Delocalization K bonding

A symmetrical allyl cation can give one product only by the S l reaction. We have already discussed the formation of the cyclohexenyl cation (Chapter 7) and that is a good example. The two delocalized structures are identical and the n bond is shared equally among the three atoms, the cyclohexenyl cation delocalized k bond... 

The unpaired electron of the allyl radical and the two electrons of the k bond are delocalized over all three carbon atoms. 

Figure 10 Annular k bonding, transannular k -k bonding (a) and additional charge delocalization through an np2 >na interaction (b)...

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). 

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. 

Examine the geometry of methylbenzyne. Measure carbon-carbon distances. Which k bonds are delocalized and which are localized Is there really a triple bond (Compare bond distance to triple bond in hexa-l,5-dien-3-yne and to partial double bonds in benzene). Are you able to draw a single Lewis structure which adequately represents the geometry of the molecule ... 

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. 

The kth order MEDNE descriptor 0ji (called electronic delocalization entropy ]Ramos de Armas, Gonzalez Diaz et al, 2004]), which is the sum over all atoms of the entropy 0j.(i) involved in the attraction of electrons at least k bonds away from any ith atom in the molecule, and defined as... 

Molecular orbital theory therefore gives a picture of [N03] in which there is one occupied MO with rr-character and this is delocalized over all four atoms giving an N—O -K-bond order of... 

He emphasized that quantum chemistry was built upon many chemical concepts, often inherited from classical chemical theories. K-electrons, L-electrons, valence electrons, 7c- or (7-electrons, localized or delocalized electrons, bonds, and so forth, were certainly incompatible with the quantum mechanical ideas of indistinguishability and nonlocality but had proved to be very useful tools for chemists and had been appropriated by quantum chemists (Daudel 1952). Hence, he argued, they should be given alternative formulations compatible with the new physics. Such was the case of the concept of "loge" introduced by Daudel and his students to explain quantum-... 

Conjugation stabilizes the allyl carbocation because overlap of three adjacent p orbitals delocalizes the electron density of the k bond over three atoms. 

CH2-CH=CH-CHCH3 These electron pairs in the two k bonds are delocalized. 

Explain why triphenylene resembles benzene in that it does not undergo addition reactions with Br2, but phenanthrene reacts with Br2 to yield the addition product drawn. (Hint Draw resonance structures for both triphenylene and phenanthrene, and use them to determine how delocalized each k bond is.)... 

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