Electron-rich sites/species

In contrast, a different reaction mechanism has been proposed by Comotti et al. for the oxidation of glucose over Au/AC (Figure 3.24) [175]. In their mechanism, the first step is the adsorption of hydrated alcoholate, which is formed in advance by alkali, onto Au catalytic sites. The electron-rich Au species might be formed by Au-alcoholate intermediates and then oxygen could be adsorbed onto Au by nucleophilic attack to form Au + -02 or Au2 + -022. Then, carboxylic adds are formed by two-electron reduction of oxygen. In this reaction, H202 must be formed as a by-product and was in fact detected experimentally. 

Electrophile (Section 8.1) An electron-poor species that seeks an electron-rich site similar to a Lewis acid. 

The regioselectivity of cof r carbenoids (and of most carbencs) is typical for an electrophilic species which preferentially attacks the electron-rich sites of the molecules. For example, competitive experiments with olefins bearing different substituents give a mixture of cyclopropanes, which is the result of both electronic and stcric control of the reaction with the electronic factors being often predominating. 

A full description of how a reaction occurs is called its mechanism. There are two general kinds of mechanisms by which reactions take place radical mechanisms and polar mechanisms. Polar reactions, the most common type, occur because of an attractive interaction between a nucleophilic (electron-rich) site in one molecule and an electrophilic (electron-poor) site in another molecule. A bond is formed in a polar reaction when the nucleophile donates an electron pair to the electrophile. This movement of electrons is indicated by a curved arrow showing the direction of electron travel from the nucleophile to the electrophile. Radical reactions involve species that have an odd number of electrons. A bond is formed when each reactant donates one electron. 

Electrophiles are electron-deficient species that can form a covalent bond by accepting two electrons from an electron-rich site. Electrophiles are often positively charged (cations), although they can also be neutral. 

A nucleophile is a species with a negative or partial negative charge. Literally, a nucleophile loves a nucleus. A nucleophile is attracted to a region of positive or partial positive charge (i.e., an electrophile). A nucleophile may be an anion, such as Cl , or the negative portion of a polar molecule, such as the Cl atom in HCl. Nucleophiles are electron-rich. Electron-rich sites and electron-poor sites are attracted to one another. 

Hydrogen binds to As sites at the surface when the GaAs electrode is electron rich when the GaAs electrode is electron poor, the hydrogen adsorbates are replaced by OH species at the As sites. Changes in potential were determined by interrupting the cyclic potential scans every 100 ms for a lmn period at various... 

Reference has already been made to electron-donating and electron-withdrawing groups, their effect being to render a site in a molecule electron-rich or electron-deficient, respectively. This will clearly influence the type of reagent with which the compound will most readily react. An electron-rich species such as phenoxide anion (36)... 

Figure 7.9 Light-dependent binding of CO to Ni at the level of the Ni -C state. After mixing of H2-reduced enzyme with CO-saturated buffer in the dark, the Nij-C state is formed within 10 ms. Carbon monoxide does not bind to Ni due to its high valence state. When illuminated at 30 K the Ni -L state is formed, where the charge density at the Ni-Fe site has greatly increased (see also the shift of the FTIR bands in Fig. 7.6). Upon raising the temperature to 200 K in the dark, the nearby CO now can bind to the electron-rich Ni.The Ni. CO species has been earlier characterized by our group.

This ready nucleophilic substitution at the 6-position is surprising since this position is electron-rich in both dihydrodiazepines and dihydrodiaze-pinium salts and is the site at which electrophilic substitution occurs. The likely explanation is that in the presence of base some prototropic rearrangement of the normal dihydrodiazepine base into a bis-imino form takes place. Although the equilibrium concentration of the bis-imine is likely to be very small (it has not been observed spectroscopically) it would be strongly electrophilic at the 6-position owing to the combined effects of the bromine atom and the two azomethine groups, and could well be the reactive species in the nucleophilic substitution of the bromine atom ... 

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