Electron-donating species

Friedel-Crafts (Lewis) acids have been shown to be much more effective in the initiation of cationic polymerization when in the presence of a cocatalyst such as water, alkyl haUdes, and protic acids. Virtually all feedstocks used in the synthesis of hydrocarbon resins contain at least traces of water, which serves as a cocatalyst. The accepted mechanism for the activation of boron trifluoride in the presence of water is shown in equation 1 (10). Other Lewis acids are activated by similar mechanisms. In a more general sense, water may be replaced by any appropriate electron-donating species (eg, ether, alcohol, alkyl haUde) to generate a cationic intermediate and a Lewis acid complex counterion. 

A cation may associate with electron-donating species to form a complex. 

A very important criterion for electron structure is the percent d-character, which characterizes the number of unpaired electrons in the rf-orbitals of the individual metal atom. Because of the vacancies existing in these orbitals, metals will interact with electron-donating species forming electron pairs. It is this interaction that determines the special features of adsorption of these species and, as a consequence, the catalytic activity of a given metal. 

A majority of chemical reactions are liable to take place at the position and in the direction where the overlapping of HO and LU of the respective reactants is maximum in an electron-donating species, HO predominates in the overlapping interaction, whereas LU does so in an electron-accepting reactant in the reacting species which have SO MO s, these play the part of HO or LU, or both"... 

RedOx electrode potentials are the result of an exchange of electrons between metal and electrolyte. In Section 5.4 we have shown that the metal/metal-ion electrode potentials are the result of an exchange of metal ions between metal and electrolyte. In the RedOx system the electrode must be made of an inert metal, usually platinum, for which there is no exchange of metal ions between metal and electrolyte. The electrode acts as a source or sink for electrons. The electrolyte in the RedOx system contains two substances electron donors (electron-donating species) and electron acceptors (electron-accepting species). One example of a RedOx system is shown in Figure 5.4. In this case the electron donor is Fe ", the electron acceptor is Fe , the electrode is Pt, and the electrode process is... 

In Chapter 8, on electroless deposition, we have shown that in the case of electroless deposition the reducing agent Red in the solution is the electron source, the electron-donating species that give electrons to the catalytic surface and the metal ions Mz+ at the interface. In this chapter we show that the substrate itself can also be the electron-donating species. 

It was found that surface derivatives modify the redox properties of Ti02 particles if a surface modifier is an electron-donating species, and that the crucial parameter for effective removal of heavy metal ions is the trade-off between enhanced redox properties of Ti02 by surface modification and the enhanced redox potential of chelated metal ions. Surface modification can lead to the appearance of a charge transfer complex with small-particle Ti02 colloids that have an optical absorption threshold at 730 nm. The red shift of the optical absorption provides improved optical properties for use of visible light, i.e., for solar energy conversion [63]. 

The discussion of Lewis acids made it clear that electrophilic centers other than a proton can react with an electron-donating species. Bond polarization, induced by attached atoms, will polarize atoms and they react with reagents that donate electrons. A simple example is the reaction of carbonyls (C=0) with the nucleophilic... 

In the previous section, atoms with an electron-deficient center that react with an electron-donating species are labeled electrophiles. With the exception of methoxide ion, labefing the electron-donating groups is avoided. When an electron-rich species donates electrons to a proton, it is called a Brpnsted-Lowry base. When that species donates electrons to another atom, such as boron or almninum, it is called a Lewis base. A specific distinction is made when the electrons are donated to carbon because any reaction with carbon is formally an organic reaction. 

Boron trifluoride (BFg) is a classic Lewis acid. Borane, BHg, is a highly reactive boron compound that also functions as a Lewis acid in the presence of a suitable electron-donating species. 

In the early sections of this chapter, alkenes reacted with acids to form a car-bocation. Once the carbocation is formed, it reacts with other electron-donating species, including the alkene itself. Continuous reaction of an alkene to form a new carbocation allows an alkene to continue reaction until a large molecular-weight material known as a polymer is generated. Alkenes also react with radicals to give new compounds. (See Chapter 7, Section 7.4.3, for the structure of radicals.) When an alkene reacts with a radical, the product is another radical. This process can be controlled in many cases to produce polymers. This section will serve as a brief introduction to the chemistry of radicals. 

A sulfonate ester can also be prepared by the reaction of an alcohol and a sulfonic acid under acidic conditions, exactly analogous to the same reaction with a carboxylic acid. In the presence of an acid catalyst, 2-methylpropane-sulfonic acid (181) reacts with ethanol to give ethyl 2-methylpropanesulfonate (187). The mechanism of this reaction is remarkably similar to that for the reaction of a carboxylic acid and an alcohol in the presence of an acid catalyst (see Section 20.5.2). The reaction proceeds by initial reaction of 181 with H+ to give 182. The oxygen of ethanol is the electron-donating species in this reaction, and the most electrophilic atom is sulfur, so reaction of 182 and ethanol generates a new S-O bond in 183. 

That 18 is formed as a product is testament to the fact that benzene reacts with Br+, but formation of Br+ requires that diatomic bromine first react with a Lewis acid. Once ferric bromide and bromine react to form Br - FeBr4, the Br is very electrophilic and may react with even a weak electron-donating species. Formation of 18 is explained by a reaction in which benzene donates electrons to Br to form a new o-covalent C-Br bond. This means that benzene reacts as a Lewis base after the cation is formed because benzene is not a strong enough Lewis base to react with Br2 directly. 

Reductant adj. The electron donating species in an oxidation reduction reaction. 

---