Delocalization K-electrons

Hiberty, P.C. The Distortive Tendencies of Delocalized k Electronic Systems. Benzene, Cyclobutadienc and Related Heteroannulenes. 153, 27-40 (1990). 

Resonance-stabilized systems include car-boxylate groups, as in formate aliphatic hydrocarbons with conjugated double bonds, such as 1,3-butadiene and the systems known as aromatic ring systems. The best-known aromatic compound is benzene, which has six delocalized k electrons in its ring. Extended resonance systems with 10 or more 71 electrons absorb light within the visible spectrum and are therefore colored. This group includes the aliphatic carotenoids (see p.l32), for example, as well as the heme group, in which 18 k electrons occupy an extended molecular orbital (see p. 106). 

In the liquid phase the topics of principal concern are adsorption and proton and/or electron transfer across the electric donble layer. Carbon materials are unique in these applications becanse they are insolnble over the entire practical range of pH, are amphoteric, and can exhibit either acidic or basic properties this was illustrated in Fignre 1.10. Furthermore, because of their more or less extensive delocalized k-electron system in the graphene layer, they can either accept or donate electrons. Snch remarkable flexibility offers, on the one hand, a nniqne opportnnity to tailor carbon s properties to specific needs in adsorption, catalysis, and electrocatalysis but, as argued in detail elsewhere [24], it is also responsible for the persistent lack of fundamental nnderstanding in the increasingly important field of carbon electrochemistry, despite the tremendous amount of research and development focused on carbon-based capacitors, batteries, and fnel cells. 

Most carbon materials behave as semiconductors, due to the k electrons and the holes present in the graphene layers, which can act as charge carriers [69]. Since the enhancement of conductivity of carbons at high temperatures is related to structural rearrangements (graphitization), the conductivity of carbons provides valuable information about the carbon surfaces. Due to the fact that the ESR is sensitive to unpaired electrons (delocalized k electrons and free radicals), it can be useful to study the chemical and electrochemical processes that occur on carbon surfaces. Despite this, only a few papers address the use of ESR spectroscopy for the surface characterization of carbon samples [59,257]. The shape... 

This circle structure best represents the six delocalized k electrons. 

Pyrrole is much more acidic than comparable saturated amines. The of pyrrolidine is about 35, but pyrrole has a pX of 16.5, making it some 10 3 times more acidic Pyrrole is about as acidic as a typical alcohol so bases stronger than alkoxides will convert it to its anion. We should not be too surprised at this as the corresponding hydrocarbon, cyclopenta-diene, is also extremely acidic, with a p/C of 15. The reason is that the anions are aromatic with six delocalized k electrons. The effect is much greater for cyclopentadiene because the hydrocarbon is not aromatic and much less for pyrrole because it is already aromatic and has less to gain. 

Robinson had also proposed two ways to represent the structures with delocalized electrons. The first representation is the method of resonance structures which we already discussed in this book. By using this method, the benzene moleeule is represented by two resonance structures. The other representation proposed by Robinson describes the six delocalized k electrons by plotting a circle inside the six-membered ring. 

K Relaxation mechanism was also proposed [130-133], The % relaxation originates from cyclic delocalization of % electrons in the double bond through the hyperconjugation with a bonds on the saturated ring atoms under control of the orbital phase property [134, 135],... 

Waggoner, K. M. Hope, H. Power, P. P. Angew. Chem. Int. Ed. Engl. 1988, 27, 1699. Six-membered heterocycles [RMER ]3 have been subject to detailed preparative and theoretical studies since they are isoelectronic to borazine B3N3Hs. However, it was demonstrated that only B3P3-heterocycles exhibit a considerable delocalization of the 6 k-electrons, whereas the delocalization in other six-membered heterocycles of the desired type contributes very little to their stabilization. Power P. P. J. Organomet. Chem. 1990, 400, 49. 

As we have already seen, delocalization of electrons by conjugation decreases the energy difference between the HOMO and LUMO energy levels, and this leads to a red shift. Alkyl substitution on a conjugated system also leads to a (smaller) red shift, due to the small interaction between the cr-bonded electrons of the alkyl group with the K-bond system. These effects are additive, and the empirical Woodward-Fieser rules were developed to predict the 2max values for dienes (and trienes). Similar sets of rules can be used to predict the A ax values for a,P-unsaturated aldehydes and ketones (enones) and the Amax values for aromatic carbonyl compounds. These rules are summarized in Table 2.4. 

Annulene has 10 k electrons, which satishes Hilckel s rule, but a planar molecule would place the two H atoms inside the ring too close to each other, so the ring puckers to relieve this strain. Because [10]-annulene is not planar, the 10 ji electrons can t delocalize over the entire ring and it is not aromatic. 

The length of each C-C bond in a polycyclic aromatic hydrocarbon is near that expected from the various resonance hybrids that can be drawn, and the bond orders that can be calculated.The main nonion-ized contributors for benzene are available from neutron studies of crystalline deuterobenzene at 15 K (-258° C) and for crystalline naphthalene at 92 K (-181 C) and are shown in Figure 11.18. The delocalization of electrons in benzene is shown by its equal bond lengths while, in naphthalene there is also evidence of some localization of double bonds. 

---