STRUCTURE SYMBOLS AS AN AID IN TEACHING INGO W. D. HACKX, COLLEGE 01 PWSICMNS AD SURGEONS, SANFRANCISCO, CALI1ORNIA
Structure symbols1are an excellent means of conveying some of the principles of organic chemistry. They are a valuable tool in visualizing isomerism, asymmetric atoms, electronic shifts, and the intricacies of terminology. My teaching experience has fully convinced me of the decided advantages of this notation and the facility with which the beginner can make use of it. The advantages of the structure symbolsZare summarized: exactness (for each symbol represents only one definite compound), simplicity (for with few rules the thousand of symbols can be constructed and their meaning understood), accuracy (for it is impossible to write a symbol for a given formula which is not theoretically correct, provided the rules are adhered to), com@ctness (for the symbol takes often less space than the name of the compound), and cleurness (for the designs of the symbols are distinct, are grasped in a single glance). The structure symbols themselves involve no new theory, they are merely the valence skeletons obtained when the well-known structural formulas are written minus the symbols for C, H, 0, and N. They adapt themselves with equal facility to the language of "bonds" or "nonpolar electron pairs," and the key to the entire system is the assumption that: hydrogen has a valency of one or shares one pair of electrons, oxygen bas a valency of two or shares two pairs of electrons, trivalent nitrogen shares three pairs of electrons, carbon has a valency of four and shares four pairs of electrons. In the structure sypbols a line indicates a valence T Structural Formula
n-H
Structure Symbols
-
-
->N-H
I I
I I
n
H>O
n
H-6-H I
>
' Science, 48,333 (1918); Chem. Age (London),3,392 (1920). 'The term "chemical shorthand" is not appropriate, for shorthand implies abbre-
viation, whereas structure symbols are more explicit than rational formulas snd on the same level as structural formulas. 836
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bond or a pair of electrons which is shared in common, and the four elements named are indicated, respectively, by points from which one, two, three, andfour lines radiate. The points are determined by the termination of a line or the junction of lines as the comparison between structural formula, electronic formula, and structure symbol on page 836 shows. All other elements, as well as tetravalent oxygen or bivalent carbon are indicated by their symbols in the proper place, thus > S for hydrogen sulfide, > P for phosphine, etc. Asymmetric atoms are indicated by a dot a t the junction of a line. Ring compounds are drawn as geometrical figures, thus a pentacyclic compound as a pentagon, etc. The lines between atoms should be of equal length; straight lines for single bonds, curved lines for double bonds. These are all the rules necessary for the complete mastery of the system, and the writer has never encountered a student who did not understand this notation after the first lecture period. The following examples may illustrate the adaptability for classroom exercises. The hydrocarbons are generally the first group of compounds which the beginner in organic chemistry encounters. The structure symbols for the first five normal hydrocarbons are,
METE-ETH- PRO- BUTANE PENTANE ANE
ANE
PANE
These signs represent, conventionally, the linkage of the carbon atom on a plane surface, but as molecules have three dimensions in space, this E straight line is rather curved and resembles a screw. Structure symbols are therefore not intended to give an actual picture of the atomic positions within the molecule, but are rather a schematic diagram of the inter-atomic linkage. With this warning to the beginner we proceed to show the isomers;
for instance for hexane: 1
2
3
4
5
None of these compounds contains an asymmetric carbon atom, hence none of them is optically active. To test for an asymmetric atom we imagine that we are standing a t the point where the atom is located and look now in succession to the north, east, south, and west: if we see in each direction a different arrangement of atoms, the carbon atom is asymmetric; if, however, in two or more directions there is the same atom or group of atoms no asymmetty exists. Applying this test to the above examples we find in No. 2 that the second carbon has two methyl groups; the third carbon of No. 3 has two groups; the second carbon of No. 4 has three methyl
