Double bonds resonance stabilization

The values in Table 2.4 show how resonance affects the strengths of bonds. For example, the strength of a carbon-carbon bond in benzene is intermediate between that of a single and that of a double bond. Resonance spreads multiple bond character over the bonds between atoms as a result, what were single bonds are strengthened and what were double bonds are weakened. The net effect overall is a stabilization of the molecule. 

Anionic polymerization is a powerful method for the synthesis of polymers with a well defined structure [222]. By careful exclusion of oxygen, water and other impurities, Szwarc and coworkers were able to demonstrate the living nature of anionic polymerization [223,224]. This discovery has found a wide range of applications in the synthesis of model macromolecules over the last 40 years [225-227]. Anionic polymerization is known to be limited to monomers with electron-withdrawing substituents, such as nitrile, carboxyl, phenyl, vinyl etc. These substituents facilitate the attack of anionic species by decreasing the electron density at the double bond and stabilizing the propagating anionic chains by resonance. 

Where the positive carbon is in conjugation with a double bond the stability is greater because of increased delocalization due to resonance and because the positive charge is... 

Electron-withdrawing substituents in anionic polymerizations enhance electron density at the double bonds or stabilize the carbanions by resonance. Anionic copolymerizations in many respects behave similarly to the cationic ones. For some comonomer pairs steric effects give rise to a tendency to altemate. The reactivities of the monomers in copolymerizations and the compositions of the resultant copolymers are subject to solvent polarity and to the effects of the counterions. The two, just as in cationic polymerizations, cannot be considered independently from each other. This, again, is due to the tightness of the ion pairs and to the amount of solvation. Furthermore, only monomers that possess similar polarity can be copolymerized by an anionic mechanism. Thus, for instance, styrene derivatives copolymerize with each other. Styrene, however, is unable to add to a methyl methacrylate anion, though it copolymerizes with butadiene and isoprene. In copolymerizations initiated by w-butyllithium in toluene and in tetrahydrofuran at-78 °C, the following order of reactivity with methyl methacrylate anions was observed. In toluene the order is diphenylmethyl methacrylate > benzyl methacrylate > methyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > t-butyl methacrylate > trityl methacrylate > a,a -dimethyl-benzyl methacrylate. In tetrahydrofuran the order changes to trityl methacrylate > benzyl methacrylate > methyl methacrylate > diphenylmethyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > a,a -dimethylbenzyl methacrylate > t-butyl methacrylate. 

The first step protonation of the double bond of the enol is analogous to the pro tonation of the double bond of an alkene It takes place more readily however because the carbocation formed m this step is stabilized by resonance involving delocalization of a lone pair of oxygen... 

Allyl alcohol, CH2=CH—CH2OH (2-propen-l-ol) [107-18-6] is the simplest unsaturated alcohol. One hydrogen atom can easily be abstracted from the aHyhc methylene (—CH2—) to form a radical. Since the radical is stabilized by resonance with the C=C double bond, it is very difficult to get high molecular weight polymers by radical polymerization. In spite of the fact that aHyl alcohol has been produced commercially for some years (1), it has not found use as a monomer in large volumes as have other vinyl monomers. 

The carbon—carbon double bond is the distinguishing feature of the butylenes and as such, controls their chemistry. This bond is formed by sp orbitals (a sigma bond and a weaker pi bond). The two carbon atoms plus the four atoms ia the alpha positions therefore He ia a plane. The pi bond which ties over the plane of the atoms acts as a source of electrons ia addition reactions at the double bond. The carbon—carbon bond, acting as a substitute, affects the reactivity of the carbon atoms at the alpha positions through the formation of the aHyUc resonance stmcture. This stmcture can stabilize both positive and... 

Reaction Mechanism. High temperature vapor-phase chlorination of propylene [115-07-17 is a free-radical mechanism in which substitution of an allyhc hydrogen is favored over addition of chlorine to the double bond. Abstraction of allyhc hydrogen is especially favored since the allyl radical intermediate is stabilized by resonance between two symmetrical stmctures, both of which lead to allyl chloride. 

At first, these highly reactive free radicals react with the antioxidant, but as the antioxidant is consumed, the free radicals react with other compounds. Hydrogens on methylene groups between double bonds are particularly susceptible to abstraction to yield the resonance stabilized free radical ( ) ... 

Clearly, in the case of (66) two amide tautomers (72) and (73) are possible, but if both hydroxyl protons tautomerize to the nitrogen atoms one amide bond then becomes formally cross-conjugated and its normal resonance stabilization is not developed (c/. 74). Indeed, part of the driving force for the reactions may come from this feature, since once the cycloaddition (of 72 or 73) has occurred the double bond shift results in an intermediate imidic acid which should rapidly tautomerize. In addition, literature precedent suggests that betaines such as (74) may also be present and clearly this opens avenues for alternative mechanistic pathways. 

A modified definition of resonance energy has been introduced by Dewar (66T(S8)75, 69JA6321) in which the reference point is the corresponding open-chain polyene. In principle this overcomes the difficulties inherent in comparing observed stability with that of an idealized molecule with pure single and double bonds, as thermochemical data for the reference acyclic polyenes are capable of direct experimental determination. In practice, as the required data were not available, recourse was made to theoretical calculations using a semiempirical SCF-MO method. The pertinent Dewar Resonance Energies are listed in Table 30. 

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