Bonding, chemical electron pair

Some other classification schemes are provided in a work by Kolthoff (Kolthoff, 1974). It is according to the polarity and is described by the relative permittivity (dielectric constant) e, the dipole moment p (in 10 ° C.m), and the hydrogen-bond donation ability Another suggested classification (Parker, 1969) stresses the acidity and basicity (relative to water) of the solvents. A third one (Chastrette, 1979), stresses the hydrogen-bonding and electron-pair donation abilities, the polarity, and the extent of self-association. A fourth is a chemical constitution scheme (Riddick et al., 1986). The differences among these schemes are mainly semantic ones and are of no real consequence. Marcus presents these clearly (Marcus, 1998). 

Molecular properties dipole moment, polarizability Chemical properties Acidity (including the abilities as proton donor, hydrogen-bond donor, electron pair acceptor, and electron acceptor)a) ... 

The convenient name covalent bond, which we shall often use in this book. in place of the more cumbersome expressions shared-electron-pair bond or electron-pair bond, was introduced by Langmuir (loc. cit. [7], 868). Lewis preferred to include under the name chemical bond a more restricted class of interatomic interactions than that giwri by ov definition f the chemical bond is at all times and in all molecules men ly a pair. i vtror s held jointly by two atoms --Lewis, op. cit. p. 78). 

In a chemical reaction, the molecule that contributes the pair of electrons that forms a new bond is called the nucleophile. To form a bond, the electron pair on the nucleophile must find an atom that lacks a bond in its reaction partner, or will soon lack a bond through breakage. This partner is called the electrophile. Thus, in the reaction shown in Figure 2.5, the oxygen in the water molecule was the nucleophilic center, while the hydrogen in H+ was the electrophilic center. 

As the understanding of chemical bonding was advanced through such concepts as covalent and ionic bond, lone electron pairs etc., the theory of intermolecular forces also attempted to break down the interaction energy into a few simple and physically sensible concepts. To describe the nonrelativistic intermolecular interactions it is sufficient to express them in terms of the aforementioned four fundamental components electrostatic, induction, dispersion and exchange energies. 

Molecular polarity of analytes is difficult to quantify unequivocally. The descriptors of polarity are expected to account for differences among the analytes regarding their dipole-dipole, dipole-induced dipole, hydrogen bonding and electron pair donor-electron pair acceptor (EPD-EPA) interactions. To find good descriptors of these chemically specific interactions is difficult, particularly since changes in analyte polarity also affect analyte geometry and its ability to take part in bulkiness-related interactions 7,l2j. 

The bonding atoms of ligands are usually non-metal elements such as oxygen, nitrogen, or chlorine. Whether alone or in molecules such as water or anuno-nia, these atoms have pairs of electrons that are not involved in chemical bonds. The electron pairs can enter the space around the metal atom and bond with it. 

If covalent, ionic and metallic bonds are explained in electrical terms, students are better prepared to accept that hydrogen bonds, van der Waals forces, solvent-solute interactions etc. are also types of chemical bonding. Where learners see covalent bonds as electron pairs attracted to two different positive cores, they have a good basis for subsequently learning about electronegativity and bond polarity. 

Gillespie RJ, Robinson EA (2006) Gilbert N Lewis and the chemical bond the electron pair and the octet rule from 1916 to the present day. J Comput Chem 28 87-97... 

Hi, pictui,. obtaLivjd is m conuast to chcniical intuition, which indicates that the electron pairs are localized within the chemical bonds, free electron pairs and inner atomic shells. The picture which agrees with intuition may be obtained after the localization of the MOs. 

The three-dimensional structures of organic and biochemical molecules play an essential role in determining their physical and chemical behaviors. Because carbon has four valence electrons ([He]2s 2p ), it forms four bonds in virtually all its compounds. When all four bonds are single bonds, the electron pairs are disposed in a tetrahedral arrangement. (Section 9.2) In the hybridization model the carbon 2s and 2p orbitals are then sp hybridized. (Section 9.5) When there is one double bond, the arrangement is trigonal planar (sp hybridization). With two double bonds or a triple bond, it is linear (sp hybridization). Examples are shown in Figure 25.1 T. 

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. 

The covalent, or shared electron pair, model of chemical bonding was first suggested by G N Lewis of the University of California m 1916 Lewis proposed that a sharing of two electrons by two hydrogen atoms permits each one to have a stable closed shell electron configuration analogous to helium... 

Chemical Properties The formation of salts with acids is the most characteristic reaction of amines. Since the amines are soluble in organic solvents and the salts are usually not soluble, acidic products can be conveniendy separated by the reaction with an amine, the unshared electron pair on the amine nitrogen acting as proton acceptor. Amines are good nucleophiles reactions of amines at the nitrogen atom have as a first step the formation of a bond with the unshared electron pair of nitrogen, eg, reactions with acid anhydrides, haUdes, and esters, with carbon dioxide or carbon disulfide, and with isocyanic or isothiocyanic acid derivatives. 

The idea put forth by G. N. Lewis in 1916 that chemical bonding results from a sharing of electron pairs between two atoms was a fundamental advance in bonding... 

There is clearly a conceptual relationship between the properties called nucleophilicity and basicity. Both describe a process involving formation of a new bond to an electrophile by donation of an electron pair. The pA values in Table 5.7 refer to basicity toward a proton. There are many reactions in which a given chemical species might act either as a nucleophile or as a base. It is therefore of great interest to be able to predict viiether a chemical species Y P will act as a nucleophile or as a base under a given set of circumstances. Scheme 5.4 lists some examples. 

Covalent — refers to a chemical bond in which there is an equal/even sharing of bonding electron pairs between atoms. This is typical of the bonding between carbon atoms and between carbon and hydrogen atoms in organic compounds. 

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. 

Mesomerism involving polarized and nonpolarized contributing enamine forms influences the enamine s spectral properties and chemical reactivity. For mesomerism to be present, a planar arrangement is required for the three atoms of enamine grouping and the five atoms immediately bound to this system. If this condition is not fulfilled, full interaction of the tt electrons of the double bond with the free electron pair on the nitrogen atom is impossible. Enamines in which mesomerism is inhibited do not show the properties characteristic of enamines, and only the mutual electrostatic interaction of the double bond and lone electron pair of the nitrogen atom can be observed. Such steric hindrance of mesomerism occurs mainly in polycyclic systems. 

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