Hill, formula index system

The Formula Index links the molecular formulae of chemical substances with their CA Index Names, CAS registry numbers, and CA abstract numbers. Molecular formulae are arranged according to the Hill system order (see Section 10.1). 

Although the Hill system is now used almost exclusively, other systems have been used in the past. For example, the early formula indexes to Beilstein used the Richter system, in which the elements are cited in the order C, H, O, N, Cl, Br, I, F, S, R... 

There are two systems of formula indexing in common use today, the Hill and the Richter systems. 

The Hill system is used in modified form today by Chemical Abstracts and Referativny Zhurnal, Khimiya (17) and indexes both inorganic and organic compounds. Beilsteins Handbuch (2) formula index to the second supplement uses the Hill system. This index covers the original work and two supplements. 

Volumes 28 and 29 of the Second Supplement are, respectively, the subject and formula indexes to the entire "Beilsteins Handbuch the three-part cumulative formula index, arranged by the Hill system, is the only index recording the known formulas of organic compounds from the beginnings of organic chemistry through 1929. (The Chemical Abstracts cumulative formula index extends this period to 1946.)... 

Each compound is characterized by its chemical name, net formula and Chemical Abstracts Service Registry Number (in square brackets). The data are ordered by the compound net formula in accordance to the Hill Indexing System [00-hil] (in each formula, carbon atoms are placed first, hydrogen atoms second, and further elements based on an alphabetical sequence). 

The Hill System has been modified for several classes of substances (the following substances on the left are not mentioned in the Formula Index, they are to be found under the substance names on the right. Sect. 2.5) ... 

According to the Hill System, the molecular formula of the above-mentioned substance is C2Cl3Na02 (Sect. 2.1.3.3). However, it is not recorded under this molecular formula in the Formula Index. 

The elements of an organic compound are listed in empirical formulas according to the Hill system [8] and the stoichiometry is indicated by index numbers. Hill positioned the carbon and the hydrogen atoms in the first and the second places, with heteroatoms following them in alphabetical order, e.g., C9H11NO2. However, it was recognized that different compounds could have the same empirical formula (see Section 2.8.2, on isomerism). Therefore, fine subdivisions of the empirical... 

The element symbols within each formula are arranged according to a modified Hill system in which the order is metal(s), C, H and then any remaining symbols arranged alphabetically. Compounds containing two (or more) different metals appear twice (or more), being listed under each metal. Common ligand abbreviations which appear in the text have also been used in the index. These include acac (acetylacetone), bipy (2,2 -bipyridyl), diars [o-phenylenebis(dimethyl-arsine)], dien (diethylenetriamine), etc. 

A single unstable compound of known composition is placed in the main first volume and is located on the basis of its empirical molecular formula expressed in the Hill system used by Chemical Abstracts (C and H if present, then all other element symbols alphabetically). The use of this indexing basis permits a compound to be located if its structure can be drawn, irrespective of whether a valid name is known for it. A representation of the structure of each compound is given on the third bold title line while the name of the compound appears as the first bold title line. References to the information source are given, followed by a statement of the observed hazard, with any relevant explanation. Cross-reference to similar compounds, often in a group entry, completes the entry. See Trifluoroacetyl nitrite p. 244. 

In the original Hill system (II) formulas are written in strict alphabetic order, omitting water of crystallization, except that with carbon compounds carbon is written first, followed immediately by hydrogen if present—i.e., trichloropropanol, C3H5CI3O. Each compound is arranged in the index by its own formula the sodium salt of propionic acid, C3H5 Na02, is entered as such. 

Last but not least, entries can be retrieved by searching gross formula in the Hill system, where the element symbols and their index numbers are separated by no blanks at all, such as CaC12 for CaCl2 or C1FH2 for CIFH2. For more complex searches, please use the exclusive Online Help of the Acrobat... 

---