Electron supply

In cationic polymerization the active species is the ion which is formed by the addition of a proton from the initiator system to a monomer. For vinyl monomers the type of substituents which promote this type of polymerization are those which are electron supplying, like alkyl, 1,1-dialkyl, aryl, and alkoxy. Isobutylene and a-methyl styrene are examples of monomers which have been polymerized via cationic intermediates. 

The free radical initiators are more suitable for the monomers having electron-withdrawing substituents directed to the ethylene nucleus. The monomers having electron-supplying groups can be polymerized better with the ionic initiators. The water solubility of the monomer is another important consideration. Highly water-soluble (relatively polar) monomers are not suitable for the emulsion polymerization process since most of the monomer polymerizes within the continuous medium, The detailed emulsion polymerization procedures for various monomers, including styrene [59-64], butadiene [61,63,64], vinyl acetate [62,64], vinyl chloride [62,64,65], alkyl acrylates [61-63,65], alkyl methacrylates [62,64], chloroprene [63], and isoprene [61,63] are available in the literature. 

Figure 10.2 shows the effect on the corrosion reaction shown in Fig. 10.1 of providing a limited supply of electrons to the surface. The rate of dissolution slows down because the external source rather than an iron atom provides two of the electrons. Figure 10.3 shows the effect of a greater electron supply corrosion ceases since the external source provides ail the requisite electrons. It should be apparent that there is no reason why further electrons could not be supplied, when even more hydroxyl (OH ) ion would be produced, but without the possibility of a concomitant reduction in the rate of iron dissolution. Clearly this would be a wasteful exercise. 

The thermodynamic and electrode-kinetic principles of cathodic protection have been discussed at some length in Section 10.1. It has been shown that, if electrons are supplied to the metal/electrolyte solution interface, the rate of the cathodic reaction is increased whilst the rate of the anodic reaction is decreased. Thus, corrosion is reduced. Concomitantly, the electrode potential of the metal becomes more negative. Corrosion may be prevented entirely if the rate of electron supply is such that the potential of the metal is lowered to the value where it is found that anodic dissolution does not occur. This may not necessarily be the potential at which dissolution is thermodynamically impossible. 

The nitration of some heterocyclic compounds by nitric acid in sulphuric acid has been studied by Katritzky et al.s0 d and the results are exactly as expected in that electron-supplying substituents in the ring favour reaction on the conjugate acid whereas electron-withdrawing substituents produce reaction on the free base. Rate coefficients and the kinetic parameters for nitration of pyridine derivatives (and some benzene analogues)50 are given in Table 4a. 

The reaction was also found to be inhibited by addition of dioxan and tetra-hydropyran, the rate decrease being proportional to the ether concentration. The results were rationalised by the assumption that 2 1 and 1 1 phenol ether complexes were formed, respectively. The inhibition was attributed to participation of the hydroxyl group in solvation of the halogen atom of the alkyl halide, though this seems much less likely than a straightforward modification of the electron-supplying effect of the substituent3 54. 

Yamase and Goto406 determined first- and second-order rate coefficients for the aluminium chloride-catalysed reaction of halide derivatives of benzoic acid (lO5 = F, 1.73 Cl, 4.49 Br, 4.35 I, 0.81) and phenylacetic acid (105fc2 = F, 12 Cl, 21 Br, 9 I, 6) with benzene. The maxima in the rates for the acid chloride are best accommodated by the assumption that a highly (but not completely) polarised complex takes part in the transition state. Polarisation of such a complex would be aided by electron supply, and consistently, the acetyl halides are about a hundred times as reactive as the benzoyl compounds (see p. 180, also Tables 105 and 108). 

Kinetic studies of mercuration have also been used as a test for hyperconjugation. Toluene and toluene-aaa-mercuric acetate (0.5 M) in acetic acid containing water (0.25 M) and perchloric acid (0.050 M) and an isotope effect, ArH/ArD = 1.00 0.03, obtained. This insignificant effect was considered as evidence against the participation of hyperconjugation in electron supply by a methyl group449. 

The first studies were concerned with deuteration of aromatics by deuterated potassamide (0.02 M) in liquid ammonia (Table 176)582,583. From this data it was not wholly apparent that electron-supplying substituents decrease the reaction rate and vice versa as has been subsequently confirmed. A further study of... 

The final aspect of the mechanism, namely the effect of different electron supply in the alkyl groups of ArSiR3 has now been settled. From the ease of cleavage of MR3 groups (M = metal) noted above, one would expect that increased electron supply from R would increase the reaction rate. The first kinetic studies664 in fact indicated the opposite, as shown by the data in Table 229, and although the... 

We can extend the Lewis symbols introduced in Section 2.2 to describe covalent bonding by using a line (—) to represent a shared pair of electrons. For example, the hydrogen molecule formed when two H- atoms share an electron pair (H=H) is represented by the symbol H—H. A fluorine atom has seven valence electrons and needs one more to complete its octet. It can achieve an octet by accepting a share in an electron supplied by another atom, such as another fluorine atom ... 

FIGURE 3.45 In an n-type semiconductor, the additional electrons supplied by the electron-rich dopant atoms enter the conduction band (forming the pink band at the bottom of the conduction band), where they can act as carriers for the current. 

Faraday s law of electrolysis The amount of product formed or reactant consumed by an electric current is stoichiometrically equivalent to the amount of electrons supplied. 

To determine the amount of electrons supplied by a given charge, we use Faraday s constant, F, the magnitude of the charge per mole of electrons (Section 12.4). Because the charge supplied is wF, where n is the amount of electrons (in moles), and Q = nF, it follows that... 

So, by measuring the current and the time for which it flows, we can determine the amount of electrons supplied. By combining the amount of electrons supplied with... 

The number of electrons required to reduce a species is related to the stoichiometric coefficients in the reduction half-reaction. The same is true of oxidation. Therefore, we can set up a stoichiometric relation between the reduced or oxidized species and the amount of electrons supplied. The amount of electrons required is calculated from the current and the length of time for which the current flows. 

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