Proton attack

Section 17 2 The carbonyl carbon is sp hybridized and it and the atoms attached to It are coplanar Aldehydes and ketones are polar molecules Nucleophiles attack C=0 at carbon (positively polarized) and electrophiles especially protons attack oxygen (negatively polarized)... 

Protonation. As expected from Tt-electron distribution, the proton attacks the negatively charged odd positions in the polymethine chain, eg,... 

Each term in this equation represents an independent pathway. The low-pH arm in the figure is equivalent to reaction (6-57), or one similar to it, in which the proton attacks the substrate directly. The high-pH pathway represents the unimolecular reaction of the substrate or else its reaction with water. As this discussion illustrates, a reaction whose pH profile shows upward bends can be analyzed in terms of separate pathways. A complex profile can be separated into regions at each upward bend each region is a distinct pathway. 

The electrostatic mixing by the positive charge polarizes rin the same direction (Scheme 12b, cf. Scheme 8a), possibly more significantly than the overlap mixing. The n orbital is the frontier orbital. The proton attacks on C. The regioselectivity is reversed. 

The demetalation kinetics of ZnTTP by an acidic aqueous phase have also been reported [61]. In this study, ZnTTP was considered to adsorb at the interface producing Zn and free base porphyrin by proton attack. The demetalation kinetics of ZnTTP were analyzed as a pseudo-first-order reaction, because the proton concentration in the aqueous phase was in large excess. The rate law was found to be described by... 

Recent calculation on pyridine shows that the HO MO is not the lone-pair orbital (or HO) but a n orbital. Nevertheless, an acceptor-like proton attacks the a HO instead of n MO. 

The kinetics of formation and dissociation of the Ca2+, Sr2+ and Ba2+ complexes of the mono- and di-benzo-substituted forms of 2.2.2, namely (214) and (285), have been studied in water (Bemtgen et al., 1984). The introduction of the benzene rings causes a progressive drop in the formation rates the dissociation rate for the Ca2+ complex remains almost constant while those for the Sr2+ and Ba2+ complexes increase. All complexes undergo first-order, proton-catalyzed dissociation with 0bs — kd + /ch[H+]. The relative degree of acid catalysis increases in the order Ba2+ < Sr2+ < Ca2+ for a given ligand. The ability of the cryptate to achieve a conformation which is accessible to proton attack appears to be inversely proportional to the size of the complexed metal cation in these cases. 

An intermediate acylnickel halide is first formed by oxidative addition of acyl halides to zero-valent nickel. This intermediate can attack unsaturated ligands with subsequent proton attack from water. It can give rise to benzyl- or benzoin-type coupling products, partially decarbonylate to give ketones, or react with organic halides to give ketones as well. Protonation of certain complexes can give aldehydes. Nickel chloride also acts as catalyst for Friedel-Crafts-type reactions. 

Reaction (I) is a bimolecular reaction involving a protonic acid on the olefin. The proton attacks the monomer and adds to a carbon atom that has the maximum electron density. 

The abundance of each element is fixed by its binding energy, which characterises its strength as an entity, and the temperature and density of free neutrons and protons attacking the nucleus (Fig. A3.1). If, as is usually the case, nuclear equilibrium is reached before a significant number of radioactive decays have had the time to occur, an auxiliary constraint can be imposed the total number density of protons and neutrons, both free and bound, must preserve the mean n/p ratio. 

A cis-coordinating ligand is apparently required to bind and activate MeOH so that a methoxy group is transferred to the polyketone chain and a hydride remains on palladium. Two mechanisms are possible for this reaction (i) nucleophilic attack by the oxygen at the acyl carbonyl with concerted formation of Pd-H (ii) formation of a Pd(acyl) (methoxy) complex and H, followed by reductive elimination and subsequent proton attack on a Pd center. No experimental evidence favoring either mechanism in ethene/CO copolymerisation has been provided so far. 

Scheme 7.1 Proton attack directed at a metal or hydride center (schematically).

Figure 7.1 Geometries of the model Re hydride and the products of its proton attack by a water molecule optimized at the DFT/B3PW91 level with a standard LANL2DZ contraction. Energies of the hydrogen-bonded complexes are given in kcaPmol.

Proton transfer to negatively charged hydrogen atoms has attracted the attention of many chemists over the last two decades. This process plays an important role in many chemical and biochemical phenomena that occnr in the gas phase, in solution, and in the solid state [1-3], For example, direct proton attack on hydride ligands generates transition metal dihydrogen complexes which are then involved in various catalytic transformations [4] ... 

On the other hand, it is well known that a dihydrogen complex can be generated through direct proton attack on a hydride ligand or initial protonation of a metal center, leading to a new classical dihydride, [MH2]", which then converts to the dihydrogen complex [M(ti -H2)]. The latter is a thermodynamic product of the protonation reaction shown in Scheme 10.4 [16,17]. 

Other substituents also influence the dissolution rate. Of these, and, in particular, Cr substantially stabilized goethite against proton attack, whereas the opposite effect was found for Mn (Fig. 12.23) (Schwertmann, 1991). Similar results were obtained by Lim-Nunez and Gilkes (1987). Unsubstituted and Cr-goethites had... 

These electron density considerations imply a preferential proton attack on position 2, which actually was observed in the experiments. Whereas only the 1,2-derivatives could be detected, after chromatographic purification, following the in... 

There has been some controversy regarding the site of attack of the proton on the basis of the dipolar structures I and I", it might be inferred that the proton attack should occur on the oxygen atom the fact that N-nitrosamines form 0-complexes and 0-alkylated products would seem to support this view (iT) However, it is currently believed that the N-protonated form is the one that leads to denitrosation, irrespective of whether or not initial protonation occurs on oxygen (18). 

The rearrangement of the 5-cyano-4,5-dihydro-l//-azepine (93) to furo[2,3-6]pyridine (95 Scheme 9) with sodium nitrate in glacial acetic acid or with silver nitrate in aqueous ethanol proceeds by initial protonation at either C-3 or C-6 followed by hydrolysis to the cyanooctanedione ester (94). By carrying out the rearrangement with an acid ion exchange resin it is now possible to isolate the dione ester (77CJC4061). Likewise, the hydrolysis of the tetrahydro-2-benzazepine (96) to an 0 -(anilinoalkyl)benzophenone is an example of proton attack at the /3 -carbon of the enamine system (77JA5045). 

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