Delocalized it Bonding

The n.m.r. spectra are consistent with structures that, like Riidorff s (Riidorff Riidorff 1947 Riidorff 1959), allow for planar carbon-atom sheets and attachment of carbon to only one fluorine atom at a time. The persistence of graphitic carbon, even at C/F ratios approaching two, suggests that delocalized it-bonding is maintained in the carbon-atom sheets. 

Alkanes contain only single bonds. Alkenes, also known as olefins, contain at least one C=C double bond, and alkynes contain at least one C=C triple bond. In aromatic hydrocarbons the carbon atoms are connected in a planar ring structure, joined by both tr and delocalized it bonds between carbon atoms. Benzene (CgHg) is the best-known example of an aromatic hydrocarbon. 

Addition reactions of benzene destruct the aromatic ring (delocalized it bond) and hence the products are less stable. 

So the six carbon atoms and the attached hydrogen atoms are held together by a skeleton of c bonds. Each carbon atom has one electron remaining in a 2p orbital, the lobes of which lie above and below the plane of the molecule. These six unhyhridized 2p orbitals combine by side-on overlap to form a delocalized it bond containing six electrons (Chapter 14). It is the presence of these delocalized n electrons that makes the benzene molecule more energetically stable than predicted for the Kekule structure, and it accounts for the fact that the molecule undergoes electrophilic attack. 

Strategy Starting with the Lewis structure, use valence bond theory and hybrid orbitals to describe the <7 bonds. Then use molecular orbital theory to describe the delocalized it bonding. 

A second isomer of [lOJannulene (the cis trans cis cis trans stereoisomer) can have bond angles close to 120° but is destabilized by a close contact between two hydro gens directed toward the interior of the ring To minimize the van der Waals strain between these hydrogens the nng adopts a nonplanar geometry which limits its ability to be stabilized by tt electron delocalization It too has been prepared and is not very stable Similarly the next higher (4n + 2) system [14]annulene is also somewhat desta bilized by van der Waals strain and is nonplanar... 

Most radicals located on double bonds (e.g. 4, 5) or aromatic systems (e.g. 6) are a-radicals. The free spin is located in an orbital orthogonal to the it-bond system and it is not delocalized. The orbital of the vinyl radical (4) containing the free spin can be cis- or trans- with respect to substituents on the double bond. The barrier for isomerization of vinyl radicals can be significant with respect to the rate of reaction. 

Hg (CN0)2, which explodes when struck. Write a Lewis structure for the CNO anion and describe its bonding, including any delocalized orbitals. 

These differences have been identified for two main classes of interactions, namely intramolecular H-bonding and charge delocalization. It has, for instance, been shown for VV-(p-methylbenzyl)hexylamine in water-DCE that its diff (log is 4.51, which is... 

It is interesting to note that the photosubstitution intermediate 11 appears to be significantly more selective toward reaction with various two electron donor substrates than is the photofragmentation intermediate 1. One speculative rationalization of this is that the Ru3(C0) intermediate has the opportunity to "delocalize" its unsaturation by having one CO bridge an edge of the metal triangle with concomitant formation of a multiple metal-metal bond. 

An aromatic ring and a double or triple bond in the a-position relative to the C—H bond weaken this bond by virtue of the delocalization of the unpaired electron in its interaction with the iT-bond. The weakening of the C—H bond is very considerable for example, D(C—H) is 422 kJ mol-1 in ethane [27], 368 kJ mol-1 in the methyl group of propene [27] (AD = 54 kJ mol-1), and 375 kJ mol-1 in the methyl group of toluene [27] (AD = 47 kJ mol-1). Such decrease in the strength of the C—H bond diminishes the enthalpy of the radical abstraction reaction and, hence, its activation energy. This effect is illustrated below for the reactions of the ethylperoxyl radical with hydrocarbons ... 

Such a difference in partial double-bond character has implications for the mechanism, and, hence, the reaction rate, of acid-catalyzed hydrolysis (Fig. 6.15). In delocalized peptide bonds (Fig. 6.15,a), protonation involves the carbonyl O-atom with its partial negative charge. In non-delocalized peptide bonds (Fig. 6.15,b), protonation involves the N-atom, which is rendered more basic by the lack of delocalization [73],... 

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