Acid proton transfer

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. 

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... 

As long as the buffer solution contains acetic acid as a major species, a small amount of hydroxide ion added to the solution will be neutralized completely. Figure 18-1 shows two hydroxide ions added to a portion of a buffer solution. When a hydroxide ion collides with a molecule of weak acid, proton transfer forms a water molecule and the conjugate base of the weak acid. As long as there are more weak acid molecules in the solution than the number of added hydroxide ions, the proton transfer reaction goes virtually to completion. Weak acid molecules change into conjugate base anions as they mop up added hydroxide. 

For amide hydrolysis in acid, proton transfer to give a cationic intermediate is easy, and breakdown to products is favored over reversion to starting material process b is hopelessly bad, but process b is better than a. 

For the acidic proton transfer of Eqn. 3-44, the proton solvation processes of Eqns. 3-32 and 3-42 are represented by the proton level versus concentration curves of Eqns. 3-39 and 3-43, respectively, as shown in Fig. 3-19. In this proton level diagram, the proton level in an acetic acid solution is given by the intersecting point (mH,o - where cross each other the occupied proton level versus concentration curve of H3O ion and the vacant proton level versus concentration curve of Ac" ion, as expressed in Eqn. 3-46 ... 

There is a continuous transition to covalent bonding structure 2, in which no distinction can be made between X—H and H—A if X=A. This situation is considered to be that of a hydrogen atom forming two covalent bonds having bond order s = Ijl. In the case of a stronger acid, proton transfer occurs to give 3. There is... 

Grignard reagents are not compatible with carboxylic acids proton transfer converts the Grignard reagent to the corresponding hydrocarbon. 

When the illustrated anion is treated with acid, proton transfer generates the final product as shown below. 

In acid, proton transfers usually occur by adding a proton in the new position, then deprotonating the old position. 

The mechanism of proton transfer for carbon acids often differs in one further way from that for oxygen and nitrogen acids. Proton transfer for these latter can occur through reaction complexes Ij and I2, and a transition state (XXIX) in which the acid and base are separated by a solvent bridge. An alternative transition state is shown in (XXX), viz. 

The pathway in Scheme 3 relates mainly to alkenes activated by keto or aldehyde groups, for reduction in hydroxylic solvents. Under these conditions, radical anions derived from carbonyl compounds are protonated at oxygen, and the resulting enolic radical, HI, is more difficult to reduce than the starting compound. Consequently, fast dimerization of the enol radicals may compete with further reduction. For other substrate types, especially in aprotic solvents containing added acids, proton transfer is to carbon... 

Acid and Base Generic Groups and the Proton Transfer Path Conjugate Bases and Acids Proton Transfer Reactions Favor Neutralization Charge Types... 

A major advance in the solubilization of polyaniline was self-doping by the attachment of a sulfonic acid side group to the backbone ring [47,48]. The resulting material is essentially a polyzwitterion with the acid proton transferring to the basic amine backbone. The polymer is then soluble in mildly basic aqueous solutions and can be coated over large areas. 

In mixed solvents formed by two aliphatic carbonic acids, proton transfer does not take place and interaction is usually limited to a mixed associate formation, according to the equilibrium [9.2]. 

Acidity in solid materials, surface acidity, condensation Acidity-basicity effects must always be taken into account to explain the properties of oxygenated material. Thus, in strong acids, proton transfer to water is complete but this is not so for weaker acids and amphoteric materials. 

Structural formula (A) contains both an amino group (a base) and a carboxyl group (an acid). Proton transfer from the stronger acid (— COOH) to the stronger base (— NH2) gives an internal salt therefore, (B) is the better representation for alanine. Within the field of amino acid chemistry, the internal salt represented by (B) is called a zwitterion (Chapter 18). 

Figure 5.61 Conductivity mechanism of phosphoric acid (PA)-doped polybenzimidazoles (a) water-acid proton transfer (b) proton transfer through a PA chain and (c) benzimidazole ring-PA proton transfer. Reproduced with permission from Ref. [98],...

Among oxygen acids, proton-transfers coupled with structural change elsewhere occur particularly in the addition reactions of the carbonyl... 

Fig. 11.16 A compound figure consisting of both experimental [75] and theoretical results [76], Lower Left Measured proton conductivities under dry conditions versus temperature of three monofunctionalized heptanes 1-heptylphosphonic acid (P-C7), magenta squares, 1-heptylsulfonic acid (S-C7), yellow triangles, and 2-heptylimidazole (I-C7), blue circles. Upper Right Computed energetic barriers for neat (i.e., acid to acid) proton transfer for methylphosphonic acid, magenta squares, methylsulfonic acid, yellow triangles, and methylimidazole, blue circles. The combined results suggest that proton conductivity is at least partially a function of the barrier for proton transfer the experimental proton conductivities are inversely related to the computed proton transfer barrier...

The study observed fliat the O2 adsorption energy for one-fold end-on was 0.43 eV and for two-fold was 0.94 eV. Two-fold bonded oxygen was more stable than one-fold. The dissociation energy for two-fold bonded O2 was 0.74 eV, while the activation barrier for the first reduction step to OOH was less than 0.60 eV at 1.23 V electrode potential. In other words, the first electron transfer has a smaller barrier than that of O2 dissociation. Furthermore, the dissociation barrier for the first electron transfer product OOH was much smaller, 0.06 eV. So, the authors eoncluded that O2 did not dissociate before the first reduction step, and OOH easily dissociated once formed after the first electron transfer step. The paper also demonstrated that the electronic field of the proton increased the electron affinity of the reactant complex and therefore facilitated the reaction. Thus, they proposed that for oxygen reduction on Pt in acid, proton transfer would be involved in the rate determining step because of the ability of its electric field to enhance the electron attracting capability of flic surface-coordinated O2. The authors concluded... 

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