Electron delocalization resonance and

Five of the next seven chapters cover the reactions of hydrocarbons—compounds that contain only carbon and hydrogen. The other two chapters treat topics that are so important to the study of organic reactions that each deserves its own chapter. The first of these is stereochemistry and the second is electron delocalization and resonance. 

Electron Delocalization and Resonance More About Molecular Orbital Theory... 

Noteworthy is the "B resonance in 103, which falls in the range 58-60 ppm, very little different from the signal of trivinylborane (55 ppm) therefore, any extensive ir-electron delocalization and density on boron can be ruled out. Thus both in chemical and physical properties, such systems as 103 can be considered antiaromatic. 

The first nitrile ylide stable enough to be isolated (i.e., 1) has been prepared by the carbene/nitrile method (1). For this dipole, the anionic component is stabilized by electron delocalization and the nitrilium component by the steric bulk of the adamantyl group to such an effect that it has a melting point of 230 °C. The X-ray structure showed that the nitrile ylide moiety is close to linear and much like the resonance structure shown below. 

The organic chemist made an important step in the understanding of chemical reactivity when he realized the importance of electronic stabilization caused by the delocalization of electron pairs (bonded and non-bonded) in organic molecules. Indeed, this concept led to the development of the resonance theory for conjugated molecules and has provided a rational for the understanding of chemical reactivity (1, 2, 3). The use of "curved arrows" developed 50 years ago is still a very convenient way to express either the electronic delocalization in resonance structures or the electronic "displacement" occurring in a particular reaction mechanism. This is shown by the following examples. 

Representative examples of ring proton and carbon chemical shifts of all known l,4-(oxa/thia)-2-azoles were reported in CHEC-II(1996). A special notice should be given for H and 13C nuclear magnetic resonance (NMR) spectra of both dithiazolium 5 (X = Y = S) and oxathiazolium salts 6 (X = 0 Y = S) and 7 (X = S Y = 0) <1996CHEC-II(4)489>. A S downfield shift for both 3-H and 5-H as well as C-2 and C-5 is correlated with a potential 7r-electron delocalization and thus the aromaticity of these ring systems <1996CHEC-II(4)498>. 

The second reason that vitamin C is used as an electron donor is that the reaction product is fairly stable and unreactive. When vitamin C gives up an electron, it becomes a free radical called the ascorbyl radical. By free-radical standards, the ascorbyl radical is not very reactive. Its structure is stabilized by electron delocalization — the resonance effect first described by Linus Pauling in the late 1920s. This means that vitamin C can block free-radical chain reactions by donating an electron, while the reaction product, the ascorbyl radical, does not perpetuate the chain reaction itself. 

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