True aromaticity

If the union occurs in such a position that loss of hydrogen with re-formation of a true aromatic nucleus is feasible, an aporphine base will result. 

Its aromaticity cannot, of course, be tested by attempted electrophilic substitution, for attack by X would merely lead to direct combination with the anion. True aromatic character (e.g. a Friedel-Crafts reaction) is, however, demonstrable in the remarkable series of extremely stable, neutral compounds obtainable from (15), and called metallocenes, e.g. ferrocene (16), in which the metal is held by n bonds in a kind of molecular sandwich between the two cyclopentadienyl structures ... 

Two other important modes of substitution require mention here. They are the SNAr and elimination-addition reactions. Actually, it is sometimes difficult to distinguish between true aromatic nucleophilic substitutions and addition-elimination processes. The second group involves pyridyne intermediates (Scheme 53). Both of these reaction types are discussed fully under substituent reactions (Chapter 2.06). 

The term "aromatic aldehyde" is usually used in perfumery, as it is here, to include both the true aromatic aldehydes and the alkyl aromatic aldehydes, in which the aldehydic group is attached to a side chain rather than onto the benzene ring. 

In the case of carbochemical oils, the BMCI may not reflect the true aromaticity of the product. For this reason, the carbon/hydrogen ratio is more favored for carbochemical products. However, as this measurement is also superior to BMCI, even for petrochemical products, the carbon/hydrogen ratio or the carbon content are becoming the preferred criteria for all carbon black feedstocks. 

The former are true aromatic sulphonic acids prepared by direct sulphonation, and reacting like benzene sulphonic acid. The latter are aliphatic sulphonic acids both in methods of preparation and reaction. 

Phenols.—The ring hydroxyl compounds take the class name of phenols from the simplest member, hydroxy benzene or phenol. These are true aromatic compounds and in methods of formation, reactions and properties are distinctly different from aliphatic hydroxyl compounds or alcohols. Their outstanding distinction is their marked acid character, the alcohols being neutral (p. 103). This is attributed to the influence of the phenyl radical (CeHs—). The same influence is present in the amino derivatives, for the ring amines, CeHs— NH2, /CH3... 

Alcohols.—The side chain hydroxyl compounds take the class name of alcohols for they are true aromatic alcohols in formation, reaction and properties. They are neutral not acid, and are formed by methods analogous to those by which the aliphatic alcohols are prepared. They may be looked upon as benzene derivatives of aliphatic alcohols, e.g. CeHs—CH2OH, benzyl alcohol or phenyl methyl alcohol. 

Davis (184) selected a large number of mutants of Escherichia coli requiring two or more aromatic amino acids for growth, and then tested a large number of substances to see if any could relieve the growth inhibition. Success was attained with shikimic acid (215, 268), at that time a relatively obscure natural product. This indicated either that shikimic acid (structure diagram 1) was a true aromatic precursor or that it could readily be transformed into a true precursor. The likelihood that shikimic acid was a true precursor was increased when other mutants were found to accumulate shikimic acid in the medium, from which it could be isolated (184, 185). 

The l,3A 5, 2,4-benzodithiadiazines 11 and 12 are mixed heterocyclic-carbocyclic compounds that combine true aromaticity in the all-carbon part (4n - - 2tt electrons) with a nonaromatic heterocyclic part, to give an overall antiaromatic structure (4n7t electrons) <2001CEJ3592>. 

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