Section 9.5.3.2. Definition of `Substructure'

The chemical environment of each bond is normally defined by a linear formulation of the substructure. The target bond is set in bold type, e.g. C_ar—C≡N (aryl cyanides); C—CH₂—O—C_ar (primary alkyl aryl ethers); (C—O)₂ —P(O)₂ (phosphate diesters). Occasionally, the chemical name of a functional group or ring system is used to define bond environment, e.g. in naphthalene, C2—C3; in imidazole, N1—C2. To avoid any possible ambiguity in these cases, we include numbered chemical diagrams in Fig. 9.5.3.2. A combination of chemical name and linear formulation is often employed to increase the precision of the definition, e.g. NH₂—C=O in acyclic amides; C=C—C(=O)—C=O in benzoquinone. Finally, for very simple ions, the accepted conventional representation is deemed to be sufficient, e.g. in , , etc.

Figure 9.5.3.2| top | pdf | Alphabetized index of ring systems referred to in the table; the numbering scheme used in assembling the bond-length data is given where necessary.

The chemical definition of substructure may be followed by brief qualifying information, concerning substitution, conformational restrictions, etc. For example: Csp³—Csp³: in cyclobutane (any substituent); X—C—F₃ (X = C, H, N, O); C_ar—NH—Csp³ (Nsp³: pyramidal). Where the generic symbol X is unqualified, it denotes any element type, including hydrogen. If the qualifying information is too extensive, then it will be given as a table footnote (see below).

The `Substructure' column is designed to convey as much unambiguous information as possible within a small space. For Csp³, we have employed the short forms C* and C^#. C* indicates Csp³ whose bonds, additional to those specified in the linear formulation, are to C or H atoms only. C*—OH would then represent the group of alcohols CH₃—OH, —C—CH₂—OH, —C₂—CH—OH and —C₃—C—OH. C* is frequently used to restrict the secondary environment of a given bond to avoid the perturbing influence of, e.g., electronegative substituents. The symbol C^# is merely a space-saving device to indicate any Csp³ atom and includes C* as a subset.