Proton transfer equivalent

Proton transfer equivalent (PTE). Protons can be transferred from various compounds in solutions according to the following dissociations. 

In Section 8, the material on solubility constants has been doubled to 550 entries. Sections on proton transfer reactions, including some at various temperatures, formation constants of metal complexes with organic and inorganic ligands, buffer solutions of all types, reference electrodes, indicators, and electrode potentials are retained with some revisions. The material on conductances has been revised and expanded, particularly in the table on limiting equivalent ionic conductances. 

Since the actual number of protons transferred between the analyte and titrant is uncertain, we define the analyte s equivalent weight (EW) as the apparent formula weight when = 1. The true formula weight, therefore, is an integer multiple of the calculated equivalent weight. 

A kinetic expression which is equivalent to that for general acid catalysis also occurs if a prior equilibrium between reactant and the acids is followed by rate-controlling proton transfer. Each individual conjugate base will appear in the overall rate expression ... 

Acidic hydrolysis of 14 occurs via protonation of the nitrogen followed by attack of water on the resulting cationic intermediate. Proton transfer followed by ring-opening affords cation 15, which is trapped by a second equivalent of water. Another proton transfer followed by loss of the amino group affords protonated carboxylic acid 16, which loses to provide the carboxylic acid product. 

Scheme 14-1. General in-line monoanionic mechanism of phosphodiester cleavage transesterification catalyzed by hairpin ribozyme the first proton transfer (PT1), the nucleophilic attack (Nu), and the exocyclic cleavage (Cl) steps are shown, and the Oip and O2p pathways are indicated by blue and red colored hydrogens, respectively. For the uncatalyzed model reaction in solution, the Ojp and O2p pathways are energetically equivalent...

During the course of the work, we also conducted further studies of the reactions of proton hydrates with CH3COCH3 and CH3COOCH3. The reaction mechanisms were found to change from proton transfer to ligand switching and ultimately to an association process, which would be equivalent to adsorption in the case of bulk systems. 

Examining Table 2, one comes to the conclusion that only Ba2+ (H20)n where n > 1 can be produced by the association reactions of M2+ with H20. For all the other ions only the monohydrate will be obtainable. For ions with high IE(M) values, even the monohydrate, M2+H20 may not be obtained because of charge transfer reactions to H20 (see equation 22). Other protic solvents will lead to charge reduction by proton transfer at different values of r. Only NH3 has been examined.71 It leads to much more facile charge reduction than H20. Many of the doubly charged ions that were observed as hydrates could not be observed as the equivalent clusters of NH3. 

A misconception that we commonly encounter is that a spectrum can be a mixture of the salt and the free base. This is an excuse that is often used by chemists to explain an inconveniently messy looking spectrum Don t be tempted by this idea - proton transfer is fast on the NMR timescale (or at least, it is when you use a polar solvent ) and because of this, if you have a sample of a compound that contains only half a mole-equivalent of an acid, you will observe chemical shifts which reflect partial protonation and not two sets of signals for protonated and free-base forms. It doesn t happen - ever ... 

Despite a very unfavorable proton transfer using (35) as an EGB, condensation of acetophenone with an aromatic aldehyde took place within a matter of hours. The reaction led to Q , 8-unsaturated ketones, which underwent the Michael addition with a second equivalent of deprotonated ketone. Scheme 30, [80, 81]. The EGB was generated ex situ. 

The first step of the reaction path involves the addition of H2O2 to the Fe " resting state to form an iron-oxo derivative known as Compound I, which is formally two oxidation equivalents above the Fe state (Fig. 2). The well studied Compound I contains a Fe" = 0 structure and a n cation radical. In the second step. Compound I is reduced to Compound II with a Fe =0 structure. The reduction of the n cation radical by a phenol or enol is accompanied by an electron transfer to Compound I and a proton transfer to a distal basic group (B), probably His 42 (Fig. 3, step 1). The native state is regenerated on one-electron reduction of Compound II by a phenol or an enol. In this process, electron and proton transfers occur to the ferryl group with simultaneous reduction of Fe" to Fe (Fig. 3, steps 2-3) and formation of water as the leaving group (Fig. 3, step 4). 

In 1903, Lapworth described his findings of the action of potassium cyanide on benzaldehyde [28], He postulated that cyanide adds to benzaldehyde to form V, followed by proton transfer of the a-labile hyd rogen, forming intermediate VI which is now referred to as an acyl anion equivalent. Addition to another molecule of benzaldehyde occurs to form VII (Scheme 1). The unstable cyanohydrin of benzoin VII then collapses to form benzoin and potassium cyanide. Additionally, Lapworth tested the reversibility of the addition of cyanide to benzaldehyde by first forming hydroxybenzyl cyanide (protonated variant of V) and subjecting it to benzaldehyde and base, in which benzoin was recovered. 

Breslow and co-workers elucidated the currently accepted mechanism of the benzoin reaction in 1958 using thiamin 8. The mechanism is closely related to Lapworth s mechanism for cyanide anion catalyzed benzoin reaction (Scheme 2) [28, 29], The carbene, formed in situ by deprotonation of the corresponding thiazolium salt, undergoes nucleophilic addition to the aldehyde. A subsequent proton transfer generates a nucleophilic acyl anion equivalent known as the Breslow intermediate IX. Subsequent attack of the acyl anion equivalent into another molecule of aldehyde generates a new carbon - carbon bond XI. A proton transfer forms tetrahedral intermediate XII, allowing for collapse to produce the a-hydroxy ketone accompanied by liberation of the active catalyst. As with the cyanide catalyzed benzoin reaction, the thiazolylidene catalyzed benzoin reaction is reversible [30]. 

The proposed catalytic cycle is shown in Scheme 35 and begins with the imida-zolylidene carbene adding to the enal. Proton transfer provides acyl anion equivalent XLVII, which may be drawn as its homoenolate resonance form XLVIII. Addition of the homoenolate to aldehyde followed by tautomerization affords L the precursor for lactonization and regeneration of the carbene. 

The final step drives the reaction to completion. Ethyl acetoacetate is more acidic than any of the other species present, and it is converted to its conjugate base in the final step. A full equivalent of base is needed to bring the reaction to completion. The /i-kctocstcr product is obtained after neutralization and workup. As a practical matter, the alkoxide used as the base must be the same as the alcohol portion of the ester to prevent product mixtures resulting from ester interchange. Because the final proton transfer cannot occur when a-substituted esters are used, such compounds do not condense under the normal reaction conditions. This limitation can be overcome by use of a very strong base that converts the reactant ester completely to its enolate. Entry 2 of Scheme 2.13 illustrates the use of triphenylmethylsodium for this purpose. 

Why are there two independent time constants corresponding to two successive proton transfer processes in such a system with an equivalent component of... 

The nicotinamide coenzymes are biological carriers of reducing equivalents (electrons). The most common function of NAD+ is to accept two electrons and a proton (H equivalent) from a substrate undergoing metabolic oxida-tion to produce NADH, the reduced form of the coenzyme. This then diffuses or is transported to the terminal-electron transfer sites of the cell and reoxidized by terminal-electron acceptors, 02 in aerobic organisms, with the concomitant formation of ATP (chapter 14). Equations (8), (9), and (10) are typical reactions in which NAD+ acts as such an acceptor. 

We have made use above of the idea that the magnitude of a (or / ) measures the extent of proton transfer at the transition state or, equivalently, of the position of the transition state along the proton-transfer reaction coordinate. Figures 8.4 and 8.5 show, respectively, the reaction coordinate diagrams drawn according to the Hammond postulate for Reaction 8.17 in the extreme cases where HX is a... 

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