The Formation of a Covalent Bond

Bonding Pairs and Lone Pairs In covalent bonding, as in ionic bonding, each atom achieves a full outer (valence) level of electrons, but this is accomplished by different means. Each atom in a covalent bond counts the shared electrons as belonging entirely to itself. Thus, the two electrons in the shared electron pair of H2 simultaneously fill the outer level of both H atoms. The shared pair, or bonding pair, is represented by either a pair of dots or a line, H H or H—H. 

An outer-level electron pair that is not involved in bonding is called a lone pair, or nnshared pair. The bonding pair in HF fills the outer level of the H atom and, together with three lone pairs, fills the outer level of the F atom as well  

In F2 the bonding pair and three lone pairs fill the outer level of each F atom  

Animation Formation of a Covalent Bond Online Learning Center 

Types of Bonds and Bond Order The bond order is the number of electron pairs being shared by any pair of bonded atoms. The covalent bond in H2, HF, or F2 is a single bond, one that consists of a single bonding pair of electrons. A single bond has a bond order of 1. 

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Numerous ionic compounds with halogens are known but a noble gas configuration can also be achieved by the formation of a covalent bond, for example in halogen molecules, X2, and hydrogen halides, HX. When the fluorine atom acquires one additional electron the second quantum level is completed, and further gain of electrons is not energetically possible under normal circumstances, i.e... 

Fig. 4.6. The formation of a covalent bond, viewed in terms of energy.

The combination of two lithium atoms to give the molecule Li2 is described as involving the formation of a covalent bond.between the atoms. In a crystal of fluorine, F2, the repulsion of the unshared outer electron pairs keeps the molecules spaced so that the minimum intermolecular... 

A roughly equivalent valence-bond theory would result from allowing the 2s electron of each lithium atom to be involved in the formation of a covalent bond with one of the neighbouring atoms. The wave function for the crystal would be... 

In addition to the ratio of concentrations olefine/HA, the donor ability, or the nucleophilicity of the anion A- is a deciding factor for the manner in which the reaction continues. This anion is formed simultaneously with the carbenium ion. When the nucleophilicity of the anion is sufficiently high, as in the case of CP, Br-, I-, for instance, the reaction proceeds as an addition by the formation of a covalent bond between A- and the carbenium ion72). 

The small size of the proton relative to its charge makes the proton very effective in polarizing the molecules in its immediate vicinity and consequently leads to a very high degree of solvation in a polar solvent. In aqueous solutions, the primary solvation process involves the formation of a covalent bond with the oxygen atom of a water molecule to form a hydronium ion H30 +. Secondary solvation of this species then occurs by additional water molecules. Whenever we use the term hydrogen ion in the future, we are referring to the HsO + species. 

Figure 16.3 Conceptual illustration of two peptides before (left) and after (right) a chemical reaction with formaldehyde. The amino acids are represented as circles. In this particular peptide, a tyrosine (Y) is located within the epitope (shaded circles). An arginine (R) is located elsewhere in the peptide. Formaldehyde results in the formation of a covalent bond between the two residues, due to a Mannich condensation reaction, as shown on the right. The new configuration prevents antibodies from binding to the epitope on the left. 

The latter is attributed to the formation of a covalent bond by the unpaired electron of O- with one of the unpaired d electrons of cobalt (1). ... 

As the formation of a covalent bond between two atoms implies a (dipolar) deformation of the density, polarizability and reactivity must be related. Indeed, Nagle demonstrated an empirical relation between the atomic polarizabilities (response to a field) and the scales of electronegativities (reactivity) [36]. More... 

Chemical modification of wood is defined as the reaction of a chemical reagent with the wood polymeric constituents, resulting in the formation of a covalent bond between the reagent and the wood substrate. 

A snbstantial body of experimental evidence indicates that the formation of a covalent bond between chemical carcinogens and cellnlar macromolecnles represents the first critical step in the multistage process, eventually leading to tumor formation (see Geacintov et al. 1997, references therein). Most chemical carcinogens are not active on their own, but require metabolic activation to produce reactive intermediates capable of binding covalently with target macromolecnles, particularly with deoxyribonucleic acid (DNA), and thereby, initiate cancer. 

