Double bond delocalization

Unlike the alkanes, however, the reaction of benzene with the halogens is catalyzed by iron. The relative lack of reactivity in aromatic hydrocarbons is attributed to delocalized double bonds. That is, the second pair of electrons in each of the three possible carbon-to-carbon double bonds is shared by all six carbon atoms rather than by any two specific carbon atoms. Two ways of writing structural formulas which indicate this type of bonding in the benzene molecules are as follows ... 

By combining high-level ab initio calculations with high-resolution infrared spectroscopy, the equilibrium bond lengths in x-frans-butadiene have been determined to an unprecedented precision of 0.1 pm. The values found for the pair of n-electron delocalized double bonds and the delocalized central single bond are 133.8 and 135.4 pm, respectively. The data provide definitive structural evidence that validates the fundamental concepts of n-electron delocalization, conjugation, and bond alternation in organic chemistry. 

Delocalized double bonding, represented by the circle within the hexagon, is responsible for the special stability of benzene and its derivatives. 

Define or identify each of the following terms (a) organic chemistry, (b) total bond order, (c) condensed formula, (d) structural formula, (e) fine formula, (/) hydrocarbon, (g) alkane, (h) aUcene, (/) alkyne, j) aromatic hydrocarbon, (k) saturated, (1) delocalized double bond, (m) isomerism, (n) cycloalkane, (o) radical, (p) functional group, (q) alcohol, (r) ether, (s) aldehyde, (f) ketone, (u) carbonyl group, and (v) ester. 

A single crystal X-ray structure of a free-base corrole was obtained in 1971, and is shown below in Figure 2.1.3. Based upon this structure and upon molecular calculations, free-base corrole was determined to bear a proton hole at N(22) (as drawn for 2.4), with the other three nitrogen atoms protonated. Where appropriate throughout this discussion, the corrole framework will be shown in this manner. This does not mean, however, that a specific location is being designated for what are, presumably, highly delocalized double bonds. 

Coji nib localized double bonds Z>2/i Ilia delocalized double bonds Ni-Ni bond... 

Because some compounds may have several copies of a target fragment distinguished by different reactivity such as delocalized double bonds in an aromatic compound, it is important to determine which site will be predominantly attacked fby the enzyme and, to do so, the META program often makes use of electronic descriptors obtained from simple quantum mechanical calculations. Finally, META will evaluate the candidate products to see which may need to be processed further to reach chemical stability and automatically perform the task when needed. 

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