Proton transfer to a base

However, the aminoazo product is formed via two pathways. The first is through the 1 1 addition complex (HAArNj )n as side-equilibrium and an intermolecular rearrangement involving redissociation of this complex into the reagents followed by formation of another 1 1 addition complex (HAArNJ )c and the classical C-o-complex (oc in Scheme 13-13). The second pathway starts from the first mentioned 1 1 complex (HAArNJ )N to which a second molecule of amine is added. This complex forms the aminoazo product by proton transfer to a base. The base may be the second amine molecule of the 1 2 complex. 

Reaction via unstable acidic or basic intermediates can be promoted by proton transfer to a base or from an acid, respectively, thus giving rise to base or acid catalysis. The concept is illustrated for nucleophilic attack on carboxamide derivatives (Scheme 11.6). With base catalysis, deprotonation of the first-formed intermediate promotes the forward reaction, as does protonation of the first-formed intermediate with acid catalysis. 

Step 3 Take a proton away. Proton transfer to a base to regenerate the aromatic ring ... 

Keep in mind that the mechanisms of electrophilic aromatic substitution reactions are all very similar. After the formation of the electrophile, attack of the electrophile on the aromatic ring occurs to give a resonance-stabilized cation intermediate.The last step of the mechanism is proton transfer to a base to regenerate the aromatic ring. The base in nitration is water, which was generated in the formation of the electrophile. 

Cyclic ethers can be prepared by the intramolecular Sj j2 reaction of a halogen-substituted alcohol such as a bromo alcohol. Proton transfer to a base such as sodium hydroxide gives a bromo alkoxide. If the solution is dilute, the alkoxide acts as a nucleophile, and an intramolecular reaction displaces a bromide ion. This process is shown below for 5-bromo-l-penten-l-ol. 

The strength of an acid depends on the extent to which a proton can be separated from it and transferred to a base. 

Atom A is more negative than atom B => the proton of H—A is more positive than that of H—B => the proton of H—A will be held less strongly => the proton of H—A will separate and be transferred to a base more readily. 

Hydrogenation of C02 to formic acid (Eq. (1)) is thermodynamically unfavorable unless a base is present (see Scheme 17.1) the proton transfer to the base drives the reaction. In the absence of a base, the reaction usually fails completely or provides only small yields [19, 20], even though the initial rate of reaction was high in one case [21]. For the reaction in water, NaOH, bicarbonates, carbonates and even dialkylamines [22] are used as the base. For the reaction in or-... 

G. Orellana, M. C. Moreno-Bondi, E. Segovia, M. D. Marazuela, Fiber-optic sensing of carbon dioxide based on excited-state proton transfer to a luminescent ruthenium(II) complex, Anal. Chem. 64, 2210-2215(1992). 

Quantitative treatment of rate constants for the hydride attack, / hp, the metal protonation, A fpm. and the exchange process in the framework of Scheme 10.7 have resulted in A hp = 2.7 x 10 M Vs and A fpm = 2.8 x 10 M /s. Thus, the hydride protonation occurs faster by a factor of 10. In earlier chapters we have shown that transition metal hydrides form dihydrogen bonds in the presence of proton donors. Now, based on the principle of microscopic reversibility, one can suggest that proton transfer to a hydridic hydrogen actually occurs via a dihydrogen bond. 

The concept of proton-transfer to the base leads to an important generalisation (18, 52), namely, this type of polymerisation may be initiated not only by aprotic bases but also by primary or secondary amines. The latter may then act in a dual way — as proton "acceptors as well as proton donors. Hence, the initiation by primary or secondary... 

HBr + H- H- + Br2 — HBr + Br. properties The characteristics of matter Examples vapor pressure color density temperature, protective oxide An oxide that protects a metal from oxidation. Example aluminum oxide, proton p A positively charged subatomic particle found in the nucleus of an atom, protonation Proton transfer to a Bronsted base. Example 2 H30+(aq) + S2-(s) — ... 

The absence of scatter in a Bronsted plot for a general base-catalysed reaction can imply that the reaction mechanism involves a rate-limiting proton transfer step. This is because proton transfer to the base in the reaction is closely similar to the equilibrium proton transfer to the base in the reaction which defines the p Ka of the conjugate acid of that base. The observation of scatter, especially for sterically hindered bases (such as 2,6-dimethylpyridine), is evidence that nucleophilic catalysis is operating as opposed to general base catalysis. 

Infrared absorption can be used to estimate the relative amounts of Lewis and Bronsted acid sites on cracking catalysts. Bases complex with Lewis acid sites while proton transfer to the base occurs at Bronsted acid sites. Each has distinct, well-resolved infrared bands. For example, pyridine forms a complex with the Lewis acid site and produces an infrared absorption band at approximately 1450 cm-1. Pyridinium ions form at Bronsted sites and produce an absorption band at approximately 1540 cm-1. The relative intensities of these two bands can be used to estimate the relative amounts of Lewis vs. Bronsted acid sites. 

Eigen (1964) found that a plot of ApR against the rate constant for proton transfer between acetylacetone and a series of bases gave a curved plot. It should be noted, however, that Eigen s explanation for curvature is quite different from the one based on Marcus theory and the reactivity-selectivity principle. The curvature discussed by Eigen is attributed to a change from a rate-determining proton transfer to a diffusion controlled reaction which is independent of the catalyst p. 

The mechanism of the NiFe hydrogenase has been treated by caleula-tional methods, with some interesting conclusions (Pavlov et al., 1998). Scheme 1 of this reference proposes a catalytic cycle based on these results. It was proposed that Fe binds H2 and that a low spin Fe is essential for het-erolytic cleavage of the H6H bond. The next step is proposed to be hydride transfer to Fe and proton transfer to a ligated cysteine thiolate, whieh leads to decoordination of the cysteine and concurrent bridging of the N of CN... 

Although a nitro group has a large effect on thermodynamic acidity, rates of proton transfer are not increased by similar amounts. For nitroethane (pA- 8.60 [99]), water catalysed ionization occurs with a rate coefficient [37] of 3.7 x 10"8 sec-1 compared with the value of 1.32 x 10 2 sec-1 for acetylacetone [89] (pif 8.9 [17]) and the value (ca. 102 sec"1) which would be observed for a normal acid [59] with pK 8.5. Since nitroparaffins ionize slowly the reaction may be followed using classical methods and rates of proton transfer to a wide variety of bases have been measured nitroparaffins [100] and ketones were among the first carbon acids to be studied. Corrections may be required for the presence of the aci-isomer. Figure 4 shows results for the rates of base catalysed ionization of ethyl nitroacetate [101] obtained by spectro-photometric measurement of the increase in concentration of the anion (74) or by measuring the rate of bromination, (74) and (75), viz. 

Proton transfers to metal anions are much slower (see Table 1) than proton transfers to other bases of comparable thermodynamic strength, reflecting the extent of the electronic rearrangement that must occur on protonation of a metal anion . A kinetic-... 

Because the pATg chart is a composite of many experimental observations under widely different conditions, it has the greatest extrapolation errors at its extremes and is most accurate in the center, between -2 and 16. While we will write for simplicity HA ionizing to give H" " + A , it is always true that the proton is transferred to a base. For our pATa chart (in the Appendix) this base is assumed to be water. 

Neutralization of the photoacids as a result of direct proton transfer to (scavenging) bases has also been explored in time-resolved studies. Whereas initial work... 

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