Proton competition

Computer simulation of such metal-ligand equilibria in bio- 

and Pb amongst 5000 complexes formed with 40 ligands was computed. However, in considering ligand design it is often more helpful to estimate Interferences by use of an interference term a (13,15,16). For example, in the case of proton interference, ol represents the fraction of ligand in its completely deproton-ated form. If Tl Is the total concentration of the uncomplexed drug in the medium, then equation (4) may be derived. 

The corresponding expressions for calcium ion and hydroxide ion interference are equations (5) and (6), respectively, and the effective binding constant, Kgff, of the iron(III)-drug complex is defined by equation (7) 

ACS Symposium Series American Chemical Society Washington, DC, 1980. 

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B5H9 also acts as a weak Brpnsted acid and, from proton competition reactions with other boranes and borane anions, it has been established that acidity increases with increasing size of the borane cluster and that arachno-boranes are more acidic than nido-horancs ... 

But even here there is a snag, for the very basic titrating agent is able to take up protons competitively and, since it exists in solution predominantly as H4penten4+, the effective reaction is... 

The nature of the nucleophile. The elimination-addition route requires a strong base which is capable of abstracting the proton. Competition between attack at carbon and at hydrogen will occur if hydrogen and carbon basicities of the nucleophile are both high. The base and the nucleophile are not necessarily the same. A strong base present in the reaction mixture, e.g. RO, may be responsible for the elimination, while a weaker one, which is a better nucleophile, e.g. RS, may add preferentially to the acetylene. The elimination-addition route... 

In 1958, Parry and Edwards predicted the acidic character of the bridge hydrogens in the lower boron hydrides and proposed that the acid strength should vary directly with the size of the boron framework for a given analogous series D. This has been confirmed for the series B5H9, BeHio, B10H14 by proton competition reactions 18) which proceed to... 

The procedure is based on the proton competition with metal ions and may therefore easily be detected by pH titration. The limitations are, however, that in itself it gives no information as to where the exact ionizing site is located on the molecule. 

Neutron-Proton-Konkurrenz, neutron proton competition 215-... 

As with EDTA, which we encountered in Chapter 9, o-phenanthroline is a ligand possessing acid-base properties. The formation of the Fe(o-phen)3 + complex, therefore, is less favorable at lower pH levels, where o-phenanthroline is protonated. The result is a decrease in absorbance. When the pH is greater than 9, competition for Fe + between OH and o-phenanthroline also leads to a decrease in absorbance. In addition, if the pH is sufficiently basic there is a risk that the iron will precipitate as Fe(OH)2. 

Other auxin-like herbicides (2,48) include the chlorobenzoic acids, eg, dicamba and chloramben, and miscellaneous compounds such as picloram, a substituted picolinic acid, and naptalam (see Table 1). Naptalam is not halogenated and is reported to function as an antiauxin, competitively blocking lAA action (199). TIBA is an antiauxin used in receptor site and other plant growth studies at the molecular level (201). Diclofop-methyl and diclofop are also potent, rapid inhibitors of auxin-stimulated response in monocots (93,94). Diclofop is reported to act as a proton ionophore, dissipating cell membrane potential and perturbing membrane functions. 

Competitive metallation experiments with IV-methylpyrrole and thiophene and with IV-methylindole and benzo[6]thiophene indicate that the sulfur-containing heterocycles react more rapidly with H-butyllithium in ether. The comparative reactivity of thiophene and furan with butyllithium depends on the metallation conditions. In hexane, furan reacts more rapidly than thiophene but in ether, in the presence of tetramethylethylenediamine (TMEDA), the order of reactivity is reversed (77JCS(P1)887). Competitive metallation experiments have established that dibenzofuran is more easily lithiated than dibenzothiophene, which in turn is more easily lithiated than A-ethylcarbazole. These compounds lose the proton bound to carbon 4 in dibenzofuran and dibenzothiophene and the equivalent proton (bound to carbon 1) in the carbazole (64JOM(2)304). 

In chlorination, loss of a proton can be a competitive reaction of the cationic intermediate. This process leads to formation of products resulting from net substitution with double-bond migration ... 

The MS2N2 complexes and their selenium analogues are readily protonated at the nitrogen attached to the metal (Eq. 7.5). Competitive studies show that the selenium complex is a stronger base than its sulfur analogue. 

If the cation has been unchanged, its ability to act as a hydrogen-bond donor has been unchanged, so why is an effect seen at all I propose that there is competition between the anion and the Reichardt s dye solute for the proton. Thus, the values of the ionic liquids are controlled by the ability of the liquid to act as a hydrogen bond donor (cation effect) moderated by its hydrogen bond acceptor ability (anion effect). This may be described in terms of two competing equilibria. The cation can hydrogen bond to the anion [Equation (3.5-2)] ... 

This competition for electrons is reminiscent of the competition for protons among acids and bases. The similarity suggests that we might develop a table in which metals and their ions are... 

The Lewis acid of choice for most of the cyclization reactions is ethylaluminum dichloride, because of its exceptional properties it is a mild Lewis acid, and, as an organometallic compound, can serve as a proton sponge , and thereby inhibit competitive protodesilyla-tion37. The desired precursors reacted smoothly with 1.1 equivalents of ethylaluminum dichloride in toluene or dichloromethane at low temperature to generate diastereoselectively the desired spiro[4.5]decanones38. 

The reversibility of aromatic diazotization in methanol may indicate that the intermediate corresponding to the diazohydroxide (3.9 in Scheme 3-36), i. e., the (Z)-or (is)-diazomethyl ether (Ar — N2 — OCH3), may be the cause of the reversibility. In contrast to the diazohydroxide this compound cannot be stabilized by deprotonation. It can be protonated and then dissociates into a diazonium ion and a methanol molecule. This reaction is relatively slow (Masoud and Ishak, 1988) and therefore the reverse reaction of the diazomethyl ether to the amine may be competitive. Similarly the reversibility of heteroaromatic amine diazotizations with a ring nitrogen in the a-position may be due to the stabilization of the intermediate (Z)-diazohydroxide, hydrogen-bonded to that ring nitrogen (Butler, 1975). However, this explanation is not yet supported by experimental data. 

Since nitration produces acetic acid, the concentration of this as well as of acetyl nitrate can be shown to depend upon the nitric acid concentration giving kinetics third-order in nitric acid (3.16 actually observed). It follows that in the presence of acetic acid the order in nitric acid should fall to 2 (2.31 observed). Likewise, in the presence of added sulphuric acid, from equilibrium (31) it follows that the order in nitric acid should fall, the observed order in this being 1.4 and 1.7 in added sulphuric acid. The retardation by added nitrate was attributed to competition by this ion for protonated acetyl nitrate, viz. 

In summary then, the kinetics and related data are most consistent with protonated acetyl nitrate as the reagent in this medium. It is unfortunate that there is doubt as to the nature of the electrophile, as this medium combines high reactivity with good solvent properties, which has made it popular for studying substituent effects in nitration. Some relative reactivities (mostly obtained under competition conditions) are given in Table 20. 

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