Aromaticity and Conjugation

Benzene has already been mentioned as a prime example of the inadequacy of a connection table description, as it cannot adequately be represented by a single valence bond structure. Consequently, whenever some property of an arbitrary molecule is accessed which is influenced by conjugation, the other possible resonance structures have to be at least generated and weighted. Attempts have already been made to derive adequate representations of r-electron systems [84, 85]. 

These examples show quite dearly the dose relationship between this new stmc-ture representation embedded in RAMSES and MO Theory. 

The treatment of conjugated systems in terms of electron systems that extend smoothly over all atoms allows the treatment of a variety of structural phenomena, as may be explained with a spedes that shows hindered rotation and with the nitro group. 

A characteristic feature of conjugated pi-bond networks is that each pi bond 7ta lies adjacent (vicinal, s-cis or s-trans) to a coplanar pi bond 7tb, a pi-type lone pair nb, or a pi-type vacancy nb (unfilled valence p orbital), e.g., 

The idealized Lewis structures are modibed in each case by donor-acceptor interactions involving the filled (Lewis) 7ta and unfilled (non-Lewis) 7ta NBOs of the formal pi bond, 7ra 7tb and7tb 7ta, nb 7ta, or7ta- nb for the three prototypes shown in (3.101). 

As described in Sections 1.4 and 1.5, a general fia— 2b donor-acceptor interaction between Lewis ( 2a) and non-Lewis (fib ) NBOs leads to perturbative corrections to the zeroth-order natural Lewis-structure wavefunction 

The starting Oa NBO is thereby perturbed to a natural localized molecular orbital (NLMO), distinguished by a tilde from its parent NBO 

The associated energy lowering is estimated from perturbation theory as 

BAD AIR Forest fires are a significant source of polycyclic aromatic hydrocarbons, which make up the solid, cancer-causing particulates that come from combustion. 

13 Special Topic TheBlo-Downslde,the Mechanism of Carcinogenesis by Polycyclic Aromatic Compounds 

Curious, though, isn t it, um, Patwardhan, that the number, er, six should be, um, embodied in one of the most, er, er, beautiful, er, shapes in all nature I refer, um, needless to say, to the, er, benzene ring with its single, and, er, double carbon bonds. But is it, er, truly symmetrical, Patwardhan, or, um, asymmetrical Or asymmetrically symmetrical, perhaps.. ..  

FIGURE 13.1 Acyclic and cyclic dienes have similar chemical properties. 

FIGURE 13.2 Benzene ( 1,3,5-cyclohexatriene ) does not undergo most of the reactions of normal alkenes. 

---

In 1937, Coulson s mentor, Lennard-Jones, wrote a series of papers on molecular orbitals applied to polyenes and aromatic molecules, discussing the variations in bond lengths in conjugated and aromatic molecules from a theoretical point of view. 92 He involved his student in the work. Coulson defined... 

Heinemann, C., Mueller, T., Apeloig, Y. et al.. On the question of stability, conjugation, and aromaticity in imidazol-2-ylidenes and their silicon analogs, /. Am. Chem. Soc., 118, 2023,1996. 

Recognize chromophores, and explain how conjugation and aromatic substituents can contribute lo chromophores... 

In Table 7-9 there are given values for carbon-carbon bond lengths for single-bond double-bond resonance, calculated by Equation 7-3 with D = 1.504 A (as corrected for the adjacent-bent-bond effect) and Dt = 1.334 A. These values are for application to conjugated and aromatic systems of single bonds and double bonds, as illustrated in the following section and in later chapter. ... 

In conclusion, all the criteria discussed above point to the existence of jr-electron delocalization and thus to some degree of aromaticity in the unsaturated carbenes, silylenes and germylenes of type 75. However, the degree of conjugation and aromaticity depends... 

Solutions of these metals in liquid ammonia effect (i) the reduction of a range of functional groups such as carbonyl and acetylenic and also conjugated and aromatic systems, and (ii) cleavage of benzyl and allyl ethers and thioethers. These reactions are usually carried out by the general procedure of adding the metal to a solution of the substrate in liquid ammonia to which dry methanol or ethanol or t-butanol has been added to provide a ready proton source (alcohols are more acidic than ammonia).34... 

Reaction (13) is typical of aliphatics and alcohols, whereas reaction (14) is common for double bonds, especially in conjugated and aromatic systems. To form the final products, the radicals R and ROH undergo additional reactions. Free radical scavengers are a very important component of Fenton systems, and their importance in pollutant degradation will be discussed further in Secs. Ill and IV. 

Q Identify conjugated and strained C = O bonds and conjugated and aromatic C = C bonds from their absorptions in the IR spectrum. 

Up-to-Date Treatment In addition to the classical reactions, this book covers many techniques and reactions that have more recently gained wide use among practicing chemists. Molecular-orbital theory is included early and used to explain electronic effects in conjugated and aromatic systems, pericyclic reactions, and ultraviolet spectroscopy. Carbon-13 NMR spectroscopy is treated as the routine tool it has become in most research laboratories, and the DEPT technique is included in this edition. Many of the newer... 

---