Bonds. .Set

Topological Strategy. The use of a particular bond, pair of bonds, set of bonds, or subunit as eligible for disconnection to guide retrosynthetic analysis conversely the designation of bonds or cyclic subunits as ineligible for disconnection (i.e. to be preserved). 

Cyclohexatriene to benzene displays a sequence of structures from 1,3,5-cyclohexatriene (withCC single and double bonds initially set to 1.5 and 1.3 A, respectively) to benzene (witb all CC bonds set to 1.4 A) and back to cyclohexatriene. Plot energy (vertical axis) vs. CC bond length (horizontal axis). How many energy minima are there Do the minima look more like 1,3,5-cyclohexatriene or benzene What is the correct interpretation of the resonance picture ... 

Foremost among the classes of the nonaromatic unsaturated systems is that of compounds in which the substituent is bonded to a carbon-carbon double bond. It is convenient to divide the data sets for carbon-carbon double bonds into five categories vinyl, vinylidene, frans-vinylene, cis-vinylene, and reactions of carbon-carbon double-bond sets. 

Results of Correlations with Carbon-Carbon Double-Bond Sets... 

We shall find it convenient to divide the data sets for substituent effects at carbon—carbon triple bonds into three categories acetylene, ethynylene, and reactions at carbon—carbon triple-bond sets. 

For the synthesis of (+)-preussin, commercially available L-pyroglutamic acid (154) could serve as an enantiomerically pure starting material. Given the key reaction we had in mind, the bond set for the synthesis of preussin was straightforward (Scheme 40). Attachment of the alkyl chain to the core... 

The coordination of oxygen to transition metal ions which occurs mostly in the side-on fashion on surfaces (Section III,A,2 and Appendix B) can be described following the model of acetylene-metal complexes (467). Both 7tu and 7tg orbitals of molecular oxygen have proper symmetry to interact with the bonding set of s, p, and d orbitals on the metal. The bonding orbitals are shown in Fig. 29. 

Cross linking - When chemical bonds set up between molecular chains, the material is said to be cross linked. Once cross linked, materials cannot be reprocessed. A form of curing. 

The 4p orbitals can also engage in n overlap, but because these are preoccupied with the more important o bonding such overlap is of little consequence. The effect on the t2g levels will be to produce a bonding set -labelled t2g- and an antibonding set, t2. Which of these is to be identified with the t2g level of CF/LF theory depends on the relative energies of the ligand n orbitals and the M 3d orbitals. We can envisage two situations ... 

While some moisture starts to leave the adhesive as soon as it is applied to the veneer, enough water must be absorbed by the wood or retained on the glue film sufficient to keep the resin molecules mobile. Otherwise, the resin will not flow properly in the bonding set time. The adhesive must gel in a continuous film. It must also cure in place. The adhesive should form a permanent durable bond to the wood. 

The equivalence of the predictions of ALPH and of PLNM are best illustrated by consideration of the reactions of axial and equatorial tetrahydro-pyranyl or pyranosyl derivatives3 (Scheme 13). The preferred conformation of the cationic intermediate is assumed to be the half-chair, and the nature of the atomic motions is best seen if one takes as a plane of reference the plane defined by C(l), C(2), 0(5) and C(5) of the oxocarbonium ion intermediate, and considers motions about the C(l)—0(5) bond, set out in Newman projections in Scheme 13. 

The only difference between this formulation and that of benzene is the historical view of benzene as a single molecule with two "main" overlapping resonance structures that were sufficiently similar that they could be coalesced into a single mental construct. On the other hand, the chemical difference between an alcohol and a ketone make such a viewpoint seem strange, if not completely untenable. Only because of the introduction of an extended bond set does such a perspective become viable. 

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