

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