Aromatic compounds position isomers

Aromatic compound Positions of substitution (and relative amounts of isomers where quoted) Ref. 

Two double bonds. This is the most important class which includes the aromatic compounds pyrazole (3), indazole (4) and isoindazole (5), their non-aromatic isomers, pyrazolenines (or 3iL-pyrazoles 6), isopyrazoles (or 4JT-pyrazoles 7) and 3JT-indazoles (8), and the carbonyl derivatives of pyrazolines with the endocyclic double bond in positions 1, 2 or 3, i.e. (9), (10) and (11), respectively. The indazolones (12) and the pyrazolidinediones (13) and (14) also belong to this group. 

Arynes are intermediates in certain reactions of aromatic compounds, especially in some nucleophilic substitution reactions. They are generated by abstraction of atoms or atomic groups from adjacent positions in the nucleus and react as strong electrophiles and as dienophiles in fast addition reactions. An example of a reaction occurring via an aryne is the amination of o-chlorotoluene (1) with potassium amide in liquid ammonia. According to the mechanism given, the intermediate 3-methylbenzyne (2) is first formed and subsequent addition of ammonia to the triple bond yields o-amino-toluene (3) and m-aminotoluene (4). It was found that partial rearrangement of the ortho to the meta isomer actually occurs. 

In many cases, however, the ortho isomer is the predominant product, and it is the meta para ratio which is close to the statistical value, in reactions both on benzenoid compounds and on pyri-dine. " There has been no satisfactory explanation of this feature of the reaction. One theory, which lacks verification, is that the radical first forms a complex with the aromatic compound at the position of greatest electron density that this is invariably cither the substituent or the position ortho to the substituent, depending on whether the substituent is electron-attracting or -releasing and that when the preliminary complex collapses to the tr-complex, the new bond is most likely to be formed at the ortho position.For heterocyclic compounds such as pyridine it is possible that the phenyl radical complexes with the nitrogen atom and that a simple electronic reorganization forms the tj-complex at the 2-position. 

Udenfriend et al. observed that aromatic compounds are hydroxyl-ated by a system consisting of ferrous ion, EDTA, ascorbic acid, and oxygend Aromatic and heteroaroinatic compounds are hydroxylated at the positions which are normally most reactive in electrophilic substitutions. For example, acetanilide gives rise exclusively to the o-and p-hydroxy isomers whereas quinoline gives the 3-hydroxy prod-uct. - The products of the reaction of this system w ith heterocyclic compounds are shown in Table XIII. 

Intermediates generated at an electrode surface may react while still near the electrode. If so, one side of the intermediate may be wholly or partly shielded from attack by other reactants by the electrode itself. Such behavior is particularly common in the electrochemical oxidation of aromatic compounds since, as we have already seen with coumarin, aromatic compounds are generally tightly adsorbed parallel to the electrode surface at potentials positive of the p.z.c. For example, electrochemical oxidation of the stilbenes in alkaline methanol affords a mixture of dl and meso-1,2 dimethoxy-1,2-diphenylethane (1) 10>. It is found that c/s-stilbene affords a mixture of isomers of 1 in which the... 

A number of l-aryl-2-thienylethylenes have been photocyclized in the presence of an oxidizing agent (usually iodine) to polycyclic aromatic compounds. Representative examples are given in Table 1. The mechanism, as with the conversion of stilbene to phenanthrene, probably involves conversion of the trans-alkene to the c/s-form, cyclization to the dihydro isomer, and oxidation of the latter to the fully aromatic compound. The yield of the cyclized product seems to decrease when the ethylene is attached to the /3-position of the thiophene. 

Analytical Properties Separation of positional isomers of aromatic compounds and geometrical isomers of sex pheromones Reference 7... 

The position of the nitro group is also an important structural factor in determining the biological activity in nitro-aromatic compounds. This is evident from studies that show markedly different mutagenic potency in structural isomers of nitro-aromatic compounds [3,15, 24, 26, 30, 31,44,46]. Structural isomers like 1-, 2-, 3-, and 6-nitrobenzo[a]pyrene have dramatically different mutagenic potency [47—49] 6-nitrobenzo[a]pyrene is a weak mutagen, while 1-, 2-, and 3-nitrobenzo [a]pyrene are potent mutagenes [47 -9]. 

An aromatic compounds containing a substituent can undergo electrophilic substitution at three different positions relative to the substituent. For example in the bromination of toluene (Fig.J), three different products are possible depending on where the bromine enters the ring. These products have the same molecular formula and are therefore constitutional isomers. The aromatic ring is said to be di-substituted and the three possible isomers are referred to as being ortho, meta, and para. The mechanisms leading to these three isomers are shown in Fig. K. 

All possible interactions between the K and L groups were taken into account and Akk = 0. The definition contains a minimum number of groups and is satisfactory for most of the binary systems studied. However, it cannot take into account the structural differences which exist between position isomers. This is the case of polycyclic aromatic compounds presenting cycle position isomers or substitute position isomers. Structural differences of this type determine the gaps between the values of certain thermophysical properties of isomers, such as, for example, the fusion temperature or sublimation enthalpy. The further the temperature falls, the more these differences are accentuated. The representation of the solid-fluid (low temperature) equilibria is consequently more difficult and the model must take into account the existing structural differences. We came across this problem in the compounds such as anthracene, phenanthrene, pyrene, methylated naphthalenes, hexamethylbenzene and triphenylmethane. As it was out of the question to increase the number of groups because... 

Whereas the separation of racemates on these two CSP are obvious, recent applications demonstrate that achiral isomers, especially aromatic compounds with substituents in different positions, are extraordinarily well separated on Chiralcel-OD and Chiralpak-AD as well (Figure 21-9). It is to be expected that further examples will follow and more and more achiral separation problems will be solved in the future on CSP. 

Acetyl hypofluorite will only fluorinate activated aromatic compounds and fluorinations arc generally performed at low temperatures. In general a mixture of products is formed, for example, the fluorination of anisole with acetyl hypofluorite results in a mixture of 2-fluoroanisole and 4-fluoroanisole (ratio 9.6 1) in 85% yield (Table 11). This preferential formation of the ortho-isomer is explained by an addition-elimination mechanism with an intermediate 15. This mechanism is support by the fact that the addition product 16 from acetyl hypofluorite and pipcronal is isolated in 55 % yield.169 Selective fluorination is achieved when the para position is blocked by a second substituent (Table 12). This approach has been used to selectively fluorinate an aromatic ring in peptide 17.170... 

The procedure described here is typical for the catalytic alkylation of aromatic ketones at the ortho position by alkenes. Aromatic ketones are readily available by Friedel-Crafts acylation and many other methods, and many of these ketones are suitable substrates for the present catalytic alkylation with alkenes affording the corresponding ortho-alkylated ketones. The present method provides a direct way to alkylate aromatics with olefins. Moreover, the C-C bond formation takes place with exclusive ortho selectivity, while mixtures of 0-, m-, p-isomers are usually obtained in the conventional Friedel-Crafts alkylation of aromatic compounds. 

Pfibram and Handl found that when viscosities at one temperature are compared, they often differ for isomeric substances those of normal esters are greater than those of isomeric esters with branched carbon chains the viscosity is increased by substitution of halogen or NO2 for hydrogen, and in aromatic compounds it is influenced by the nature and position of the substituent. Cauquil found the viscosities of trans- greater than those of cw-isomers. Schulz reviewed the relation between viscosity and constitution. 

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