Ammonium protonated

The first four values are for the carboxyl protons, and the remaining two values are for the ammonium protons. A ladder diagram for EDTA is shown in figure 9.26. The species Y becomes the predominate form of EDTA at pH levels greater than 10.17. It is only for pH levels greater than 12 that Y becomes the only significant form of EDTA. 

After 19 hours, no reaction between the zinc chelate 2 and benzaldehyde can be detected at 20 °C. However, 10 mol % of the zinc chelate effectively catalyzes theenantioselective addition of diethylzinc to aromatic aldehydes. The predominant formation of the S-configurated products, effected by this conformationally unambiguous catalyst, can be explained by a six-mem-bered cyclic transition state assembly17. The fact that the zinc chelate formed from ligand M is an equally effective catalyst clearly demonstrates that activation of the aldehyde moiety does not occur as a consequence of hydrogen bond formation between the ammonium proton of the pyrrolidine unit and the aldehydic oxygen. 

Fig. 9. 600 MHz EXSY spectrum showing the exchange of magnetization between ammonium protons and two distinct water components in a peptidyl-water cluster bound to -aminomethyl polystyrene resin swollen in DMF-d1. Reproduced with permission from Ref. 88. Copyright 2001 American Chemical Society.

The first four pAi values apply to carboxyl protons, and the last two are for the ammonium protons.11 The neutral acid is tetraprotic, with the formula H4Y. A commonly used reagent is the disodium salt, Na2H2Y 2H20.12... 

Presumably, the Si—H bond in the cationic species points toward one of the ammonium protons, thus favoring an easy hydrogen elimination by their thermolysis (equation 187)413. 

A similar simplification is commonly used in the description of the dynamic effects observed for protons which are attached to carbon atoms which in turn are bonded to an ammonium group where the ammonium protons are exchanging with the environment. (87)... 

The equilibrium concentrations are also known accurately in most of the intermolecular exchange processes. This is the case in the exchange process of ammonium protons in acidified aqueous solutions of ammonium salts. In such circumstances, the mole fractions of the species involved in the equilibrium are unambiguously determined by the composition of the sample under investigation. In other cases equilibrium parameters (concentrations) have to be determined experimentally. They are usually as interesting as are the corresponding kinetic parameters. 

An alternative explanation is based on the charge type of the reaction. The zwitterions which are initially formed on reaction with amines show two characteristic features (a) The oonformers (112) and (114) with eclipsed nucleophile and electron pair have extra stabilization resulting from the interaction of opposite charges, (b) An ammonium proton is available in the vicinity of the negative charge. 

Because a protonated amino acid has at least two dilfeent protons that can be removed, a value is reported for each of these protons. For example, the p/fg of the carboxy proton of alanine is 2.35 and the pATa of the ammonium proton is 9.87. Table 28.1 lists these values for all 20 amino acids. 

The solution stabilities of the rare earth complexes with N-methyliminodiacetic acid, N-henzyliminodiacetic acid, N-phenyliminodiacetic acid, N-methoxyethyliminodiacetic acid, and N-methylmercaptoethyliminodiacetic acid have been measured at 25°C. and /x = 0.1 (KNOj). The linear relationship between the pk values for the ionization of the ammonium proton and the log Kj values for terdentate iminodiacetate ligands is demonstrated. This relationship is used to provide a measure of the free energy of formation of the third chelate ring in quadridentate iminodiacetates. 

At appreciable ionic strength these microscopic constants should be written in terms of activities rather than concentrations. The constants are normally estimated by spectral means. There are three microscopic ionization constants for the loss of the first proton, six for the loss of the second, and three for the loss of the last. In Table 3-2 the subscripts 1, 2, and 3 denote the carboxyl, sulfhydryl, and ammonium protons thus 32 is the microscopic constant for the ion formed by loss of the sulfhydryl proton from the species that has already lost the ammonium proton. 

In vacuo the gauche conformer is not an energy minimum a fiiUy geometry optimization would lead to the transfer of one ammonium proton to the carboxylate group. On the other hand, this conformer becomes a true minimum... 

A transition-state model of the Orito reaction is presented at the end of the section although various models have been proposed on many occasions, the structure shown in Figure 10.2 is currently the most reliable. The CD is adsorbed onto the catalyst surface with the quinoline ring facing against the surface, while the hydroxy group stays near the surface. In this conformation (the so-called Open-3 ), the ammonium proton at the quinuclidine interacts with the carbonyl and is reduced. 

The ammonium protons split the methyl resonance into a quartet (n + 1 = 4) intensities 1 3 3 1... 

While adding base to 46, the pH of the solution can be measured after one equivalent of base has been added (the isoelectric point), but the pH can also be measured when half of the molar equivalents of base have been added. This is the point where half of 46 is converted to 45, and the pH of the solution at this point is equal to pKj. Adding a second molar equivalent of base to 45 removes the ammonium proton to give 47. Once again, the point at which half of the... 

The acidic amino acids have a third pK value listed pKg (shown in Table 27.2). This pK value refers to the acidity of the acidic unit on the side chain. The usual equilibrium for an amino acid is shown for glycine 52, which is in equilibrium with 74 and 75. The equilibrium for an amino acid with an acidic side chain is more complex. In the case of glutamic acid, the zwitterion form is 66. The value for pK results from loss of the carboxyl proton from the a-amino acid unit in 76 to form zwitterion 66. The next most acidic proton is the carboxyl proton on the side chain, and loss of this proton generates 77 from 66. The equilibrium between 66 and 77 is represented by Kg and leads to the value of pKg in Table 27.2. Loss of the ammonium proton from 77 gives 78, represented by pKg. When the side chain has an acidic proton that is less acidic than the ammonium proton or the carboxyl proton—such as the phenolic proton in tyrosine (60)—the equilibrium is slightly different. 

Amino acid 60 is the zwitterionic form and it is in equilibrium with 79, represented by pK. The ammonium proton is more acidic than the phenolic proton, so 60 is in equilibrium with 80 for PK2 this is followed by loss of the phenolic proton to give 81. The equilibrium between 80 and 81 is represented by Kg, which is measured as pKg in Table 27.2. The real point is that pKg measures the acidity of the side chain acid, but does not necessarily indicate the... 

Chemical shifts of amine protons lie around 0.5-5 ppm depending on solvent, concentration, and hydrogen bonding. Those of ammonium protons are found between ca. 6 and 9 ppm ... 

In acidic media (e.g., in trifluoroacetic acid as solvent), the exchange of the ammonium protons is slowed down to such an extent that the vicinal coupling H-N -C-H generally becomes observable. In other media, signals are usually broad owing to intermediate exchange rates. 

The signals of amine and especially of ammonium protons are often broadened additionally because the coupling is only partly eliminated by the... 

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