Acidity of proton donor

E. Acidity of Proton Donor. Effect of Iron in Birch Reductions... 

Origin of by-products in Birch reductions / 18 Acidity of proton donor. Effect of iron in Birch reductions / 19 Requirements of Birch reduction procedures / 22 Experimental conditions for Birch reductions / 25... 

To study the stereochemistry of protonation reactions, substituted indenes have been cathodically reduced in DMF/TBAP with added water or phenol [170b,173,174]. In the presence of water, a formal anti addition of protons was observed, whereas addition of phenol led to the prevalent formation of products, formally deriving from syn protonation. Obviously, steric effects and/or acidities of proton donor and the dihydroproduct play an important role. A detailed discussion of stereochemical aspects is found in Chapter 26. 

Hammet s acidity constant, Hq A logarithmic thermodynamic scale used to relate acidity of proton donor acids. 

A useful definition of acids and bases is that independently introduced by Johannes Bronsted (1879-1947) and Thomas Lowry (1874-1936) in 1923. In the Bronsted-Lowry definition, acids are proton donors, and bases are proton acceptors. Note that these definitions are interrelated. Defining a base as a proton acceptor means an acid must be available to provide the proton. For example, in reaction 6.7 acetic acid, CH3COOH, donates a proton to ammonia, NH3, which serves as the base. 

The role that acid and base catalysts play can be quantitatively studied by kinetic techniques. It is possible to recognize several distinct types of catalysis by acids and bases. The term specie acid catalysis is used when the reaction rate is dependent on the equilibrium for protonation of the reactant. This type of catalysis is independent of the concentration and specific structure of the various proton donors present in solution. Specific acid catalysis is governed by the hydrogen-ion concentration (pH) of the solution. For example, for a series of reactions in an aqueous buffer system, flie rate of flie reaction would be a fimetion of the pH, but not of the concentration or identity of the acidic and basic components of the buffer. The kinetic expression for any such reaction will include a term for hydrogen-ion concentration, [H+]. The term general acid catalysis is used when the nature and concentration of proton donors present in solution affect the reaction rate. The kinetic expression for such a reaction will include a term for each of the potential proton donors that acts as a catalyst. The terms specific base catalysis and general base catalysis apply in the same way to base-catalyzed reactions. 

A competing reaction in any Birch reduction is reaction of the alkali metal with the proton donor. The more acidic the proton donor, the more rapid IS the rate of this side reaction. Alcohols possess the optimum degree of acidity (pKa ca. 16-19) for use in Birch reductions and react sufficiently slowly with alkali metals in ammonia so that efficient reductions are possible with them. Eastham has studied the kinetics of reaction of ethanol with lithium and sodium in ammonia and found that the reaction is initially rapid, but it slows up markedly as the concentration of alkoxide ion in the mixture... 

Benzylic, allylic and propargylic positions enhance the cathodic cleavage rate of C— heteroatom bonds as, for example, in the reduction of benzylic and allylic halides or alcohols56. Similar activated sulphones, due to their acidity, are in a class apart. Figure 8 shows the similitude between the cathodic behaviour of an allylic sulphone and its isomer, i.e., the corresponding vinylic sulphone when the electrolyses are run in an aprotic solvent. However, in the presence of an excess of proton donor, discrepancies appear. 

What Are the Key Ideas Bronsted acids are proton donors Bronsted bases are proton acceptors. The composition of a solution of an acid or base immediately adjusts to satisfy the values of the equilibrium constants for all the proton transfer reactions taking place. 

Bronsted acid A proton donor (a source of hydrogen ions, H+). Examples HC1 CH COOH HCO - NH4 ... 

Weak acids are compounds that donate protons quantitatively to hydroxide ions but not to water. All acids are proton donors, but whereas a strong acid quantitatively donates protons to water, a weak acid does not. Aqueous solutions of weak acids contain small concentrations of hydronium ions, making the solutions acidic, but nearly all the weak acid molecules remain intact. Representative of strong acids, HCl generates H3 O and Cl" quantitatively when dissolved in water. Representative of weak acids, HF remains predominantly as HF molecules when dissolved in water. However, HF donates protons quantitatively to OH ions to give H2 O molecules and F ions, as shown in Figure 4-9. 

Bronsted theory a theory of acids and bases that defines acids as proton donors and bases as proton acceptors. 

Monoprotonation of the [2.1.1]-cryptand occurs rapidly but protonation of the monoprotonated species by hydronium ion and other acids can be followed kinetically in various solvents (Cox et al., 1982, 1983). In methanol, protonation of ii+ species by substituted acetic and benzoic acids to give i+i+ has been studied using the stopped flow technique with conductance detection. The values of the rate coefficients (kHA) for protonation (81) vary with the acidity of the donor acid from kHA = 563 dm3mol-1s-1 (for 4-hydroxy-benzoic acid) to kHA = 2.3 x 105 dm3mol 1s 1 (for dichloroacetic acid). 

The role of proton donors in the closing of the catalytic loop is revealed by the experiments carried out in a nonprotic medium after addition of an acid. The results are summarized in Figure 4.9. Similar results are again obtained with cobalamin. In the absence of substrate, a two-wave system... 

Lowry is best known to chemistry students through the tradition of eponymony, since the proton theory of acidity is known as the "Bronsted/Lowry theory" of proton donors. His most important experimental investigation likely was a long series of studies on optical rotatory dispersion.49 For our purposes, there is special interest in his discovery of mutarotation in camphor derivatives and his theory of dynamic tautomerism, which led him to an ionic theory of organic reaction mechanisms. 

Some complications arise from the presence of proton donor-acceptor interactions134 when the donor is a protic amine. The separate evaluation of the two kinds of interactions may be a difficult problem. Similarly, if the electron acceptor is also a proton donor, the overlapping of salification and complexation processes makes the separate investigation of the interactions very difficult. This is the case in the complexes between amines and picric acid or other related phenols. For complexes of 2,4,6-trinitro-3-hydroxypyridine135 and... 

The niobinm trihydride (1 to 7) illustrates clearly the influence of proton-donor strength The dihydrogen bonding energy increases from the weak proton donor CH3OH (—6.7 kcal/mol), to the relatively strong acid CF3OH (—12.3 kcal/mol). 

In the mechanism of the pancreatic hydrolase ribonuclease, a specialized histidine within the active site acts as a general acid or proton donor to begin cleavage of the phosphodiester linkage of the substrate RNA. 

Cationic polymerization has a history dating back to the early 1800s and has been extensively investigated by Plesch [1,2], Dainton and Sutherland [3], Evans et al. [4,5], Pepper [6,7], Evans and Meadows [8], Heiligmann [9], and others [10-13]. Whitmore [14] is credited with first recognizing that carbonium ions are intermediates in the acid-catalyzed polymerizations of olefins. The recognition of the importance of proton-donor cocatalysts for Friedel-Crafts catalysts was first reported by Evans and co-workers [4,5]... 

A very effective tosylamide cleavage seems to be possible by the cooperative action of the electro-generated anthracene anion radical as electron transfer agent and of ascorbic acid as proton donor and additional reducing agent (Eq. (97))... 

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