Searching the Literature of Organic Compounds G. F. HEMYION University of Notre Dame, Notre Dame, Indiana INTRODUCTION
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0 DOUBT one of the most disturbing problems to confront the advanced student and the beginning investigator in organic chemistry is the search of the vast accumulated literature of the subject for particular information. As he was gradually introduced to the endless variations in which carbon combines with hydrogen and other elements, he was quickly impressed with the conviction that his texts and manuals present but a very small fraction of the total knowledge. It is therefore a matter of great concern to know exactly how to conduct thorough and reliable literature searches. Fortunately indeed this is not a haphazard process. Perhaps no other field of science is so well systematized and so abundantly provided with guides to original sources of information. I t is the purpose of this discussion to identify these with brief directions for their use. Let us first presume that i t is desired to review the literature of a particular compound, since this is the simplest problem presented to an investigator. We will assume too that the compound is fairly well known so that we may justify covering the literature in chronological sequence. This then is the recommended order at search. 1. Richter's Lezikon dn Kohlensloffverbindungen. 2. Stelzner's Literatur-Regisfer d n orgonixhen Chemic. 3. Formula indexes of the Chemisches Zalralblatt, beginning with the year 1922 in the sixth collective index, or with 1925 in the seoarate volumes. 4. Formula index& of Chemical Abdracfs, beginning with 1920 in each volume. 5. The fourth edition of Beilstein's "Handbuch der organischen Chemie." 6. The subject and author indexes of Chemical Abstrads, and perhaps also the Zentralbkztt. The coilective indexes are preferred for the periods covered. The author indexes are useful for following the work of an investigator over long periods.*
This order presumes that a thorough survey is indicated. Admittedly there is considerable overlapping. Very frequently either step 3 or 4 may be omitted entirely. Some workers generally ignore the first three sources and examine only the last three for the reason that Beilstein covers the entire literature through the year 1019. Others prefer to work in reverse of the chronological order on the presumption that the most recent information is the most reliable and will generally indicate gross discrepancies with older work. From this point of view the order should be Chemicul Abstrads, formula and subject indexes, Zentralblatt, thence to Beilstein, etc.
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'Occasionally valuable use may he made of Poggendorfl's "Bi-phisch-Litera~isches Handwijrterbuch," which gives a complete bibliography of each chemist included. 33
It should be emphasized that the above sources are chiefly literature while they furnish considerable information of immediate utility, if our prime purpose is to collect original references our study is not complete until we have referred to the primary sources. Since the literature of most organic compounds may be covered practically to date by means of formula indexes, i t is well to know a t this stage how these are arranged. Two methods are employed: the Richter system in the German works and the Hill system in our own Chemical Abstracts. In the former method compounds are tabulated according to their molecular formulas beginning with those of one carbon atom. The elements in combination with carbon are given in the order H, 0 , N, C1, Br, I, F, S, P. Any others are in alphabetical sequence following these. At the top of each index page there appears a "guide number" of which the Arabic numeral refers to the carbon content and the Roman numeral to the number of other elements for the compounds on that page. For example, the notation "5 111" shows that compounds of five carbon atoms and three other elements follow. To find a compound indexed this way, one should write its molecular formula with the elements placed in the above order. and determine the euide number. The com~oundis then located auicklv amona its isomers, if &ch are reported. In &e ~ i system, i which provides also forinorganic compounds,the entries ar; by molecular formulas, alphabetical by symbols, eztc9t that C and H, if present, are cited first. While there are no guide numbers, the organic compounds are grouped according to carbon content as in Richter. Further information may be had from the 1942 index of chemical Abstracts, page 8443. Before proceeding further i t is necessary to become acquainted with the works mentioned.