The effectiveness of the coating has been investigated by separating phenolic compounds in the nonaqueous media. The EOF was found to be anodic and dependent on the pH of the separation buffer. In anofher study [66], imidazole containing zwitterionic salt (N-3-(-triethoxysilylpropyl)-4,5-dihy-droimidazole) was attached to the silica capillary wall via the formation of a covalent bond (Figure 6.8). 

Three representations of the formation of a covalent bond in a dihydrogen molecule. 

The missing electron can be provided by the formation of a covalent bond with a univalent atom, e.g. hydrogen or a halogen, in compounds like ClCo(CO)4 and HCo(CO)4. 

Thus, Professor Sokolov s mathematical treatment of the hydrogen bond is based already on the assumption that in a system A—H O— the A—H bond is mainly ionic A H+, although it is known that this is far from true. Ab a result of this assumption, the repulsions between the electrons of the A-—H bond and the non-bonding electrons of O are ignored. This treatment km little relationship to the problem of the hydrogen bond, but rather corresponds to the interaction of a proton with the acceptor system O—, which already m absence of any cal dilations, could be predicted to result in the formation of a covalent bond H—0+—. 

Quantum mechanics has made important contributions to the development of theoretical chemistry, e.g. the concept of quantum mechanical resonance in the interpretation of the perturbation in the excited states of polyelectronic systems, the concept of exchange in the formation of a covalent bond, the concept of non-localized bonds (though, in my view, unsatisfactory and only arising from a neglect of electronic repulsions), the concept of dispersion forces etc., but it is noteworthy that all these ideas owe their success and justification to their ability to account qualitatively for previously unexplained experimental facts rather than to their quantitative mathematical aspect. 

All of these early studies, however, contained, in addition to suggestions that have since been incorporated into the present theory, many others that have been discarded. The refinement of the electronic theory of valence into its present form has been due almost entirely to the development of the theory of quantum mechanics, which has not only provided a method for the calculation of the properties of simple molecules, leading to the complete elucidation of the phenomena involved in the formation of a covalent bond between two atoms and dispersing the veil of mystery that had shrouded the bond during the decades since its existence was first assumed, but has also introduced into chemical theory a new concept, that of resonance, which, if not entirely unanticipated in its applications to chemistry, nevertheless had not before been clearly recognized and understood. 

Among the hypotheses formulated, the most likely mechanism calls for the formation of a covalent bond between carbon 4 of the 3,4-flavan-diol (leucoanthocyanidin) and carbons 6 or 8 of another flavan molecule. Benzylic alcohol is a reactive electrophile (loss of OH"), and it donates readily in acid media leucoanthocyanin (25) functions similarly at position 4. The phenolic group is a mesomeric structure which displays negatively charged nucleophilic centers in the ortho and para positions analogous centers may be found at positions 6 and 8 of flavan molecules. This would allow the possibility of covalent bond formation between carbon 4 of 25 and carbons 6 or 8 of 26a or 26b. This bond is attributable to the elimination of a water molecule. 

Like antithrombin, heparin cofactor II inhibits proteases by forming a I I stoichiometric complex with the enzyme. The protease attacks the reactive site of heparin cofactor II located on the C-terminus, resulting in the formation of a covalent bond. Heparin cofactor II has higher protease specificity than antithrombin. Of the coagulation enzymes, heparin cofactor II is known only to inhibit thrombin (92). Additionally heparin cofactor II has been shown to inhibit chymotrypsin (93) and leukocyte cathepsin G (94), This protease specificity appears to be due to the active site bond present in heparin cofactor II. Whereas antithrombin contains an Arg-Ser bond as its active site, heparin cofactor II is unique in containing a Leu-Ser bond. This suggests than another portion of the heparin cofactor II molecular may be required for protease binding,... 

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