Hydrocarbon alternant

Pariser R 1956 Theory of the electronic spectra and structure of the polyacenes and of alternant hydrocarbons J. Chem. Rhys. 24 250-68... 

Figure 8-6 Approximate x-y Coordinates for the Alternant Hydrocarbons Ethene Through 1,3,5.7-Octatetraene.

Unsubstituted PMDs (2) or dyes containing odd alternate hydrocarbon residues as end groups can exist in two relatively stable forms distinguished by a TT-electron pair, eg, a,CO-diphenylpolymethines (9). 

W. Dukek and J. P. LongweU, Alternative Hydrocarbon Fuels for Aviation, Esso Air Wodd(b)29 No. 4, Exxon International Co., Florham Park, N.J., 1977. 

The calculation of localization energies in heteroaromatic systems derived from alternant hydrocarbons has been simplified by Dewar and Maitlis (57JCS2521). This approach has had considerable success the results provide a somewhat empirical index of reactivity. 

An important distinction for conjugated hydrocarbons is the classification into alternant and non-alternant hydrocarbons. Alternant hydrocarbons are those like ethene, hexatriene, benzene and naphthalene where we can divide the carbon atoms into two sets called starred and unstarred , such that no member of one set is directly bonded to another member (Figure 7.4). 

Huckel calculations on alternant hydrocarbons have the following characteristics ... 

A consequence of these findings is that the jr-electron charges are all exactly equal to 1, for the ground state of every alternant hydrocarbon. (Coulson and Rushbrooke, 1940). 

Trigollenine (18) and the pseudo-cross-conjugated Homarine (19) are isoconjugate with the isopropenylbenzene anion, which is an odd alternant hydrocarbon anion. Therefore, these betaines belong to class 9 and 13, respectively. 

Demethylvasconine (85) (9-methoxy-5-methyl-phenanthridin-8-olate) presented in Scheme 31 was found in Crinum kirkii (95P1291) (Amaryllidaceae). Although published as cation, no information about the anion of this alkaloid is given. Its relationship to other alkaloids of this class, however, makes a betainic structure more than likely and this is confirmed by a comparison of the NMR data of 85 with the cationic and betainic alkaloids presented in Table III. This betaine is isoconjugate with the 2-methylphenanthrene anion and thus defined the alkaloid as a member of class 1 (odd alternant hydrocarbon anions). Whereas substitution of the isoconjugate phenanthridinium moiety at the 1-position with an anionic fragment results in zwitterions (cf. Section III.D), the phenanthridinium-2-olate is a mesomeric betaine. 

Benzo[c]phenanthridine alkaloids are widespread in Papaveraceae, Fumariaceae, and Rutaceae. Fagaridine (118), the structure of which had to be revised, is a derivative of the unknown 5-methyl-benzo[c]phenan-thridine-8-olate (119) which is isoconjugate with the 2-methyl-chrysene anion (Scheme 43). Thus, Fagaridine is a member of class 1 of conjugated heterocyclic mesomeric betaines, which are isoconjugate with odd alternant hydrocarbon anions. 

The oxoaporphine alkaloids Teliglazine (126), Corunnine (127), Nandazurine (128), PO-3 (129), A-Methylliriodendronine (130), and A,(9-Dimethylliriodendronine (131) contain the 6-methyl-7-oxo-dibenzo [Je,g]quinolinium-l-olate ring system 125 which is isoconjugate with the l-methyl-7-methylene-7H-benzo[Je]anthracene anion (Scheme 46). Therefore, these alkaloids belong to class 1, i.e., heterocyclic mesomeric betaines isoconjugate with odd alternant hydrocarbon anions. Another... 

Methylisoquinolinium 2-carboxylate (230), originally prepared by Quast (70LA64), was recently identified as a defensive betaine from Photuris versicolor fireflies (99JNP378). It is a pseudo-cross-conjugated mesomeric betaine isoconjugate to the odd alternant hydrocarbon 2-isopropenyl-naphthalene anion which is an odd alternant hydrocarbon anion. This compound therefore is a member of class 13, which is very rare. The UV absorption maxima Imax (methanol) were found at 235 (4.35), 320 (shoulder, 3.97), and 326 (3.99) nm. This compound undergoes similar reactions as Homarine 19 (Scheme 75). The NMR data are presented in Table VIII. 

Many chemicals are produced from synthesis gas. This is a consequence of the high reactivity associated with hydrogen and carhon monoxide gases, the two constituents of synthesis gas. The reactivity of this mixture was demonstrated during World War II, when it was used to produce alternative hydrocarbon fuels using Fischer Tropsch technology. The synthesis gas mixture was produced then hy gasifying coal. Fischer Tropsch synthesis of hydrocarbons is discussed in Chapter 4. 

Aromatic hydrocarbons can be divided into two types alternant and nonalternant.In alternant hydrocarbons, the conjugated carbon atoms can be divided into two sets such that no two atoms of the same set are directly linked. For convenience, one set may be starred. Naphthalene is an alternant and azulene a nonaltemant hydrocarbon ... 

FIGURE 2.7 Energy levels in odd- and even-alternant hydrocarbons. The arrows represent electrons. The orbitals are shown as having different energies, but some may be degenerate. 

In alternant hydrocarbons (p. 55), the reactivity at a given position is similar for electrophilic, nucleophilic, and free-radical substitution, because the same kind of resonance can be shown in all three types of intermediate (cf. 20,22, and 23). Attack at the position that will best delocalize a positive charge will also best delocalize a negative charge or an unpaired electron. Most results are in accord with these predictions. For example, naphthalene is attacked primarily at the 1 position by NOj, NHJ, and Ph, and always more readily than benzene. 

Encouragingly, the orbital phase predictions on ground-state spin of the alternant hydrocarbon diradicals, 1, 5, 7, and 13, are in agreement with those proposed by Borden and Davidson [64,71, 78-80], by Ovchinnikov [72], and by Radhakrishnan [49, 50]. For the non-altemant systems and hetero-derivatives, 16-20, the orbital phase theory performs as well as the Radhakrishnan s rule [49, 50]. 

In the HMO or extended Hiickel approach, the individual ionization potentials should be set equal to orbital energies. The inadequacy of the HMO treatment is apparent with odd alternant hydrocarbons (e.g., allyl, benzyl), where a constant value is obtained, in disagreement with the experiment. Streitwieser and Nair (105) showed, however, that reasonable results can be obtained with the co technique. 

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