As substituent groups

As mentioned earlier, nucleophilic attack at ring carbon atoms a and y to a heteroatom is a reaction characteristic of the ir-deficient six-membered heterocycles. Within this context we do not regard hydrogen atoms attached to ring carbon atoms as substituent groups, and hence proton removal to generate a carbanionic centre is classified as a reaction at a ring carbon atom. There are thus six major reaction types, summarized in equations (21)—(26) in terms of the initial mechanistic steps. 

If non-identical ring systems are joined together by single or double bonds, one is considered as parent structure and the other(s) as substituent group(s). To select the most senior parent system the following descending order of priorities is obeyed ... 

Conjunctive nomenclature is particularly suitable for systems multiply substituted with identical functional parents but is generally used by Chem. Abstr. for any type of saturated linear monocarboxylic acid bearing a ring as substituent group. 

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