(1995; 53 pages)
2. ACYCLIC STRUCTURES
2.1 In acyclic structures, a single bond is shown as a dash, unless a broken line, a wedge or a wavy line is used to depict stereochemistry,1 a carbon-carbon or carbon-heteroatom double bond is shown as double dash, and a triple bond is shown as a triple dash:
2.2 In computer-aided drawing, because it takes time to insert dashes, a single bonding dash between the atoms of an aliphatic chain need not be used. Nevertheless, in this compact form:
- a dash is used to show a single bond between a substituent and a chain or between a chain and a ring;
- a double dash and a triple dash are used to show a double bond and a triple bond respectively;
- dashes, broken lines or wedges are used to depict stereochemistry.1
1 A bond that lies below the plane of the paper is shown by a broken line, one that lies above that plane by a wedge, and one whose configuration is not known by a wavy line.
Sometimes dashes are replaced by dots, but this practice is not recommended:
Not used by the INN Programme
2.3 Acyclic chains can be represented either in linear way or in the form of lines at an angle to one another; the latter option is preferred because it sometimes makes it easier to show an atom next to the atom to which it is linked and offers a better configuration for structures having chiral centres (see section 7):
The latter representation may be simplified by omitting the letter “C” from the central skeleton and the letter “H” for hydrogen atoms, which can be understood to be present. The carbon chain is then represented by a series of lines at an angle to one another, with all terminal groups set out in full:
This form of representation is particularly useful for drawing long carbon chains and is often used in chemical literature. Moreover, this is the form that computer drawing programs are designed to use.
2.4 The groups at the left-hand end of the formula are always inverted, except in the compact form without dashes.
2.5 Single substituents (whether mono- or polyatomic) are not shown in parentheses and included in the structure, but are linked to it with dashes:
Not used by the INN Programme
2.6 When several identical groups are linked to the same atom, they are often shown in parentheses, a subscript on the right indicating their number; dashes are not used to show bonding in this case:
2.7 In acyclic chains that contain a large number of identical groups, such groups can be placed in square brackets, their number being indicated by a subscript on the right:
In general, the points of bonding between adjacent repeated groups are not shown, but there are situations in which they may be indicated to avoid ambiguity (see section 15).
2.8 In a polymethylene chain, when one extremity is linked to a heteroatom, the methylene group linked to that heteroatom may be left outside the brackets if a contracted group such as hydroxymethyl, aminomethyl, etc., is to be shown: