Bonding electron-pair delocalization

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). 

The greater rate of dehydration of 2-exo- over 2-endo-norbornanol can be interpreted by an anchimeric assistance which involves the delocalization of C(l)-C(6) bonding electron pair this helps in the removal of a hydroxyl ion and facilitates dehydration. This delocalization is probably responsible for the formation of norbornene as one of the primary products of dehydration. 

The t3C resonances of xanthone (21) have been assigned on the basis of shift analysis of the structurally related compounds anthrone and xanthene (77P735) and by correlation with the component molecules diphenyl ether and benzophenone (78T1837). Delocalization of the non-bonding electron pairs at oxygen, leading to an increase in aromaticity of the pyranone ring, accounts for the deviation of the chemical shifts of C-4a, C-8a and the carbonyl carbon atom from the calculated values. 

Esters and Related Functions 57 Thus, primary electronic effects (n-n interaction) form the conjugated system of the ester function whereas secondary electronic effects (n-o interaction) are the result of the orientation of non-bonded electron pairs anti peri planar to the o C-0 bonds of the ester function. Clearly, the primary are energetically more important than the secondary electronic effects and this terminology is justified by the fact that these two effects have their origin in the same chemical principle, orientation in space of electron pairs with resultant electronic delocalization. 

The N-N distance in planar N2O4 is 175 pm, with a rotation barrier of about 9.6 kJ mol-1. Although the N-N bond is of the a type, it is lengthened because the bonding electron pair is delocalized over the entire N2O4 molecule with a large repulsion between the doubly occupied MOs on the two N atoms. 

Pericyclic reactions are a class of reactions that include some of the most powerful synthetically useful reactions such as the Diels-Alder reaction. Pericyclic reactions often proceed with simultaneous reorganization of bonding electron pairs and involve a cyclic delocalized transition state. They differ from ionic or free radical reactions as there are no ionic or free radical intermediates formed during the course of the reaction. They proceed by one-step concerted mechanisms and have certain characteristic properties (although there are some exceptions to all these rules). 

Pyrrole is cyclic and planar, with a total of four ji electrons from the two 7i bonds. Is the non-bonded electron pair localized on N or part of a delocalized ji electron system The lone pair on N is adjacent to a double bond. Recall the following general rule from Section 16.5 ... 

The trigonal planarity of BH3 is easily deduced from VSEPR considerations of the dispositions of the three bonding electron pairs. The delocalized... 

A satisfactory localized view of ferrocene can be developed by noting from the Lewis structure that there are nine electron pairs around iron three from each cyclopentadi-enide ring and three from the metal atom. From our delocalized MO picture we note that the occupied orbitals in Fig. 21 were generated from a set of GO s composed of an s, all three p s and all of the d s. These GO s were seen to be of the same type as die valence shell AO s actually employed by the iron atom in forming MO s. The Lewis structure suggests that we should localize three bonding electron pairs between each ring and the metal atom. To accomplish this we first hybridize the GO s pO, pi, pi", dO, dl, dl" associated with the lowest six delocalized MO s in Fig. 21 which yields... 

It is thus clear that orbital localization permits justification of the classical concepts of localized binding and lone electron pairs, as opposed to delocalized n systems, as well as the notion of directed valency, a foundation of stereochemistry. Nevertheless, it must be noted that even if localization were perfect, the notion of bond electron pairs would be conventional In fact, if the descriptions in terms of the initial and the localized orbitals are fuUy equivalent in the sense that the total wave function is the same, quantities associated with a single pair of electrons are not invariant under the transformation. It has no absolute... 

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 ... 

Electron delocalization diffuses electron density over a greater volume, which reduces electron-electron repulsions and thus stabilizes the molecule. Resonance is very common, and many molecules (and ions) are best depicted as resonance hybrids. Benzene (CsHe), for example, has two important resonance forms in which alternating single and double bonds have different positions. The actual molecule has six identical carbon-carbon bonds because there are six C—C bonds and three electron pairs delocalized over all six C atoms, often shown as a dashed circle (or simply a circle) ... 

These carbenes have not been trapped by alkenes. Presumably the presence of an electron-rich phosphorus atom next to an electron-deficient carbon atom results in some delocalization of the phosphine non-bonded electron pair, tending to produce a dipolar species as shown above, and reducing the carbene character of the intermediate. 

Hyperconjugation Delocalization of a bonding electron pair, into an empty or a partly empty atomic orbital, stabilizing the molecule. 

In order to explain the observed substituent and solvent effects on the rate of the Ciaisen rearrangement, Coates et al. [28] suggested a dipolar character for the transition state "... partial delocalization of a non-bonded electron pair from the donor substituent generates a significant degree of enolate-oxonium ion-pair character that stabilizes the TS more than the ground state,... (Scheme 11.22). 

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