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21.2 - The names of hydrocarbons containing five or more fused benzene rings in a straight linear arrangement are formed from a numerical prefix as specified in Rule A-1.1 followedby "-acene".
Examples to Rule A-21.1
21.3 - "Ortho-fused" or "ortho- and peri-fused" polycyclic hydrocarbons with maximum number of non-cumulative double bonds which contain at least two rings of five or more members and which have no accepted trivial name such as those of Part .1 of this rule, are named by prefixing to the name of a component ring or ring system (the base component) designations of the other components. The base component should contain as many rings as possible (provided it has a trivial name), and should occur as far as possible from the beginning of the list of Rule A-21.1. The attached components should be as simple as possible.
Examples to Rule A-21.2
21.4 - The prefixes designating attached components are formed by changing the ending "-ene" of the name of the component hydrocarbon into "-eno"; e.g., "pyreno" (from pyrene). When more than one prefix is present, they are arranged in alphabetical order. The following common abbreviated prefixes are recognized (see list in Part.1 of this rule):
Examples to Rule A-21.3
(not Naphthophenanthrene: benzo is "simpler" than naphtho, even through there are two benzo rings and only one naphto)
21.5 - Isomers are distinguished by lettering the peripheral sides of the base component a, b, c, etc., beginning with "a" for the side "1,2", "b" for "2,3" (or in certain cases "2,2a") and lettering every side around the periphery. To the letter as early in the alphabet as possible, denoting the side where fusion occurs, are prefixed, if necessary, the numbers of the positions of attachment of the other component. These numbers are chosen to be as low as is consistent with the numbering of the component, and their order conforms to the direction of lettering of the base component. When two or more prefixes refer to equivalent positions so that there is a choice of letters, the prefixes are cited in alphabetical order according to Rule A-21.4 and the location of the first cited prefix is indicated by a letter as early as possible in the alphabet. The numbers and letters are enclosed in square brackets and placed immediately after the designation of the attached component. This expression merely defines the manner of fusion of the components.
Examples to Rule A-21.4
The completed system consisting of the base component and the other components is then renumbered according to Rule A-22, the enumeration of the component parts being ignored.
Examples to Rule A-21.5
21.6 - When a name applies equally to two or more isomeric condensed parent ring systems with the maximum number of non-cumulative double bonds and when the name can be made specific by indicating the position of one or more hydrogen atoms in the structure, this is accomplished by modifying the name with a locant, followed by italic capital H for each of these hydrogen atoms. Such symbols ordinarily precede the name. The said atom or atoms are called "indicated hydrogen". The same principle is applied to radicals and compounds derived from these systems.
Example to Rule A-21.5
See Recommendations'93 R-2.4.1, R-9.1, Tables 20 and 20(a)
Examples to Rule A-21.6
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