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APRIL,1930
groups, and in No. 5 the second and thud carbon atoms have each two methyl groups; thus none of these compounds exhibits optical activity. The same structure symbols shall now be used to'illustrate and practice terminology. Accordingly we shall try to find as many names for the compounds represented by these symbols. We shift our compound of reference from hexane, to pentane, to butane, to propane, to ethane and finally to methane, and prepare a list something similar to the following: 1. Hexane, 1-methyl-pentane, 1,4-dimethyl-butane, 1-methyl-3-ethylpropane. 2. 2-methyl-pentane, 1,l-dimethyl-butane, isopropyl-propane, 1methyl-1-propyl-ethane, dimethyl-propyl-methane. 3. 3-methyl-pentane, 1,2-dimethyl-butane, 1-methyl-1-ethyl-propane, l,l-diethyl-ethane, diethyl-methyl-methane. 4. 2,2-dimethyl-butane, 2-ethyl-2-methyl-propane, 1,l-dimethyl-lethyl-ethane, trimethyl-ethyl-methane. 5. 2,3-dimethyl-butane, 1,1,2-trimethyl-propane,1,1,2,2-tetra-methylethane, dimethyl-isopropyl-methane. These names do not exhaust all the possible ones, hut they are suggestive of the system of terminology. Reconstructing the structure symbol from these names is simple; thus 1,1,3-trimethyl-propane requires first the writing of the structure symbol for propane:
*
*
and, second, adding the
PROPANE
three methyl groups to the first and third carbon atom, respectively; thus:
Remembering that there may be free rotation of the single bond, all these symbols are identical with No. 2 above. Similarly the aromatic hydrocarbons are readily visualized; for instance, the isomers of CsHlz are:
and the corresponding names:
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Propyl-benzene. Isopropyl-benzene, cumene. l-ethyl-2-methyl-benzene,2-ethyl-toluene. l-ethyl-3-methyl-benzene, 3-ethyl-toluene. l-ethyl-4-methyl-benzene, 4-ethyl-toluene. 1,2,3-trimethyl-benzene,-hemimellitene. 1,2,4-trimethyl-benzene, pseudocumene. 1,3,5-trimethyl-benzene, mesitylene.
Substituting the hydroxyl group for a hydrogen atom, that is considering the alcohols or phenols derived from these hydrocarbons, greatly increases the number of possible isomers, as is shown by the following structure symbols for C~HIJOH:
Of these compounds, 15, 16, 17, 21, and 22 contain each one asymmetric carbon atom, and 20 contains two asymmetric carbon atoms. Hence there are the corresponding dextro- and levo-forms which are conveniently and conventionally written by reversing the symbol. The primary alcohols are those containing the -CHzOH group; as, 14, 16, 17, 18, 22, 23, and 24. The secondary alcohols are 15, 20, and 21, while 19 is a tertiary alcohol. Using a circle for any hydrocarbon radical, there are the charac-
teristic symbols
+&-%
PRIMARY SECONDARY TERTIARY ALCOHOLS
and tertiary alcohol, and likewise
for primary, secondary,
for priPRIMARY SECONDARY TERTIARY AMINES
mary, secondary, and tertiary amines. These general structure symbols are extremely useful in recognizing rapidly the type of a compound and establishing a mental picture of their relationship. Thus we have
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Am&, 1930
R.OH R.CHO RCOOH R.0.R RJ2O.R RCO0.R ALCO-ALDE- ACIDS ETHERSKETONES ESTERS HOLS
HYDES
With little practice these symbols will fix themselves upon the mind and enable a student to recognize them in the stntcture of a more complex compound. During ten years of experience there was found no structural formula which could not be represented by a structure symbol. They have been found to be useful in showing the relationship among the terpenes and alkaloids3 as well as other large molecules; e. g., dyes and peptides. A systematic list of some radicals and c o m p n d s has also been published4 and shows the facility with which the principle of bold-faced type can be utilized by making the lines heavier. So far the orthodox treatment of organic chemistry has been considered. The future sees the application of electronic concepts to the organic molecule. Various attempts have and are made to picture, to some extent, the distribution of the electrons which constitute a non-polar bond. Thus in different notations this non-polar bond is: A-B, A-o-B, or A-:-B for those cases where the electron pair is half-way between