Resonance stabihzation

GopolymeriZation. The importance of VDC as a monomer results from its abiHty to copolymerize with other vinyl monomers. Its Rvalue equals 0.22 and its e value equals 0.36. It most easily copolymerizes with acrylates, but it also reacts, more slowly, with other monomers, eg, styrene, that form highly resonance-stabiHzed radicals. Reactivity ratios (r and r, with various monomers are Hsted in Table 2. Many other copolymers have been prepared from monomers for which the reactivity ratios are not known. The commercially important copolymers include those with vinyl chloride (VC),... 

Cycloheptatriene might be expected to be even more acidic, since seven resonance contributors can be drawn for its conjugate base. However, the fact that several resonance contributors can be drawn for a molecule does not necessarily guarantee that it will actually be resonance stabihzed (see also Chapter 12, Problem 9). 

The exact enthalpy of polymerization for a particular monomer will depend on the steric and electronic effects imposed by the substituents attached to the E=E double bond. For olefins, resonance stabihzation of the double bond and increased strain in the polymer due to substituent interactions are the most important factors governing AHp For example, propylene has a calculated AH of -94.0 kJ moT, whereas the polymerization of the bulkier 2-methylpropene is less exothermic (-78.2 kJ moT ) [63]. Due to resonance effects, the experimentally determined AH of styrene (-72.8 kJ mol ) is less exothermic than that for propylene, while that for bulkier a-methylstyrene is even less favorable (-33.5 kJ moT ) [63]. In general, bulky 1,2-disubstituted olefins (i.e., PhHC= CHPh) are either very difficult or impossible to polymerize. 

There is one other way to stabilize a carbocation (other than alkyl gronps)— resonance. If a carbocation is resonance stabihzed, then it will be easier to form that... 

There are two kinds of systems that you should learn to recognize a LG in a benzylic position and a LG in an aUyhc position. Compounds like this will be resonance stabihzed when the LG leaves ... 

The base thus formed from the olefin has a negative charge on a carbon atom, and so may be referred to as a carbanion. The allylic hydrogen is the most acidic because removing it results in a resonance-stabihzed intermediate. 

The different hole injection and transport materials can be compared relative to TPD. The spiro derivative Spiro-TAD (36a) has a lower first oxidation potential that can be explained by the better resonance stabihzation of the radical cation [87]. The material exhibits two successive one-electron oxidations (0.23 and 0.38 V vs. Fc/Fc ) and one subsequent formal two-electron oxidation (0.58 V) to the tetracation (Fig. 3.28). 

In contrast, ethyl 3-oxo-4-(triphenylphosphoranylidene)butyrate behaves as a C,0-bis(nucleophile) when reacted with unsaturated 5(4//)-oxazolones 584 with a leaving group at the exocyclic p-carbon. In this case, initial Michael reaction generates 585 that eliminates HX to produce a resonance stabihzed ylide 586. Cyclization of 586 with ring opening leads to the interesting ylide intermediate 587 used for the synthesis of 2//-pyran-2-ones (Scheme 7.185). 

Many carbon acids, upon losing the proton, form carbanions that are stabilized by resonance. Structural reorganization (movement of atoms to different positions within the molecule) may accompany this. Chloroform, HCN, and 1-alkynes do not form resonance-stabihzed carbanions, and these behave kinetically as normal acids.It has been reported that carborane acids, such as H(CHBnH5Cl6), are the strongest isolable (Lewis-free) Brpnsted acids known. 

In this reaction, the nascent hydroxyl radical, breaking away from the hydroperoxide molecule, abstracts the weakly bonded aH-atom from cyclohexanone, forming water and a resonance-stabihzed ketonyl radical (Figure 1.2). This reaction features a significantly lower activation barrier (i.e., 28 kcal/mol) and prevents facile in-cage radical recombination. The theoretically predicted rate constant of this reaction (i.e., 0.6 X10 at 418 K) quantitatively... 

The degree of ionic character depends on the nature of the metal, the medium, and the substituents on the carbanionic carbon atom. For simple alkyls (those in which resonance stabiHzation of the negative charge is not expected), the nature of the metal is especially important. The percent ionic character (alternatively, the percent contribution of the ionic resonance structure to the hybrid) increases with increasing difference in electronegativity of the two atoms. 

Changes in the transition metal atom of the complex are expected to have larger effects on the physical properties [15]. Attempts to prepare compounds, where such effects should be present (namely with Pt, Ni and Cu), failed, due to the instability of those complexes in the +3 formal oxidation state (lack of resonance stabihzation in this structure) [10]. 

The stability of charge-transfer complexes depends upon internal resonance stabihzation. This degree of stabilization determines how easily the diradicals open up [183]. Consequently, the stabihty also determines how the copolymerization occurs. It can occur spontaneously, or under the influence of light or heat, or because of an attack by an initiating free radical. 

It is important to emphasize that 1,3-butadiene is not resonance stabihzed that is, there is no resonance delocalization in 1,3-butadiene. Although there is some interaction of the p-orbitals on C2 and C3 (as discussed for 4B), there is no resonance. A colloquial way to express this states that one C=C unit knows what the other is doing, but the Ji-bonds remain locahzed on each individual C=C unit. 

Phenyl and alkenyl (-CH=CH2) substituents, although electron-pushing inductively, cau resonance stabihze the anionic propagating species in the same manner as a cyano group Eq. (8.4)]. Monomers such as styrene and 13-butadiene can therefore undergo both anionic and cationic... 

