Alkenes resonance effects

Substituent effects (substituent increments) tabulated in more detail in the literature demonstrate that C chemical shifts of individual carbon nuclei in alkenes and aromatic as well as heteroaromatic compounds can be predicted approximately by means of mesomeric effects (resonance effects). Thus, an electron donor substituent D [D = OC//j, SC//j, N(C//j)2] attached to a C=C double bond shields the (l-C atom and the -proton (+M effect, smaller shift), whereas the a-position is deshielded (larger shift) as a result of substituent electronegativity (-/ effect). 

In the El cb mechanism, the direction of elimination is governed by the kinetic acidity of the individual p protons, which, in turn, is determined by the polar and resonance effects of nearby substituents and by the degree of steric hindrance to approach of base to the proton. Alkyl substituents will tend to retard proton abstraction both electronically and sterically. Preferential proton abstraction from less substituted positions leads to the formation of the less substituted alkene. This regiochemistry is opposite to that of the El reaction. 

The reactivities of carbenes toward alkenes have been correlated with the inductive and resonance effects of the carbene substituents, log k — a Eat + fcEaR+ + c.m Analogous correlations cannot be obtained for the reaction rates of carbenes with alcohols, neither with the substituent parameters used by Moss,109 nor with related sets.110 In particular, the substituent parameters do not describe the strong, rate-enhancing effect of aryl groups. For a detailed analysis, see the discussion of proton affinities (Section V.A). 

Model studies discussed in previous chapters show that the reactivity of cations and alkenes are very strongly affected by inductive and resonance effects in the substituents. Correlation of the rate constants of addition of benzhydryl cation to various styrenes with Hammett substituted benzhydryl cations to a standard alkene (2-methyl-2-pentene) gave also good correlation and p+ = 5.1 [28]. The large p value signals difficult copolymerizations between alkenes, even of similar structures. Thus, in contrast to radical copolymerization which easily provides random copolymers, cationic systems have a tendency to form either mixtures of two homopolymers or block copolymer (if the cross-over reaction is possible). 

Like other compounds with carbon-carbon double bonds, enols are electron rich, so they react as nucleophiles. Enols are even more electron rich than alkenes, though, because the OH group has a powerful electron-donating resonance effect. A second resonance structure can be drawn for the enol that places a negative charge on one of the carbon atoms. As a result, this carbon atom is especially nucleophilic, and it can react with an electrophile to form a new bond to carbon. Loss of a proton then forms a neutral product. 

Hammett (sigma) Reactions of para- or meta-substitu-ted aromatic compounds hydrolysis, hydration of alkenes, substitution, oxidation, enzyme-catalyzed oxidations some Type II photooxidations Electron withdrawal and/or donation from/to reaction sito by substituents on aromatlo rings via resonance effects... 

The properties of dienes depend upon the arrangement of the double bonds and the other substituents. If the double bonds are widely separated(as in isolated dienes),the compound behaves like a simple alkene. If the compound is conjugated, there is potential delocalization of the tt electrons. Conjugated double bonds are, in general, more stable than isolated double bonds because of the resonance effect. Cumulated double bonds (allenes) are more unstable than isolated double bonds because of their geometry and electron density. 

Attack at C2 is more sterically hindered due to the eminaZ-dimethyl group, so 75 may be formed in greater amormt. This observation is difficult to predict without experimental data, so in the oxymercuration ofunsymmetrical alkenes, anticipate a mixture of both possible alcohols. In other words, assume that oxymercuration of unsymmetrical internal alkenes will give a 1 1 mixture of two alcohols unless there is a compelling reason for one to predominate, such as electronic stabilization, severe steric hindrance, or resonance effects. Exceptions to this assumption will rarely be encountered in this book. 

Addition to a terminal alkene shows only a P effect. Both electron withdrawing and electron donating groups increase the rate of radical addition when in this position, because the orbital resulting after radical addition is lower in energy due to delocalization of the radical by either inductive, hyperconjugative, or resonance effects. The sensitivity to the Taft steric parameter is relatively small (S = 0.28), meaning that sterics play very little role in the p-position. 

---