Orbital unsymmetrization overlapping

Such secondary bonds are formed by donation from the lone pair of a nucleophile into the a orbital of a covalent bond ( n —> a attack ). Weak covalent bonds (implying low-energy a levels) are easier attacked by n - a overlap, leading to unsymmetric or symmetric 3c le bonds, than strong bonds this is why supramolecular arrays, due to secondary interactions or 3c-4e bonds, play a particular role in the chemistry of the heavier main group elements. 

These studies indicate that the selectivity of a-aminoalkyl radical formation is Me > Et /-Pr, which is the opposite of that expected on the basis of radical stability. For example, in diisopropylmethylamine, methyl adducts are formed exclusively. Similar selectivity has been observed for oxidation of unsymmetrical amines by ferricyanide [43, 123] and in anodic processes [124]. This selectivity has been attributed to requirement of overlap between the half-vacant nitrogen p orbital and the a-CH orbital of the a-carbon. From the Newman projections below it can be seen that the conformation necessary for methyl deprotonation (a) is lower in energy than that for the isopropyl deprotonation (b) [5, 122]. 

Four ijf hybrid orbitals (green), oriented to the corners of a regular tetrahedron, are formed by combination of an atomic s orbital (red) and three atomic p orbitals (blue). The sp hybrids are unsymmetrical about the nucleus, giving them a directionality and allowing them to form strong bonds when they overlap an orbital from another atom. 

The concept of hybridization explains how carbon forms four equivalent tetrahedral bonds but doesn t explain why it does so. Looking at an sp hybrid orbital from the side suggests the answer. When an s orbital hybridizes with three p orbitals, the resultant spi hybrid orbitals are unsymmetrical about the nucleus. One of the two lobes is much larger than the other and can therefore overlap better with an orbital from another atom when it forms a bond. As a result, sp hybrid orbitals form stronger bonds than do unhybridized s or p orbitals. 

From a molecular orbital standpoint, it is possible to do an analysis of C-H OA similar to that for addition of H2 (Figure 7-8). The non-spherical nature of the (t C-H bonding orbital, however, makes its overlap with the metal s empty dz2-like orbital or c/.v/ -hybridized orbital (dIT) of comparable symmetry less favorable than that of the a (H2) orbital (Figure 7-10). Also, the interaction of the filled d,-like metal orbital (d ) with the C-H a orbital (back-bonding) is weaker because the latter orbital is unsymmetrical, not directed toward the metal, and less spherical in nature than the comparable dihydrogen orbital ... 

The former reaction is unsymmetrical, like many Diels-Alder reactions, and the transition structure will also be unsymmetrical. This will make f3, which is distance-dependent, different for each of the CC bonds being formed, and will effectively increase the difference between the Zc2-valucs calculated above. Asynchronicity involving initial overlap of the two largest lobes will also be enhanced by the stabilisation of whatever radical character the transition structure has, and hence will augment the frontier-orbital explanation for the site selectivity in several other conjugated systems. 

For example, heptatriene 6.290 and 1-cyanobutadiene 6.294 react with maleic anhydride and isoprene 6.293 to give mainly the lower-energy adducts 6.291 and 6.295, respectively, rather than the alternatives 6.292 and 6.296.837 These reactions are similarly governed both by the formation of the thermodynamically favoured products and by the initial overlap with the larger frontier orbital coefficient in each component leading to the unsymmetrical transition structure. 

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