Acid-base reactions present

Reviewing the three acid-base reactions presented, we find that all fit into the general equation for a Brpnsted-Lowry proton-transfer reaction ... 

The electrophilic character of boron is again evident when we consider the oxida tion of organoboranes In the oxidation phase of the hydroboration-oxidation sequence as presented m Figure 6 11 the conjugate base of hydrogen peroxide attacks boron Hydroperoxide ion is formed m an acid-base reaction m step 1 and attacks boron m step 2 The empty 2p orbital of boron makes it electrophilic and permits nucleophilic reagents such as HOO to add to it... 

Using Environmental Examples to Teach About Acids. Acid-base reactions are usually presented to secondary students as examples of aqueous equilibrium (2). In their study of acids and bases, students are expected to master the characteristic properties and reactions. They are taught to test the acidity of solutions, identify familiar acids and label them as strong or weak. The ionic dissociation of water, the pH scale and some common reactions of acids are also included in high school chemistry. All of these topics may be illustrated with examples related to acid deposition (5). A lesson plan is presented in Table I. 

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... 

Another way of analyzing these combinations is by type of substance. Parts a and c involve mixing of two aqueous solution, parts b and d involve adding a metal to an aqueous system, and part e is the interaction of a metal with O2 gas. When solutions mix, we look first for acid-base reactions (part c), then for formation of a precipitate (part b). When a metal contacts an aqueous system, the most likely reaction, if any, is oxidation of the metal. Any time molecular oxygen is present, we can expect oxidation to be one possibility. 

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. ... 

The fact that complex 38 does not react further - that is, it does not oxidatively add the N—H bond - is due to the comparatively low electron density present on the Ir center. However, in the presence of more electron-rich phosphines an adduct similar to 38 may be observed in situ by NMR (see Section 6.5.3 see also below), but then readily activates N—H or C—H bonds. Amine coordination to an electron-rich Ir(I) center further augments its electron density and thus its propensity to oxidative addition reactions. Not only accessible N—H bonds are therefore readily activated but also C—H bonds [32] (cf. cyclo-metallations in Equation 6.14 and Scheme 6.10 below). This latter activation is a possible side reaction and mode of catalyst deactivation in OHA reactions that follow the CMM mechanism. Phosphine-free cationic Ir(I)-amine complexes were also shown to be quite reactive towards C—H bonds [30aj. The stable Ir-ammonia complex 39, which was isolated and structurally characterized by Hartwig and coworkers (Figure 6.7) [33], is accessible either by thermally induced reductive elimination of the corresponding Ir(III)-amido-hydrido precursor or by an acid-base reaction between the 14-electron Ir(I) intermediate 53 and ammonia (see Scheme 6.9). 

When proteins undergo hydrolysis, you wind up with 22 a amino acids, 20 of which are regulcir amino acids and 2 of which are derived amino acids. Amino acids are amphoteric (they possess the characteristics of both acids and bases and can react as either) because both acidic and basic groups are present. An internal acid-base reaction produces a dipolar ion known as a zwitterion (you can see the general structure of one in Figure 16-33). 

According to the Br0nsted-Lowry definitions, any species that contains hydrogen can potentially act as an acid, and any compound that contains a lone pair of electrons can act as a base. Therefore, neutral molecules can also act as bases if they contain an oxygen, nitrogen or sulphur atom. Both an acid and a base must be present in a proton transfer reaction, because an acid cannot donate a proton unless a base is present to accept it. Thus, proton-transfer reactions are often called acid-base reactions. 

With the exception of B = OH-, which relates in fact to an acid-base reaction, the other nucleophiles are potential reductants. After forming the reversible adducts [Eq. (5)], redox reactions are usually operative, leading to the reduction of nitrosyl and oxidation of the nucleophile in Eq. (6). Nevertheless, we will consider first the reaction with B = OH- for the sake of simplicity, and also because it allows for some generalizations to be made on the factors that influence the electrophilic reactivities of different nitrosyl complexes (51). We continue with new results for some N-binding nucleophiles (62,67), which throw light on the mecanisms of N20/N2 production and release from the iron centers. A description of the state of the art studies on the reactions with thiolate reactants as nucleophiles will be presented later. 

Two complications can prevent a simple determination or the concentration of each species from a measurement of absorbance at a chosen frequency. Although most of the acid-base reactions of interest result in a one-to-one stoichiometry, one cannot assume this a priori, and two-to-one and three-to-one adducts might also be present. Fortunately, this is usually an easy point to Tesolve. The presence of an isosbeslic point or point of constant absorbance (see Fig. 9.1) is usually a reliable criterion that only two absorbing species (the free acid or base and a single adduct) are present.26... 

I once mentioned in a lecture that acid-base properties across the interface were not important in developing water resistant bonds to metal surfaces and cited the data of Table 7 [17]. Professor Fowkes was present and commented that there are numerous examples where acid-base reactions across the interface... 

What is it that makes an acid an acid and a base a base We first raised those questions in Section 4.5, and we now take a closer look at some of the concepts that chemists have developed to describe the chemical behavior of acids and bases. We ll also apply the principles of chemical equilibrium discussed in Chapter 13 to determine the concentrations of the substances present in aqueous solutions of acids and bases. An enormous amount of chemistry can be understood in terms of acid-base reactions, perhaps the most important reaction type in all of chemistry. 

Given these definitions, it is crucial to emphasize again that no compound is inherently an acid or a base. A compound only functions as an acid (in the Bronsted sense) if it donates a proton to something that is, there must be a base present (a proton acceptor) to have an acid-base reaction. Thus a compound acts as an acid only in the presence of a base, and a substance can only act as a base in the presence of an acid. A good example of this concept is the fact that HC1 in the vapor phase is an undissociated, covalent molecule. This... 

While the necessity of having both and acid and a base present in order to have an acid-base reaction is axiomatic, it is surprising how often this concept is neglected. However, if these conditions are met, then a wide variety of organic compounds can donate protons to appropriate bases (they are deprotonated) and a wide variety of compounds can accept protons from appropriate acids (they are protonated). 

The nucleophilic substitution of carboxylic acids is complicated due to the fact that an acidic proton is present. Since most nucleophiles can act as bases, the reaction of a carboxylic acid with a nucleophile results in an acid-base reaction rather than nucleophilic substitution. 

Secondly, the acidic proton is no longer present and so an acid-base reaction is prevented. Thirdly, the original OH group is converted into a good leaving group and is easily displaced once the chloride ion makes its attack. The reaction of a carboxylic acid with thionyl chloride follows the general mechanism shown below ... 

It is necessary to protect any carboxylic acids present when carrying out organolithium reactions since one equivalent of the organolithium reagent would be wasted in an acid-base reaction with the carboxylic acid. 

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... 

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