Canonical form

The contributions from the canonical forms have been calculated, and the contributions from the equivalent polar stmctures (2) and (3) dominate those of (4) and (5) (6). Thus, electrophilic substitution is predicted to occur in the a-position, and this is proven experimentally in most cases. Nitrosation and selenocyanation occur at the P-position (7,8). 

We classify compounds as aromatic, if there is continuous conjugation around the ring, or non-aromatic. Aromatic compounds are further subdivided into those without exocyclic double bonds and those in which important canonical forms containing exocyclic double bonds contribute. 

The five possible azole monoanions are shown (one canonical form only) in Scheme 2 all heteroatoms are now nitrogens. 

These compounds are usually written in the unionized form as in (8 Z = NH, NR, O, S). Canonical forms of types (9) or (10) are important, i.e. these compounds can also be considered as betaines formally derived from azolium ions. Many compounds of this type are tautomeric and such tautomerism is discussed in Section 4.01.5.2. 

Canonical forms of type (442b) facilitate proton loss from the amino groups the anions formed react easily with electrophilic reagents, usually preferentially at the exocyclic nitrogen atom e.g. 443 -> 444) (79HC(34-2)9). 

Hydroxy-imidazoles, -oxazoles and -thiazoles (484 Z = NR, O, S) can isomerize to 2-azolinones (485a). These compounds all exist predominantly in the azolinone form and show many reactions similar to those of the pyridones. They are mesomeric with zwitterionic and carbonyl canonical forms e.g. 485a 485b Z = NR, O, S). 

It has very recently been suggested that this species is not oxirene, but the authors do not make it clear how their intermediate differs from an oxirene canonical form (82JA216). 

Ollis and Ramsden state that A compound may be appropriately called mesoionic if it is a five-membered heterocycle which cannot be represented satisfactorily by any one covalent or polar structure and possesses a sextet of electrons in association with the five atoms comprising the ring . From the point of view of systematic nomenclature, compounds of this type are difficult to deal with, since most available nomenclature systems are designed so as to name one particular bond- and charge-localized canonical form. 

Trioxo-l,3,5-triazinanetriide ( isocyan-urate, a canonical form of cyanurate)... 

Keep R1 integrals in memory in canonical form, NReq=... 

Many of the quaternary salts discussed in this section and in Section V may be represented by two or more canonical forms. Such resonance, where it occurs, is obvious and therefore only one of the structures is given. 

The urea-type resonance is illustrated, as the neutral species, by 33. Resonance of this type does not operate in anionic species because one of the two possible canonical forms would have to carry the negative charge on the oxygen atom and the other on the nitrogen atom, and these forms would be unequivalent. The urea-type resonance is exemplified by the neutral species of 2-hydroxypteridine, which is strongly hydrated and has an anhydrous anion. 

The chemical reactions of this compound were recently reconsidered, and both structures 64 and 65 were rejected in favor of the zwit-terion formulation 66, which is supported by the presence of a band at S.lfx (3226 cm ) in the infrared spectrum and is merely an alternative canonical form of 64. On the other hand, the ultraviolet spectrum of 4-hydroxypyrrole-2-carboxylic acid (67) resembles that of its ethyl ether, possibly indicating that the 2-acid exists in the hydroxy form. -... 

A criterion for the position of the extent of the mesomerism of type 9 is given by the bond order of the CO bond, a first approximation to W hich can be obtained from the infrared spectrum (v C=0). Unfortunately, relatively little is known of the infrared spectra of amide anions. How-ever, it can be assumed that the mesomeric relationships in the anions 9 can also be deduced from the infrared spectra of the free amides (4), although, of course, the absolute participation of the canonical forms a and b in structures 4 and 9 is different. If Table I is considered from this point of view, the intimate relationship betw-een the position of the amide band 1 (v C=0) and the orientation (0 or N) of methylation of lactams by diazomethane is unmistakeable. Thus the behavior of a lactam tow ard diazomethane can be deduced from the acidity (velocity of reaction) and the C=0 stretching frequency (orientation of methylation). Three major regions can be differentiated (1) 1620-1680 cm h 0-methylation (2) 1680-1720 cm i, O- and A -methylation, w ith kinetic dependence and (3) 1730-1800 em , A -methylation, The factual material in Table I is... 

EXAMPLE Cousklo.r a oiie-dimeiisioiial lattice of si/.e N = 5. The adjacency matrix, L, and Smith Canonical Form of L — xl, are, respectively,... 

---