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RICHTER
Richter's "Lexikon" (third edition) lists in four volumes all the organic compounds known up to the time of publication (1910), in the Richter arrangement, beginning with C in volume I. The melting point or boiling point, a few references to the original literature, and the reference to the third edition' of Beilstein are given. The four volumes are classified as follows: volume I, C, to Co (1 I to 9 111); volume 11, Cpto C13 (9 111to 13 IV); volume 111, CD to CZ0(13 IV to 20 11); volume IV, C20 and higher (20 I1 to 1039 IV). t The searcher likely will not use this edition of Beilstein. However, it may be well to know that the third edition was published in four main volumes and four corresponding suppiementary volumes, along with a fifth supplement which is an index to the whole work. The arrangement is best learned by examining the tables of contents of the main volumes and from the discussion of the arrangement in the fifth supplementaryvolume
STELZNER
The period 1910-21, inclusive, is similarly covered by the "Literatur-Register," published in five volumes. The system of classification is much the same as in Richter, except that each volume covers a period of time rather than a group of compounds. The references are far more adequate than in Richter. The periods covered are: 1910-11, volume I ; 1912-13, volume 11; 1914-15, volume 111; 1916-18, volume IV; 1919-21, volume V. Chemisches Zentralblatt Chemisches Zentralblatt first appeared in 1830 and is the oldest of our abstract journals. There have been a number of minor changes in the title. Also the number of volumes per year has changed frequently: one per year through 1888, two per year from 1889-1918, four each year from 1919-23 inclusive, and two volumes per year since. Five-year collective indexes are now published. An interesting feature of Chem. Zentr. is the continuation of the Richter system for indexing abstracts, begun in the sixth collective index (1922-24, inclusive). This method is continued in the subsequent collective indexes, each covering five years. Beginning with 1925 each volume contains its own Richter type formula index. Therefore, where the collective indexes are not available, there is a three-year gap in this arrangement. Chemical Abstracts This "key to the world's chemical literature" first appeared in 1907 and a t present is unquestionably the most important chemical abstract journal in the world. While the subject and'author indexes are continuous since 1907, a formula index was added in 1920, and a patent index in 193.5. The formula index lists all new compounds and all those for which new data are given. Inorganic compounds are included. The three decennial indexes to Chemical Abstracts (1907-16, 1917-26, and 1927-36) are, of course, important when making subject and author searches, but unfortunately these do not contain formula tabulations. Among the various special features of Chemical Abstracts, i t is important to note that the 1936 index lists 2808 periodicals abstracted with proper abbreviation, publisher, price, location in this country, etc. The 1942 index includes a supplement of 1132 periodicals. BEIISTEIN
The "Handbuch," frequently called the "organ'ic chemist's bible" is by far the most important work devoted to organic chemistry. The current fourth edition available a t this time embraces 59 volumes in 49 covers for a total of about 38,000 pages. Though still incomplete, the published portion is the most essential part. The so-called "main series" is in 29 volumes, of which the first 27 cover organic compounds of known structure through the year 1909. There are 27 supplemen-
tary volumes (in 15 covers), each for the period 191019, inclusive. There is also a second supplement to volume I (1920-29, inclusive). Beilstein now may be used very easily by taking advantage of the two index volumes (in 4 covers). Volume XXVIII (2 covers) is an alphabetic compound index; volume XXIX (2 covers) is a formula index arranged in the Richter system. Since compounds may be located readily in this manner, it is not necessary to understand the elaborate system of classification and arrangement employed.f Although considerable experience is required to appreciate it fully, the following may be helpful. According to this scheme the sum total of all organic compounds is divided into four major groups or series: (1) carbouchain compounds (acyclic series); (2) carbon rings (isocyclic series); (3) compounds containing rings in which elements other than carbon appear (heterocyclic series); (4) natural compounds of unknown or partially known constitution. Of these, each of the first three major groups is subdivided into 28 principal classes. Some of the more common ones are as follows: 1. 2. 3. 4. 5.
Stem nuclei Hydroxy compounds Keto compounds Carhorylic acids Sulfinic acids 6. Sulfonic acids 7. Seleninic and selenonic acids 8. Amines 9. Hydroxylamines 10. Hydrazines 11. Am compounds 16-22. Compounds with three or four nitrogen atoms 23. Compounds with phosphorus, arsenic, bismuth or antimony attached to carbon 24. Compounds with silicon, germanium, tin, or lead attached to carbon.
Before considering the exact location of a compound in Beilstein it is necessary to understand the following. (a) Stem nuclei are the hydrocarbons and the various heterocyclic rings. (b) A substituenl is an inorganic group containing no hydrogen (F, C1, Br, I, NO, NOz, N3). Substitution products are formed by replacing hydrogen bound to carbon with one or more of these. (c) A fundional group is an inorganic radical containing replaceable hydrogen. Such are -OH, -NHe, S O z H , S O s H , -NHOH, -NH-NH2, -NzOH, -0OH of carboxylic acids and =O of aldehydes and ketones (considered as dehydrated diol). (d) Functional derivatioes are compounds formed by the interaction of a functional group with an organic hydroxy compound or with an inorganic hydrogen compound so as to eliminate water ("anhydro-synthesis"). (e) Mother substances or indexed compounds are defined as stem nuclei and those containing functional groups only. All other compounds are derived from f The special arrangement of compounds in Beilstein is explained s t the front of Volume I. Main Series, pp. 1-40,
