Partial bonds

Stereoselective All lations. Ben2ene is stereoselectively alkylated with chiral 4-valerolactone in the presence of aluminum chloride with 50% net inversion of configuration (32). The stereoselectivity is explained by the coordination of the Lewis acid with the carbonyl oxygen of the lactone, resulting in the typ displacement at the C—O bond. Partial racemi2ation of the substrate (incomplete inversion of configuration) results by internal... 

Ionic Bonding, Partial Ionic Character, and Electronegativity... 

Chapter 2. Ionic Bonding, Partial Ionic Character, and Electronegativity Papers SP 23 to SP 27 255... 

Certain types of bond, whilst nominally being considered as single , have in fact, sufficient double bond character , to render rotation about their axis, restricted . The one you are most likely to encounter, is the amide bond. Partial double bond character exists between the carbonyl, and the nitrogen, and may be represented as in Structure 6.12 ... 

When examining other work published in this field [1-20], one sees that in different forms, in different combinations and not always together, lipophilicity, H-bonding, partial atomic charge and volume have been identified as determinants of aqueous solubility. Even lipophilicity represents a combination of volume and H-bonding as shown by El Tayar and Testa [32] and by Raevsky et al. [33]. Thus,... 

The C2-C3 overlap gives the central bond partial double bond character and allows the four tt electrons of 1,3-butadiene to be delocalized over all four atoms. 

According to Thiele, the explanation of the phenomenon of 1 4-addition is that the fields of force, which surround the carbon atoms separated by double bonds, partially cancel each other between C2 and C3 because of the proximity of these atoms in space, so that a higher chemical potential exists at C4 and C4 than at C2 and C3. Consequently the places at which addition occurs preferentially are C4 and C4. 

The catalytic homogeneous oxidation at low temperatures is therefore economically interesting, but also very difficult to achieve due to the high stability of CH-bonds. Partial oxidation is particularly hard in alkanes as classical oxidation procedures tend to over oxidize them. In the case of methane this would result in the formation of CH2O, CO and CO2. Low valent transition metals, however, are capable of activating the CH bond and rendering that problem less important as the difference in reactivity between the CH bond in methane and methanol is not that big. 

In americium, the occupation number is n = 6 (actually, n = 6.4 from band calculations ). If we assume full spin-polarization for this metal, then the 5 f bonding partial pressure is very small, and we may be reconducted to the lanthanide case for this metal also. 

Free pyrimidines and purines are weakly basic compounds and are thus called bases. They have a variety of chemical properties that affect the structure, and ultimately the function, of nucleic acids. The purines and pyrimidines common in DNA and RNA are highly conjugated molecules (Fig. 8-2), a property with important consequences for the structure, electron distribution, and fight absorption of nucleic acids. Resonance among atoms in the ring gives most of the bonds partial double-bond character. One result is that pyrimidines are planar molecules purines are very nearly... 

Broadly, then, if the substitution site is primary, and therefore access to it is not hindered sterically, the nucleophile approaches it and, by donation of its electron pair, forms a partial bond to. carbon while the leaving-group—carbon bond begins to break (Equation 4.7). At the transition state, both bonds partially... 

Certain oxides, first of all zinc oxide17, as well as copper chromite18 (a mixed copper-chromium oxide), are also active in the saturation of the C=C bond. Partial sulfur treatment under controlled conditions of metals or oxides can result in sulfided catalysts which exhibit specific activity and selectivity in hydrogenations19-21. 

The use of a strong nucleophile capable of coordinating with the silicon atom has provided the key for cleaving the Si-C bond. Partial desilylation reaction of BSMA derivatives has already been described (see Section IV.C.2). Desilylation-protonation of MSMA derivatives is presented here. 

As regards their chemical properties, halodisilanes are (as expected) to be very reactive thermally labile nucleophiles. Acid hydrolysis occurs to give polymeric compounds (see p. 92) retaining Si-Si bonds alkaline hydrolysis yields silicic acid with disruption of the Si-Si bonding. Partial hydrolysis carried out with Si2Cl6 led to S OCljo, Si602Cl4 and Si803Cli8 (see p. 93). 

It is noteworthy that among the formulae proposed for benzene in the 19 century, only the first one, that of Loschmidt, is not far from being correct. The next acceptable formula only appeared with Thiele s suggestion of fractional carbon-carbon bonds (partial valences) in 1899-1900. This formalism did not explain why cyclooctatetraene is not aromatic, however, as shown experimentally (for historical accounts, see refs. [15, 16]). 

Transition states have high energies because bonds must begin to break before other bonds can form. The following equation shows the reaction of a chlorine radical with methane. The transition state shows the C — H bond partially broken and the H—Cl bond partially formed. Transition states are often enclosed by brackets to emphasize their transient nature. 

For <7-bond donors such as H2, forming the M-L <7-bond partially depletes the H-H a-bond because electrons that were fully engaged in keeping the two H atoms together in free H2 are now also delocalized over the metal (hence the name 2-electron-3-center bond for this type of interaction). Back bonding into the H H a causes additional weakening of... 

The inherent basis of these procedures is the Zintl Kleimn concept and the Mooser-Pearson extended (8 — N) rule. Formerly applied only to classical two-center-two-electron bonds, the extended procedures comprise all varieties of bonding (multiple bonds, partial bonds, multicenter systems, radicals, and free electrons). Generally, for a compound AmB , an electron transfer A- -A +,... 

Once reaction (1) has taken place, collision of the oxygen of an ambient normal THF molecule with one of the positively charged carbon atoms leads to a reaction characterized by the fact that a new bond is formed with the positively charged carbon atom while the latter abandons one of its original bonds. In the case considered, the bond abandoned turns out to be the bond to the original trivalent oxygen, since this is the weakest of all the bonds around. The C-H bonds are very much stronger than C-C and C-0 bonds, and the C-0 bond partially depleted of electrons is weaker than the C-C bond. 

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