A and B, and A+-B, A-o-B, or A-:-B, for those cases where the electron pair is displaced more toward B, that is, where A has become positive and B negative in relation t o one anoiher. In the Lewis symbols, pictured a t the beginning of this article, all of the valence electrons are shown, but no account is taken of the relative distance of the electrons in a nonpolar bond. The "electronic" structure symbols enable a combination of the advantages of both types of notation, thus: dots upon a line are the valence electrons of the non-polar bonds, that is, the electron pair which is shared in common, each atom contributing one electron. Dots outside the lines are the valence electrons which are not shared but which complete the octet. The relative position of the dots on the lines is an indication of the "positiveness" or "negativeness" of the atom, the closer the more negative. The typical non-polar bond is in 25, where the electrons are assumed to he half-way between carbon-carbon and carbon-hydrogen. In 26 the carbon has become slightly positive with respect to nitrogen. In 27 still more positive and in 28 most positive. As a result the hydrogen atoms have hecome successively more negative, due to the nearness of the electron pairs. "Some Constituents of Essential Oils and Their Structural Relations," I. Am. Phornz. Assoc., 9, 948 (Oct., 1920). "Structural Relations among Opium, Berberis, Corydalis and Hydrastis Alkaloids," Ibid., 10, 680 (Sept., 1921). 4 Chcm. Nms, 136, 17 (Jan. 13, 1928); also in Qulmicrr e Induslr&z, 6 , 22 (Feb., J
1929).
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In 28 are distinctly negative hydrogen atoms, which means the hydrogel1 atoms can be readily replaced by negative groups; as, halogens.
In the case of methanol, No. 27, it 'is noticed that there are two kinds of hydrogens, the methyl-hydrogen which is relatively negative; replaceable by negative atoms or groups, and the hydroxyl-hydrogen which is relatively positive; replaceable by positive atoms or groups. In asimilar way the electron shifts of Lncas theory and the Pauling structure of benzene can be visualized. So far the structure symbols have stood well the test of time and they promise some useful applications in the future. As a help to the beginner and as a tool to the student, they are recommended for extensive use.
Sex Hormone Isolated by Gennan Chemist. The i m ~ o r t a nsex t hormone has iust been obtained in pure form for the first time by a German scientist, Dr. M. Butenandt, working a t the laboratory of a recent Nobel Prize winner. Professor Adolf Windaus. a t the University of GBttingen. This hormone, which has been known to the medicel profession for some years, is thought to he capable of restoring the functioning of the reproductive organs. Heretofore it has been obtained only in combination with other compounds, hut Dr. Butenandt has been able to produce the hormone itself in pure form, as a aystalline substance which he has named progynon. The sexual hormone is one of a number of curious and as yet little understood substances which are secreted by the ductless glands of the human body. Each of these special chemicals is responsible for the proper functioning of certain bodily activities, and physiological chemists believe that a systematic study of these secretions will lead not only t o an understanding of the physical operations of the body, hut even to an explanation of mental characteristics and that elusive property called "character." The importance of obtaining a hormone in a Dure state is that it is the first s t e ~ toward the determination of its structure and its synthetic production in the laboratory. The action of the hormones within the body and their influence on other chemical orocesses of the body may be bettcr understood after the structure of the hormones has been determined. Progynon belongs chemically to the group of stearates, or fats, and it is related to the artificial vitamin, vigantol, discovered by Professor Windaus. This also puts it in the same class as the poison of toads and the bile acids. On the other hand, adrenalin, the hormone of the suprarenal glands, is related chemically to the plant drugs known as the alkaloids, of which morphine is a well-known example. Adrenalin was the first hormone isolated in a pure state, and it has since been produced synthetically.-Science Service