these--by substitution or reaction of functional groups. Louting a compound in Beilstein thus becomes a matter of determining the mother substance. For this purpose the following rules must be applied to each case. 1. Replace all substituents with hydrogen. Find ethyl bromide under ethane, bromoacetic acid under acetic acid, chloropyridines under pyridine, etc. 2. Hydrolyze functional derivatives. Find methane sulfone chloride under methane sulfonic acid, diphenyl ether under phenol. 3. When hydrolysis of a functional derivative yields two or more functional compounds, the mother substance is that one which has the latest position in the system. Find ethyl acetate under acetic acid (caxboxylic acids occur later than alcohols); phenyl acetate is under phenol (carbocylic compounds follow the acyclic~). 4. The imaginary hydrolysis must be such that the --OH attaches to the coupling compound and not to the indexed compound. Thus monomethyl aniline is converted to aniline and methyl alcohol, not to phenylhydroxylamine and methane. 5 . Sulfur when not part of a snlfinic or sulfonic acid group is replaced by oxygen. For example, diphenyl sulfide is sought under phenol. Thiophene is in volume XVII, following furan by only two pages. 6. Salts are located under the acids or bases to which they are most closely related. 7. When several functions, or a functional group and a substituent, axe attached to the same carbon atom, replace with hydroxyl groups. Thus acetyl chloride is sought under acetic acid because the chlorine is now to be replaced by OH rather than by H. 8. Compounds containing nitrogen, sulfur, and iodine in their higher states of valence must be regarded as functional derivatives. For example, tetramethyl ammonium hydroxide is located under methyl amine, obtained by hydrolyzing off three methyl groups according to rule 4. The application of these rules determines immediately the mother substance for the compound sought and thus the correct Beilstein volume. One may now consult the index to that volume or he may locate the mother substance itself from the table of contents. The mother substance is followed by the functional derivatives permitted under rule 3 and by the substitution products. The latter in turn are followed by their functional derivatives. For example, acetic acid is found in volume 11, page 96, ethyl acetate on page 125, and chloroacetic acld and ethyl chloroacetate on pages 194 and 197, respectively. Having obtained the desired information in the main series, the examination should be continued in the supplementary volumes. In using these i t is well to be guided by the bold-face numbers given a t the top center of the pages of the supplements. These numbers refer to the corresponding pages of the main series. For example VI-200 identifies the material of that page as corresponding to the subjects discussed on page 200, volume VI, of the main series.
Let us consider now a more difficult literature survey, namely, one concerned with a class of compouuds or with a type of reaction. The formula indexes enumerated above will be useful only in so far as we are able to reduce our problem to specific examples. Under any arcurnstances a good assortment of these might well be followed through the formula indexes. However, considerable reliance will have to be placed on subject indexes, particularly to the abstract journals. The immediate problem is to arrange a broad list of subject index entries. These generally fall into some six rather definite categories as follows. All suitable names for each substance involved. The family name or class to which the compounds belong. e. g., hydrocarbons, oldins, antimalarials, fuels, etc. Particular features of the compounds involved as triple bands, resonance, etc.. as may apply. 4. Principal derivatives. 5. Reaeents leadine " to the comomnds concerned. 6. Named reactions and procsscs which may be involved IS addition, hydrolyric, chlorination, pyrolysis, ctc.
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When a proper assortment of entries has been determined, they should be put in alphabetical order and each index used should be examined systematically for them. As the library study progresses, need may be found for abstract journals other than the two already mentioned, not only for subject and author searches, but also because certain original sources are sometimes not available and recourse must be had to abstracts. Among the more important sources the following should be remembered: Journal of the Chemical Society, 18711925, Journal of the Society of Chemical Industry, 18821925, British Chemical Abstracts, 1926-, Bulletin de la Sociite chimique de France, 1863-, and the Berichte der deutschen chemischen Gesellsckaft (Referate), 1880-95. Quite obviously no literature search need be continued after finding the desired information. On the other hand, complete and systematic library study, while tedious and time consuming, is an absolute necessity before undertaking extensive laboratory work. Many an ardent young investigator has labored long and diligently only to learn subsequently that his "discoveries" were long before published for all to read. BIBLIOGRAPHY
(1) Hm~mss,"A Brief Introduction to the Use of Beilstein's Handbuch der oreanischen Chemie." . Tahn Wilev and Sons. New York, 1938." (2) SonE, "Library Guide for the Chemist,'' McGraw-Hill Book Company, Inc.. New York, 1938, cspcchlly pp. 12361. (3) REID,"Iutrodu~tionto Organic Research," D. Van Nostrand *Company, Inc., New York, 1924, especially pp. 60-
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C-E AND PATTERSON, "A Guide to the Literature of Chrmistry;' John Wiley and Sons, New York, 1927. MCARDLE,"Classif;ing organic compounds by the PragerJacohsen system, C k m . Met. Eng., 22,256 (1920). HIBBERT," k t of searching chemical literature," ibid.. 20, 578 (1919). SMITH,"Searches in chemical literature," J. CHEM.Eouc.. 4, 1522 (1927). "Symposium on technical library techniques," a collection of 16 articles, ibid., 20, 531-53, 58(t608 (1943).
