Line formula conventions

The resonance interaction of chlorine with the benzene ring can be represented as shown in 13 or 14, and both of these representations have been used in the literature to save space. However, we shall not use the curved-arrow method of 13 since arrows will be used in this book to express the actual movement of electrons in reactions. We will use representations like 14 or else write out the canonical forms. The convention used in dashed-line formulas like 14 is that bonds that are present in all canonical forms are drawn as solid lines, while bonds that are not present in all forms are drawn as dashed lines. In most resonance, a bonds are not involved, and only the n or unshared electrons are put in, in different ways. This means that if we write one canonical form for a molecule, we can then write the others by merely moving n and unshared electrons. 

It is important to discuss cell notation and conventions. Instead of drawing a complete diagram to present electrochemical cells, it is convenient to specify a cell in line formula form or, as may be said, shorthand form. The Cu-Zn cell is thus presented as ... 

Substances are listed alphabetically by systematic name. The Notes column gives further information on the form of the substance and the basis to which the TLV is referred. This column also includes common synonyms and acronyms in brackets (e.g., [MTBE]). The Eormula column gives the molecular formula in the Hill convention for organic compounds and the customary line formula for inorganic compounds. The TWA and STEL limits appear in the last two columns. 

The following are two three-dimensional bond-line formulas for 2,3-dibromopentane. Notice that in drawing these formulas we have followed the conventions above. 

Tn structural formulae, metal-metal bonds are drawn for simplicity as a single line, but this need not imply a conventional single bond. This depiction is only used to indicate the presence of a connectivity between the two atoms, and does not necessarily denote a particular distribution of electrons in these zwitterionic compounds. 

In traditional two-dimensional structural formulas (A1), atoms are represented as letter symbols and electron pairs are shown as lines. Lines between two atomic symbols symbolize two bonding electrons (see p. 4), and all of the other lines represent free electron pairs, such as those that occur in 0 and N atoms. Free electrons are usually not represented explicitly (and this is the convention used in this book as well). Dashed or continuous circles or arcs are used to emphasize delocalized electrons. 

The absence from a formula of any one of the horizontal or vertical lines at a chiral or prochiral carbon atom (as in examples on pages 27 and 32), or of or Z designations at double bonds, indicates that the configuration of that stereoisomeric centre is not known. Also, as in our previous document [2], the convention of orienting polymer structures (and the corresponding constitutional and configurational units) from left to right is used. Thus, the two bracketted constitutional units in... 

If the bond valence b calculated from (13.4.1) and (13.4.2) for a cluster M matches the number of connecting lines drawn between pairs of adjacent atoms in a conventional valence bond structural formula, the cluster is termed electron-precise. ... 

Skeletal formula The representation of an organic compound s carbon-to-carbon bonds by lines. A single line represents a single bond with double and triple lines for double and triple bonds, respectively. The carbon-to-hydrogen bonds are assumed but not shown apart from the outline, but other functional groups or elements use their conventional representation. 

The stereogenic centers in sugars are often depicted following a different convention than is usually seen for other stereogenic centers. Instead of drawing a tetrahedron with two bonds in the plane, one in front of the plane, and one behind it, the tetrahedron is tipped so that horizontal bonds come forward (drawn on wedges) and vertical bonds go behind (on dashed lines). This structure is then abbreviated by a cross formula, also called a Fischer projection formula. In a Fischer projection formula ... 

Access to all this information is provided by the Index Chemicus Registry System (ICRS), which contains records of new compounds and associated data on magnetic tape (6). Structures are described by the Wiswesser Line Notation (7), a system for describing chemical formulas in terms of a linear groups of symbols. A recent development is the ICRS Substructure Index (G5), which enables manual searches to be made for new chemical information, and is based on the occurrence of fragments of structures, the most common of which are illustrated as conventional structural diagrams. 

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