Resonance stabilization, and the

Copolymers of VF and a wide variety of other monomers have been prepared (6,41—48). The high energy of the propagating vinyl fluoride radical strongly influences the course of these polymerizations. VF incorporates well with other monomers that do not produce stable free radicals, such as ethylene and vinyl acetate, but is sparingly incorporated with more stable radicals such as acrylonitrile [107-13-1] and vinyl chloride. An Alfrey-Price value of 0.010 0.005 and an e value of 0.8 0.2 have been determined (49). The low value of is consistent with titde resonance stability and the e value is suggestive of an electron-rich monomer. 

Resonance The compound on the left has resonance stabilization and the compound on the right does not. Based on this factor alone, we would say the compound on the left is more stable. 

Direct oxidation of benzene to phenol is of great interest not only for its industrial importance, but also from a purely scientific point of view. Apart from many earlier reports [35] on the oxidation of benzene to phenol by hydroxyl radicals generated by the reaction of Fe2+ salt (Fenton reagent) with H202 not much is known about the homogeneously catalysed oxyfunctionalization of aromatic C-H bonds. The lack of studies is largely attributable to the fact that the activation of the C-H bond in benzene is difficult owing to its resonance stability and the reactivity of phenol, which is consecutively oxidized to quinones and other by-products. 

The second resonance form puts the positive charge on the electronegative oxygen atom, but it has more covalent bonds, and it provides each atom with an octet in its valence shell. This type of stabilization is called resonance stabilization, and the oxygen atom is called resonance-donating or pi-donating because it donates electron... 

Stability of triphenylmethylium (trityl) salts is due to the resonance stabilization and the lack of /3-protons which could undergo elimination. Several trityl salts are commercially available, they may also be easily prepared by direct mixing of components or by the silver salt method, e.g. [17,18] ... 

Of the various plant sterol conjugates, SFs are known to prevent lipid oxidation in various systems. The activity is based on the capability of ferulic acid to donate hydrogen from the phenolic hydroxyl group to a radical. The resulting SF radical formed is resonance stabilized, and the SF radicals may still effectively interfere with the chain reaction of... 

The polymer can be produced in either cyclohexane or ethylbenzene using butyllithium, preferably sec-butyllithium. Since the addition of a styrene molecule to a DPE chain end is slow (decrease of resonance stability) and the addition of a DPE monomer to a growing styryl chain is fast (increased resonance stability), the polymerization rate decreases with increasing DPE content in the polymerizing monomer mixture. The reactivity ratio rs(K s/Ksd) in cyclohexane was found to be between 0.44 (50 °C) and 0.72 (70 °C). 

The benzylic hydrogen will be abstracted in preference to a 2° hydrogen because the benzylic radical is both 3° and resonance-stabilized, and the 2° radical is neither. 

In the structure shown, galvinoxyl has the radical on the lower oxygen. It has another resonance structure, equivalent to this one, which has the radical located on the upper oxygen. In addition, it has a number of resonance structures which have the radical located on the carbons ortho and para to the two oxygens, and on the carbon connecting the two rings. Because it has considerable resonance stabilization, and the positions where the radical electron density is located are sterically hindered, the radical is relatively unreactive. 

Most copolymerizations in the presence of a free radical initiator obey the simple copolymerization equation. Equation (22-22). Consequently, the copolymerization parameters calculated from this equation can be interpreted directly as the ratios of two rate constants. Since they are relative reactivities, they must be influenced by the polarity, the resonance stabilization, and the steric effects of the monomers. In these cases, resonance stabilization effects are generally stronger than polarity influences, and these, in turn are greater than effects due to steric hindrance. 

If, however, one polymer free radical is resonance stabilized and the other is not, the resonance-stabilized monomer is preferentially added on to the resonance stabilized free radical, since, a new resonance species is formed. That is why styrene has a copolymerization parameter much greater than unity and vinyl esters have copolymerization parameters of much less than unity when these two monomers are copolymerized together. 

In conclusion, it may thus be said that the quantum-mechanical analysis carried out for the mechanism of action of the principal group transfer coenzymes enables us to establish the existence of conunon electronic features responsible for the catalytic activity of a number of them. These features are related to the resonance stabilization and the electronic activation of the transitional intermediates of the catalyzed reactions. This situation is in no way accidental but arises from the fact that such molecules appear to be particularly well suited to play the role of reaction sites, as it is in this type of compound that the simultaneous effects of energy stabilization and electronic activation of the transition forms may be obtained with particular ease. Their choice by nature is thus one of the manifestations of quantum effects in biochemical evolution. 

The relation between the amount of resonance stabilization and the energy difierenee of the resonating states is given, for example, by L. Pauling, The Nature of the Chemical Bond, p. 18. Cornell University Press, Ithaca, N. Y. 1940. 

The delocalization energy is obtained from the difference in the heats of combustion of the monomer and the corresponding monomeric unit of the polymer. In a few systems, such as cyclosiloxanes/poly(siloxanes) or phosphazenes/poly(phosphazenes), the delocalization energies are equal in monomer and polymer. If the monomer is resonance-stabilized and the polymer is not, for example, through absence of planarity, then the resonance energy becomes negative. In the polymerization of benzene to (—CH=CH—) , therefore, A// p will be positive, since is zero. 

When the oxocarbenium is drawn as resonance contributor 52A, it is an oxonium salt, clearly related to oxonium salts derived from ethers or alcohols in the preceding sections. Oxonium ion 52 is actually more stable than 13 or 47 (see preceding discussion) because it is resonance stabilized, and the positive charge can be delocalized on carbon (see 52A and 52B). The resonance form that has a positive charge on carbon (52B) is considered to be an oxygen-stabilized carbocation (hence the term oxocarbenium ion see Chapter 16, Section 16.3) and it can react with nucleophiles. Oxocarbenium ions will be generated in several chemical reactions to be presented in later chapters, and their reactions with suitable nucleophiles will be examined at that time. Ketones behave similarly. 

ANSWER Look at the final products of the two reactions. Reaction with hydroxide leads to a carboxylic acid (pA 4.5) and an alkoxide ion. These two species must react very rapidly to make the more stable carboxylate anion (resonance stabilized) and the alcohol (pA 17). The hydroxide reagent is consumed in this reaction, and the overall process is so thermodynamically favorable that it is irreversible in a practical sense. 

In certain circumstances the formation of a carbinol from a pyrrole and a carbonyl compound can be followed by a second step different from those already mentioned. For example, 3-formyl-2,5-dimethylpyrrole and 2,5-dimethylpyrrole give, in acid conditions, the first-discovered member (24) of an important class of compounds, the pyrromethene salts These highly coloured salts are deary resonance-stabilized, and the reaction, of... 

N-substitution affects polymerizability of the ds-lactams much more than does C-substitution. The largely reduced reactivity given by substituents with +I effect on nitrogen has been attributed to the loss of resonance stabilization and the cis character of the amide bond. ... 

The conjugate base derived from ethanal (acetaldehyde) is more stable than the conjugate base of ethane. Explain why. Answer The conjugate base of ethanal is resonance stabilized, and the conjugate base of ethane is not. 

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