Kim YS (2011) Resonance-stabihzed anion exchange polymer electrolytes. US DOE hydrogen and fuel cells program and vehicle technologies program... 

Carbocation rearrangements generally do not occur when the carbocation is already tertiary unless a rearrangement will produce a resonance-stabihzed carbocation for example ... 

BHA is a mixture of constitutional isomers. These compounds function as radical inhibitors, because they react with radicals to generate resonance-stabihzed radicals ... 

The lone pair occupies a p orbital and is delocalized by the aromatic system. This resonance stabihzation is lost if the lone pair is protonated, and as a result, the nitrogen atom of an aryl amine is less basic than the nitrogen atom of an alkyl amine. If the aromatic ring bears a substituent, the basicity of the amino group will depend on the identity of the substituent (Table 23.2). Electron-donating groups, such as methoxy, sHghdy increase the basicity of aryl amines. 

FIGURE 22.39 All OH groups in a sugar can be reversibly protonated. In addition to the protonation at C(3) shown, the top reaction represents protonation at C(2), C(4), and C(6).The bottom reaction shows protonation of the OH at the anomeric C(l). Only in this intermediate can an oxygen kick out water and provide resonance stabihzation for the resulting cation. Addition of alcohol at C(l) followed by a deprotonation gives the glycoside. 

To understand why some substiments make a benzene ring react faster than benzene itself (activators), whereas others make it react slower (deactivators), we must evaluate the rate-determining step (the first step) of the mechanism. Recall from Section 18.2 that the first step in electrophilic aromatic substimtion is the addition of an electrophile (E ) to form a resonance-stabihzed carbo-cation. The Hammond postulate (Section 7.15) makes it possible to predict the relative rate of the reaction by looking at the stability of the carbocation intermediate. 

Radical polymerization of CH2=CHZ is favored by Z substituents that stabilize a radical by electron delocahzation. Each addition step occurs to put the intermediate radical on the carbon bearing the Z substituent. With styrene as the starting material, the intermediate radical is benzylic and highly resonance stabihzed. Figure 30.3 shows several monomers used in radical polymerization reactions. 

Carbon-carbon bonds next to the carbonyl group of an aldehyde or ketone break readily becanse resonance-stabihzed ions called acylium ions are produced ... 

Keto and enol forms (Sections 18.1-18.3) Tautomeric forms of a compound related by a common resonance-stabihzed intermediate. An enol structure consists of an alcohol functionahty bonded to the sp carbon of an alkene. Shifting the hydroxyl proton to the alkene and creation of a carbon-oxygen 7r-bond results in the keto form of the species. 

Furthermore, there is a fine balance between the reactivity of the monomer and the stabihty of the macroradical, quantities typically inversely related. For example, styrene is a very reactive monomer but produces a more stable (i.e., less reactive) chain end in the form of a resonance-stabihzed... 

This reaction proceeds by the addition-ehmination mechanism. The first step is attack by JNH, at C2, a process that resembles 1,2-addition to an imine function. Expulsion of a hydride ion, H , from C2 is followed by deprotonation of the amine nitrogen to give H, and a resonance-stabihzed 2-pyridineamide ion. Protonation by aqueous work-up furnishes the final product. Note the contrast with electrophilic substitutions, which include proton loss, not expulsion of hydride as a leaving group. 

Attack at C2 and C4 produces the more highly resonance stabihzed anions (only the most important resonance forms are shown). The relatively slightly diminished reactivity of C2 versus C4 is ascribed to steric disruption of solvation of the partial negative charge on the adjacent N. 

Allylic and benzylic carbocations are primary, but allyl and ben l halides tend to react by an S l mechanism. Since they are resonance stabihzed, these primary carbocations are approximately as stable as secondary alkyl carbocations. Furthermore, secondary resonance-stabilized allylic and benzylic carbocations are as stable as tertiary alkyl carbocations. Any substrate that can form these secondary... 

Both compounds have a tertiary carbon—chlorine bond, and they react by an Sj l mechanism to form tertiary carbocations. However, the carbocation from the unsaturated compound is also allylic, so the charge on the carbocation intermediate is resonance stabihzed. 

Acyl cations are resonance stabihzed. The more stable form has an octet of electrons on both the carbon and oxygen atoms, and a formal positive charge on the oxygen atom. However, to give a stable product, reaction of the acyl cation with an aromatic ring must occur at the acyl carbon atom. 

Alcohols that contain only one hydroxyl group can also be protected by forming an acetal. Alcohols react regiospedficaUy with dihydropyran in an acid-cataly2ed addition reaction. The carbocation formed is resonance stabihzed by the lone pair electrons of the ring oxygen atom. Subsequent attack of the carbocation by the nucleophilic oxygen atom of the alcohol followed by deprotonation yields an alkoxy-substituted tetrahydropyran known as aTHP derivative. 

Figure 21.2 shows energy profiles for reactions of a nucleophile with an amide and with an acid chloride. The relative energies of transition states and tetrahedral intermediates for the two reactions are about the same because neither intermediate is resonance stabilized. However, because the amide reactant is resonance stabihzed and the acid chloride is not, the reaction of the acid chloride is faster because it is the less stable reactant. 

Claisen condensations in cells result from the condensation of thioesters. The sulfur atom of the thioester is part of a relatively large molecule called coenzyme A. The pA of an a-hydrogen atom of a thioester is about 8.5. It is a hundred times more acidic that the a-hydrogen of P-keto ester. The increased acidity of thioesters results from the ineffective resonance stabihzation of the positive charge of the carbonyl carbon atom by sulfur compared to oxygen. 

---