Carbon conjugation

Isolated double bonds are separated from other double bonds by at least one sp hybridized carbon Conjugated double bonds are joined by a single bond ... 

We generally write a for a diagonal element and p for a bonded off-diagonal element in particular uq and Pec for carbon atoms and carbon-carbon conjugated bonds. The physical interpretation is that each of the r-electrons experiences an average field due to the nuclei, the cr-electrons and the remaining TT-electrons. 

Kresge et a/.498 have drawn attention to the fact that detritiation of [3H]-2,4,6-trihydroxy- and [3H]-2,4,6-trimethoxy-benzenes by concentrated aqueous perchloric acid gives correlations of log rate coefficient with — H0 with slopes of 0.80 and 1.14 respectively. Protonation to give the carbon conjugate acids is, however, governed by h0lA0 and h0l 9S, respectively, which suggests that the difference in kinetic acidity dependence is a property of the substrate and should not be interpreted as a major difference in mechanism. The kinetic difference can be eliminated by an appropriate comparison of kinetic and equilibrium acidity dependencies. In equation (230)... 

The characteristic valence four of the C-atom is the origin of the enormous variety of these systems. Purely steric and topological considerations indicate that carbon conjugated polymeric structures can be either one- or two- dimensional since the conju-... 

Vargaftik, M.N. et al., Catalysis with a Pd giant cluster phenol oxidative carbonylation to diphenul carbonate conjugated with reductive nitrobenzene conversion, J. Mol. Catal. A Chem., 108, 77, 1996. 

Due to the necessity for aromatic structure in the commodity isocyanates, and the intensive chemical conversions required to derive the isocyanate reactant, no high-carbon frameworks derived principally from plant sources have been developed into isocyanates. The majority of the derivation of raw materials from high carbon conjugates has been focused on the development of polyols, for which the structural demands are a better fit for available renewable feedstocks. Although there are some notable exceptions [1], as a general rule modification of the... 

Examples 10.1 and 11.1-11.4 are used, along with a number of similar examples presented below, to generate all the required C sp )—C sp ), C sp )—C sp ), and C sp )—H bonds, including those formed by aryl carbons. Conjugation must therefore also be considered. 

An all carbon conjugated ladder polymer (graphite ribbon) was synthesized by a novel electrophile-induced cyclization reaction to provide fused benzenoid aromatic hydrocarbon in quantitative yield [161]. Suzuki cross-coupling of dieneyne 105 with 1,4-didode-cylbenzene-2,5-diboronic acid (106) gave rigid-rod polymers 107, which was further treated with TFA to produce 108 graphite ribbon as a yellow/orange solid. 

Scheme IV, the loss of one H atom from the naphthalene trimer leads to a molecule containing a 19-carbon conjugated system and a single sp3 carbon atom. The free radical is stabilized by delocalization of the unpaired electron over the 19-carbon atom network. Loss of an additional H atom results in a fully condensed aromatic hydrocarbon containing 20 carbon atoms.

Figure 10.1 gives an orbital overlap view of bonding in allyl cation. The planar structure of H2C=CHCH2" (Figure 10.1a) provides a framework that allows for continuous overlap of the 2p orbitals of the three adjacent 5/7 -hybridized carbons (Figure lO.lfo and c). Until now, we have only seen rr orbitals involving two adjacent carbons. Conjugated systems are characterized by extended rr orbitals that encompass three or more atoms. The electrostatic potential map (Figure 10.Id) shows the equal sharing of positive charge between the first and third carbons of 2C...

Saffron is a mixture of substances. Those that make it desirable as a spice and dye are among the ones the plant uses to attract insects. Two of them, crocetin and crocin, are mainly responsible for its color, another (safrana ) its odor, and another picrocrocin) its taste. The same 20-carbon conjugated polyene unit is the chromophore that gives crocetin and crocin their yellow color. The difference between the two is that crocin is a glycoside in which both carboxylic acid functions of crocetin are attached to a disaccharide igentiobiose) by ester linkages. 

Most of the reported conjugated polymers with nonlinear optical or electrical activities are carbon-carbon conjugated systems, such as the extensively studied polyenes (e.g., polyacetylenes [15], polydiacetylenes [16,17], poly-/ -phenylenes) and heteroaromatic polymers (e.g., polypyrroles, polythiophenes, polycarbazoles, polyanilines) [18]. The synthetic methods, applications, and the structure-property relationships of these polymers have also been substantially investigated [19-21]. 

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