Delocalized electronic pair

Our need for more than one Lewis structure to depict the ozone molecule is the result of electron-pair delocalization. In a single, double, or triple bond, each electron pair is attracted by the nuclei of the two bonded atoms, and the electron density is greatest in the region between the nuclei each electron pair is localized. In the resonance hybrid for O3, however, two of the electron pairs (one bonding and one lone pair) are delocalized their density is spread over the entire molecule. In O3, this results in two identical bonds, each consisting of a single bond (the localized electron pair) and a partial bond (the contribution from one of the delocalized electron pairs). We draw the resonance hybrid with a curved dashed line to show the delocalized pairs ... 

Reading the equation from left to right, we can say that anilinium ion is a stronger acid than cyclohexylammonium ion because loss of a proton from anilinium ion creates a delocalized electron pair of aniline and biases the equilibrium toward the right. 

Enol ethers (Figure 2-58a) have two electron pairs on the oxygen atom in two different orbitals, one delocalized across the two carbon atoms, the other strictly localized on the oxygen atom (Figure 2-58b). Ionization ftom either of these two orbitals is associated with two quite different ionization potentials, a situation that cannot be handled by the present connection tables. 

Boranes are typical species with electron-deficient bonds, where a chemical bond has more centers than electrons. The smallest molecule showing this property is diborane. Each of the two B-H-B bonds (shown in Figure 2-60a) contains only two electrons, while the molecular orbital extends over three atoms. A correct representation has to represent the delocalization of the two electrons over three atom centers as shown in Figure 2-60b. Figure 2-60c shows another type of electron-deficient bond. In boron cage compounds, boron-boron bonds share their electron pair with the unoccupied atom orbital of a third boron atom [86]. These types of bonds cannot be accommodated in a single VB model of two-electron/ two-centered bonds. 

Neighboring group participation (a term introduced by Winstein) with the vacant p-orbital of a carbenium ion center contributes to its stabilization via delocalization, which can involve atoms with unshared electron pairs (w-donors), 7r-electron systems (direct conjugate or allylic stabilization), bent rr-bonds (as in cyclopropylcarbinyl cations), and C-H and C-C [Pg.150]

There exist a number of d -synthons, which are stabilized by the delocalization of the electron pair into orbitals of hetero atoms, although the nucleophilic centre remains at the carbon atom. From nitroalkanes anions may be formed in aqueous solutions (e.g. CHjNOj pK, = 10.2). Nitromethane and -ethane anions are particularly useful in synthesis. The cyanide anion is also a classical d -synthon (HCN pK = 9.1). 

The N-basicity of the commonly used amines (pyrrolidine > piperidine > morpholine) drops by 2-3 orders of magnitude as a consequence of electron pair delocalization in the corresponding enamines. This effect is most pronounced in morpholino enamines (see table below). Furthermore there is a tendency of the five-membered ring to form an energetically favorable exocyclic double bond. This causes a much higher reactivity of pyrroUdino enamines as compared to the piperidino analogues towards electrophiles (G.A. Cook, 1969). 

Additionally sp hybridization of the hydroxyl oxygen allows one of its unshared electron pairs to be delocalized by orbital overlap with the tt system of the carbonyl group (Figure 19 1) In resonance terms this electron delocalization is represented as... 

Tetrahydroisoqumoline is a stronger base than tetrahydroquinohne The unshared electron pair of tetrahydroquinohne is delocalized into the aromatic nng and this substance resembles am line m its basicity whereas tetrahydroisoqumoline resembles an alkylamine... 

Aromatic compound (Section 113) An electron delocalized species that is much more stable than any structure wntten for It in which all the electrons are localized either in cova lent bonds or as unshared electron pairs... 

Delocalization (Section 1 9) Association of an electron with more than one atom The simplest example is the shared electron pair (covalent) bond Delocalization is important in conjugated tt electron systems where an electron may be associated with several carbon atoms... 

The hydroxyl group stabilizes the carbocation by delocalization of one of the unshared electron pairs of oxygen ... 

Working against this electron-delocalization is the fact that both carbonyl groups are competing for the same electron pair-. Thus, the extent to which each one is stabilized is reduced. 

Just as aniline is much less basic than alkylamines because the unshared electron pair of nitrogen is delocalized into the tt system of the ring, p-nitroaniline is even less basic because the extent of this delocalization is greater and involves the oxygens of the nitro group. 

The increase in the proportion of the tetrasubstituted isomer in the cases of the morpholine and piperidine enamines of 2-methylcyelohexanone has been ascribed to both steric and electronic factors. The authors propose that the overlap of the electron pair on the nitrogen atom and the v electrons of the double bond is much more important in the case of the pyrrolidine enamines and much less with the others. Support for this postulate was provided by the NMR spectra of these enamines, wherein the chemical shifts of the vinylic protons of the pyrrolidine enamines were at a higher field than those of the corresponding morpholine and piperidine enamines by 20-27 Hz. The greater amount of overlap or electron delocalization, in the case of pyrrolidine enamine, is in accord with the postulate of Brown et al. (7- ) that the double bond exo to the five-membered ring is more favored than the double bond exo to the six-membered ring. 

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