Heterocycle aromaticity

Jeanmaire D L and Van Duyne R P 1977 Part I heterocyclic, aromatic and aliphatic amines adsorbed on the anodized silver electrode J. Electroanal. Chem. 84 1-20... 

Cyclic compounds that contain at least one atom other than carbon within their ring are called heterocyclic compounds, and those that possess aromatic stability are called het erocyclic aromatic compounds Some representative heterocyclic aromatic compounds are pyridine pyrrole furan and thiophene The structures and the lUPAC numbering system used m naming their derivatives are shown In their stability and chemical behav lor all these compounds resemble benzene more than they resemble alkenes... 

Heterocyclic aromatic compounds can be polycyclic as well A benzene ring and a pyridine ring for example can share a common side m two different ways One way gives a compound called quinoline the other gives isoquinoline... 

A large group of heterocyclic aromatic compounds are related to pyrrole by replacement of one of the ring carbons p to nitrogen by a second heteroatom Com pounds of this type are called azoles... 

The structural types described m this section are but a tiny fraction of those pos sible The chemistry of heterocyclic aromatic compounds is a nch and varied held with numerous applications... 

Section 11 22 Heterocyclic aromatic compounds are compounds that contain at least one atom other than carbon within an aromatic ring... 

Section 11 23 Huckel s rule can be extended to heterocyclic aromatic compounds Unshared electron pairs of the heteroatom may be used as tt electrons as necessary to satisfy the 4n + 2 rule... 

Acridine is a heterocyclic aromatic compound obtained from coal tar that is used in the syn thesis of dyes The molecular formula of acndine is C13H9N and its ring system is analogous to that of anthracene except that one CH group has been replaced by N The two most stable reso nance structures of acridine are equivalent to each other and both contain a pyndine like struc tural unit Wnte a structural formula for acridine... 

Section 12 18 Heterocyclic aromatic compounds may be more reactive or less reactive than benzene Pyridine is much less reactive than benzene but pyrrole furan and thiophene are more reactive... 

Halides derived from certain heterocyclic aromatic compounds are often quite reac tive toward nucleophiles 2 Chloropyridme for example reacts with sodium methoxide some 230 million times faster than chlorobenzene at 50°C... 

Nucleic acids are acidic substances present m the nuclei of cells and were known long before anyone suspected they were the primary substances involved m the storage transmission and processing of genetic information There are two kinds of nucleic acids ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) Both are complicated biopolymers based on three structural units a carbohydrate a phosphate ester linkage between carbohydrates and a heterocyclic aromatic compound The heterocyclic aro matic compounds are referred to as purine and pyrimidine bases We 11 begin with them and follow the structural thread... 

Two nitrogen containing heterocyclic aromatic compounds—pyrimidine and purine— are the parents of the bases that constitute a key structural unit of nucleic acids... 

Section 28 1 Many biologically important compounds are related to the heterocyclic aromatic compounds pyrimidine and purine... 

Pyranose form (Section 25 7) Six membered ring ansing via cyclic hemiacetal formation between the carbonyl group and a hydroxyl group of a carbohydrate Pyrimidine (Section 28 1) The heterocyclic aromatic com pound... 

UK Ministry of Agriculture Fisheries and Food and Flealth and Safety Executive, Annual Report of the Working Party on Pesticide Residues in Food (1997), MAFF Publications, London, 1997. lARC, Monographs on the Evabiation of the Carcinogenic Risk of Chemicals to Hnmans Volume 56 Some Naturally Occurring Substances Food Items and Constituents, Heterocyclic Aromatic Amines and Mycotoxins, WFIO, Geneva, 1979, p. 397. 

Section 11.23 HiickeTs rule can be extended to heterocyclic aromatic compounds. 

Pyridine and imidazole were two of the heterocyclic aromatic compounds described in Section 11.22. 

Imidazole and its derivatives form an interesting and important class of heterocyclic aromatic fflnines. Imidazole is approximately 100 times more basic than pyridine. 

From the preparative point of view, reactions of heterocyclic aromatic compounds with nucleophilic reagents are very important, especially the reactions of their quaternary salts containing a formal enamine grouping in the molecule. 

First heterocyclic aromatic analogue of pyridine (Ph3C5H2P) prepared by G. MUrkl, followed by the parent compound C3H3P in 1971 (A. J. Ashe). 

The three possibilities of synthesizing a C5 chain, namely, C2 +Ci + C2. C2 -t C2 -I- Cl, or Cl + C3 + Cl, lead to pyrylium salts having identical substituents in positions 2 and 4 in the second case, or 2 and 6 in the first and third cases. Despite this limitation, such syntheses are very convenient because they make the pyrylium salts easily accessible (more so than other six-membered heterocyclic aromatics) from aliphatic starting materials. 

The following are the particular situations arising in heterocyclic aromatic chemistry which can be, and have been, treated in terms of the Hammett equation and which will be discussed in the following sections. 

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