Tautomerism and Resonance

There is no sharp distinction which can be made between tautomerism and resonance but it is convenient in practice to make a distinction between the two which is applicable to all except the border-line cases. 

It is evident that we may define tautomerism and resonance in the following reasonable way When the magnitudes of the electronic resonance integral (or integrals) and of the other factors... 

Each of the tautomeric forms of a substance may show electronic resonance tautomerism and resonance are not mutually exclusive. Let us discuss 5-methylpyrazole as an example. This substance exists in two tautomeric forms, A and B, differing in the position of the N-hydrogen atom. 

The net products of this reaction are a protonated hydroxy-x-triazine and Cl. The Cl- is electrostatically repelled from the surface due to the dominance of the negative charge on soil surfaces. On the other hand, the protonated hydroxy-.v-triazine is held on the surface substantially more tightly than the original chloro-x-triazine due to the added contribution of the electrostatic interaction. The pKa values of the hydroxy-.v-triazines are all greater than 5 (Weber, 1970a, b) hence with the help of surface acidity, adsorbed protonated hydroxy-x-triazines tend to remain protonated. Protonated hydroxy-.v-triazines are further stabilized by both tautomerism and resonance (Russell et al., 1968 Laird, 1996). 

It is important not to confuse the properties of tautomerism and resonance. They are quite different effects and the differences between them are summarised in Table 3.1. Although the enol form of warfarin is present to... 

It is well known from chemical history that the discoveries of the first stable organic radicals, such as triphenylmethyl, diphenyl-picrylhydrazyl, tri-tert-butylphenoxyl, and nitroxides are very significant contributions to theoretical chemistry. The relative stabilities of these radicals were attributed by chemists to the participation of an unpaired electron in conjugated ir-electron systems. Classical stable radicals can thus be thought of as a superposition of many resonance structures with different localizations of an unpaired electron. The first stable radical obtained by Pilotti and Schwerin in 1901 in the pure state can be described by a variety of tautomeric and resonance structures as shown in Scheme 1. 

Scheme 1. Tautomeric and resonance structures of a stable free radical.

Treatment of Tautomerism and Resonance. The Registry CT must provide for a satisfactory treatment of the phenomena of tautomerism and resonance, so that different valence bond descriptions of the same molecule can be recognised as equivalent and can be represented in the Registry file by the same Registry CT. 

It is possible, on the other hand, to define tautomerism and electronic resonance in a way which gives structural significance to them individually. 

Although Eibner elucidated the structure between 1904 and 1906, it was only through IR and nuclear magnetic resonance spectroscopy (NMR) that the chro-maticity of these molecules could be attributed to keto-enol tautomerism and simultaneous hydrogen bond formation (structures 137a = 137b) [2]. 

In 1934 Ingold defined the chemical term "mesomerism" to mean stable intermediate states explained physically by quantal resonance.41 He stressed a preference for the word "mesomerism" over the term "tautomerism" or "resonance" on the grounds that it did not lead to the false impression that alternate structural formulas are passing into each other very rapidly, like tautomerides. If such rapid motion were going on, he argued, every molecule must spend most or all of its time in transforming itself, and the term molecular "state" simply loses its meaning.42... 

It is important to distinguish tautomerism from resonance, a term used to indicate that the properties of a given molecule cannot be represented by a single valence structure but can be represented as a hybrid of two or more structures in which all the nuclei remain in the same places. Only bonding electrons move to convert one resonance form into another. Examples are the enolate anion, which can be thought of as a hybrid of structures A and B, and the amide linkage, which can be represented by a similar pair of resonance forms. 

The bands show significant bathochromic shifts in base solution (Fig. 5.2C) relative to neutral (Fig. 5.2A) or acidic (Fig. 5.2B) solutions. This may be due to tautomeric [39] and resonance effects taking place in basic medium. Interactions occurred between the nonbonded electrons on the nitrogens and the n electrons of the fused rings as evidenced by the blurring of the bands. Furthermore, the enolic configuration... 

The relationship between tautomerism and aromaticity is strong both in carbocyclic chemistry (e.g., phenol) [37, 38] and in heterocyclic chemistry (e.g., 2-hydroxypy-ridine) [39, 40], We consider that the cycle should be formed by covalent bonds, thus excluding the possibility that rings comprising hydrogen bonds, the so-called resonance assisted hydrogen bonds (RAHB), could be aromatic [41 45],... 

This review covers the Pd(0)-catalyzed allylations of aromatic ambident heterocyclic compounds, including all rings for which an aromatic tautomeric or resonance form can be written. Cases of C vs. O, C vs. N, N vs. O, and S vs. N allylation are discussed from all available viewpoints regioselectivity, kinetic vs. thermodynamic control, mechanism, stereo-... 

This type of isomerization, occurring by the migration of a proton and the movement of a double bond, is called tautomerism, and the isomers that interconvert are called tautomers. Don t confuse tautomers with resonance forms. Tautomers are true isomers (different compounds) with their atoms arranged differently. Under the right circumstances, with no catalyst present, either individual tautomeric form may be isolated. Resonance forms are different representations of the same structure, with all the atoms in the same places, showing how the electrons are delocalized. 

Fig. 4-7. Endwise polymerization (Adler, 1977). A guaiacylglycerol-/3-aryl ether structure (1) is dehydrogenated and after resonance, radical c is coupled with a coniferyl alcohol radical b (cf. Fig. 4-4). The /3-5 coupling product (3) is tautomerized and undergoes intramolecular ring closure (a phenylcoumaran structure, 5).

Compounds whose structures differ in the arrangement of hydrogen atoms and which are in rapid equilibrium are called tautomers. It is important to draw a distinction between resonance forms and tautomers. Whereas it is possible to obtain spectroscopic information on the existence of the individual tautomeric forms, resonance forms are not distinguishable. The difference can be illustrated by considering an amide (1.52). The resonance form (1.53) shows a difference in the position of charge, while the tautomer (1.54) shows a difference in the position of a hydrogen atom. 

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