Benzene mesomeric structures

The problem of the structure of 1,2- and 1,4-quinone diazides was investigated by Le Fevre s group (1949, 1954) by measuring dipole moments. The observed moments in benzene are in the range 2.9 to 5.0 D, compared with calculated values of 1.6 to 4.0 D for the quinone diazide structure and 15.7 and 27.4 D respectively for the 1,2-and 1,4-zwitterionic forms. No attempts were made by Lowe-Ma et al. (1988) to calculate dipole moments for the mesomeric structure 4.4 that they proposed. 

Fig. 1 Mesomeric structures of a para-substituted benzene intramolecular charge transfer (ICT) complex in the ground state and in the dipolar excited state...

The benzene ring is usually depicted as one of two mesomeric structures (2.1). The double arrow indicates that the true structure of the molecule lies somewhere in between the two drawn structures. It is therefore more accurate to use structure 2.2, since the six C-C bonds of the ring are identical, with the 7t-electrons delocalized over the entire ring. The configuration shown in 2.2 is, however, less convenient for drawing reaction mechanisms. 

The most pronounced effect of bond localization is observed in the t]2 -complexes due to a loss of equivalence of the mesomeric structures in the benzene ring, i.e. ... 

The term protomeric structure was obviously introduced in analogy to the well-known mesomeric structures , which are used to describe the electronic ground state of aromatic compounds such as benzene in terms of a resonance hybrid [323],... 

When the imidazole ring is considered to be something resembling a pyrrole-pyridine combination (1) it would appear that any electrophilic attack should take place preferably at C-5 (pyrrole-or, pyridine-j8). Such a model, though, fails to take account of the tautomeric equivalence of C-4 and C-5 (Section 4.06.5.1). The overall reactivities of imidazole and benzimidazole can be inferred from sets of resonance structures in which the dipolar contributors have finite importance (Section 4.06.2) or by mesomeric structures such as (2). These predict electrophilic attack in imidazole at N-3 or any ring carbon atom, nucleophilic attack at C-2 or C-1, and also the amphoteric nature of the molecule. In benzimidazole the acidic and basic properties, the preference for nucleophilic attack at C-2 and the tendency for electrophiles to react at the fused benzene ring can be readily rationalized. 

Benzene is described as a resonance hybrid of the two extreme forms which correspond, in terms of orbital interactions, to the two possible spin-coupled pairings of adjacent p electrons structures 1 and 2. These are known as resonance contributors , or mesomeric structures , have no existence in their own right, but serve to illustrate two extremes which contribute to the real structure of benzene. Note the standard use of a double-headed arrow to inter-relate resonance contributors. Such arrows must never be confused with the use of opposing straight fish-hook arrows that are used to designate an equilibrium... 

Structure of benzene resonance contributors (mesomeric structures)... 

The situation resembles the unusual reduction mechanism of p-dicarbonyl benzenes [24] published in 1960s, where, in aqueous solutions, a two-electron reversible process was found. The explanation was based on the assumption that the two equal carbonyls provide the planar shape of the molecule where an extended quinoid mesomeric structure takes place involving both carbonyls. In our case, however, the experiments proceed in aprotic media, only one carbonyl is present, and the substitution in p-positions is nonsymmetric and complicated by the carbene function. Nevertheless, the second carbonyl can be mimicked either by the formal Cr=C double bond or by the hypothetic C=N double bond and the observed anomalous reduction potentials can be explained by the model of p-diearbonyl benzene. 

This mesomerism (or resonance )59 between equivalent Kekule structures was recognized as the quintessential feature underlying the aromaticity of benzene, conferring highly distinctive symmetry, stability, and reactivity patterns. 

Annelation on to a benzene ring increases considerably the complexity of the spectra, and indole has absorptions at 216 (4.54), 266 sh (3.76), 270 (3.77), 276 (3.76), 278 (3.76) and 287 (3.68) nm in ethanol solution. Because of the widespread occurrence of the indole ring system in nature and the sensitivity of absorption band position and intensity to substitution type, considerable use has been made of electronic spectroscopy in the past for structure identification. An extensive tabulation of data, primarily for monosubstituted derivatives, is available (71PMH(3)67,p.94). As expected, whereas the effects of alkylation are comparatively slight, introduction of groups capable of mesomeric interaction with the indole it -system may cause profound changes in the appearance of the spectrum representative examples are given in Table 24. 

RESONANCE. 1. In chemistry, resonance (or mesomerism) is a mathematical concept based on quantum mechanical considerations (i.e.. die wave functions of electrons). It is also used to describe or express the true chemical structure of certain compounds that cannot be accurately represented by any one valence-bond structure. It was originally applied to aromatic compounds such as benzene, for winch there are many possible approximate structures, none of which is completely satisfactory. See also Benzene. 

The question of the contribution of the benzene rings to the indigo chromogen led to a search for the basic chromophore. Consecutively removing parts of the mesomeric system leaves a cross-conjugated structure 7 that, as the basic chromophore, still exhibits the typical deep color and redox properties of indigo. 

The stable borazole or triborine-triamine B3N3H6 is completely analogous to and isoelectronic with benzene with the same structure, stability, mesomerism and even the same smell. Also derivatives analogous to symm. trichlorobenzene and meta-dibromobenzene are known. 

This mesomeric (or M) effect is seen when aniline is placed in a solution of pH 8-14, i.e. when the basic aniline is unionised. When aniline is placed in a solution of pH < 7, the A,max returns to virtually the value obtained for benzene (203 nm). What is happening is that aniline in acidic solution reacts to form the anilinium salt. The lone pair of electrons on the nitrogen is now involved in bond formation to an H+ ion and can no longer function as an auxochrome. The structure of aniline hydrochloride is shown in Figure 7.8. 

Groups such as trifluoromethyl, CF3, and trialkylammonium R3N+, are unable to interact with the n-system, but withdraw electrons as a result of the electronegativity of the fluorine atoms and the positively charged nitrogen, respectively. A study of the canonical forms for electrophilic attack at the three sites indicates a situation similar to that discussed above for mesomerically withdrawing groups (Scheme 2.14). The intermediates are overall destabilized by electron withdrawal, but structures 15 and 16 are particularly unfavourable because the positive charge is adjacent to the electron-deficient atom of the substituent. Thus, attack occurs preferentially at the 3-position, but is more difficult than electrophilic attack on benzene. 

More than anyone else it has been Linus Pauling (b. 1901) who has been responsible for the development and application of the valence bond theory. In the early 1930s he deduced from quantum mechanics the tetrahedrally directed valencies of carbon, and he introduced the concept of the hybridisation of atomic orbitals. He introduced the idea of resonance as the quantum-mechanical counterpart of mesomerism. The wavefiinction for the molecule must contain terms for all possible structures, and the molecule is said to resonate between them. In 1933 Pauling described the benzene molecule as a resonance hybrid between the two Kekule structures and the three possible Dewar structures (Figure 11.22). 

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