NEW MODELS of OLD MOLECULES* Their Consfruction and Use in Chemical Education
THOMAS H. HAZLEHURST, JR.,
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HARVEY A. NEVILLE
Lehigh University, Bethlehem, Pennsylvania
Molecular models which reflect recent knowledge of atomic structure and provide a versatility not found in older models have been constructed. Each atom (except hydrogen) is represented by a cube with truncated corners, making a fourteen-sided figure with eight faces of equilateral triangles and six faces of squares. This permits the conctruction of models hawing
all the known types of symmetry in molecular configuration. Various uses of these models for instruction and study i n the sederal fields of chemistry are suggested, and it is indicated that a study of the models themselves may in certain cases aid i n resolwing questions of molecular structure and properties.
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HE chemical concept of atoms and molecules, originating in the mind of John Dalton, of necessity found its first expression in the symbols which he invented for these units of matter. The subsequent development of chemical theory was closely associated with, and to an extent dependent upon, the parallel development of graphical and visual representation of atoms and molecules. The various types of isomers and other feat~resof stereochemistry require more than empirical formulas, particularly for the purpose of instruction. Likewise the nature of the valence bonds and thespatial relations of atoms to each other in chemical combination are not clearly shown eveu by two-dimensional graphic formulas. Wherever it is desired to illustrate direction and extent in molecular structure three-dimensional models obviously possess great advantages. However, in view of the inertia of most minds toward new modes of thought, eveu the beginning student in chemistry will find molecular models giving a,cloak of reality to his first chemical formulas and thus satisfying a real need by supplying tangible evidence for+hypothetical objects. It is.an inherent defect in models of all kinds that they imply more than they should and fail to represent adequately all that is intended. A model "best seems the thing it is" and only second best the thing it is devised to portray. We must be prepared therefore to find in all models radical departures from the originals they represent. Certain features will be emphasized and reproduced with considerable fidelity; others will be obscured or even falsified. With regard to molecular models, it is essential that they should exhibit the valence characteristics of the individual atoms and the existence and peculiarities of the special groups, such as hydroxyl, carbonyl, and amino groups, which permit us to classify the molecule chemically. Every development of our knowled~e - of
valence bonds should be reflected in a corresponding development in the molecular models. The present authors offer for consideration a modification of the usual molecular models, endeavoring to take into account as far as is practicable the more recent advances in the field of subatomics. We are all familiar with the balls and rods by means of which our teachers of organic chemistry demonstrated the structure, synthesis, and decomposition of organic compounds. The new models differ from these in several important respects, so that, without sacrificingthe quality of demonstrating all that the old models could, they are able to portray additional important features of molecular structure. The chemical valence bond which may be represented adequately in models by a rod or wire is the non-polar bond. All valence bonds may be classed as polar, nonpolar, or of an intermediate type. The polar bond is an electrostatic hond between two oppositely charged fragments of the molecule. It .is characterized by a high degree of ionization, the non-polar bond by absence of ionization. In terms of the electronic theory of valence, a polar hond results from the actual transfer of an electron from the outer or valence shell of one atom to that of another atom; a na-polar bond is produced when a pair of electrons is shared equally between two atoms. The various intermediate conditions of polarity are due to the shared electron pair being attracted more strongly by one of the atoms (or groups) so that the pair of electrons which constitutes the bond is displaced in some degree toward that atom. Langmuir has defined a non-polar molecule as one in which the center of gravity of its electrons coincides with that of its protons. Such a molecule does not exhibit a dipole moment. If,however, an electron is transferred from one atom to another, or if a shared pair of electrons is displaced toward one of the atoms, the centers of gravity of electrons and protons do not coincide and a molecule having a defini'te degree of polarity results. I t is Presented before the Division of Chemical Education. probable that examples could be found to illustrate any Eighty.eighth Meeting of the American Chemical Society, degree of polarity from strong electrolytes with nearly Cleveland, Ohio. September 11, M34. 128
purely polar bonds (e. g., KCl) to inert organic molecules (e. g., the para&s). Most organic bonds and quite a number of bonds in inorganic compounds, particularly those in complex ions, may be safely classified as non-polar. It is now well established that the non-polar bond consists in almost every case of a pair of electrons shared by two neighboring atoms. In the familiar models the "bonds" are rods several times as long as the diameters of the "atoms" they join. This may be justifiable if we are to regard the ball as the nucleus of the atom, but not if, as is usually the case, it represents the nucleus plus the electron shells. Since the outer shells of the two joined atoms actually have a pair of electrons in common, it is evident that the atoms are in contact and that the rod or bond should be as short as is practicable.
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meant by the words. The atoms ordinarily present in organic compounds, namely, carbon, hydrogen, oxygen, nitrogen,andsulfur, haveapproximately the same radius, except hydrogen. The radii to be considered in constructing inorganic compounds containing the heavier atoms are somewhat larger, but for the purposes of the model the differences are unimportant. Hydrogen is the outstanding exception. The radius of a proton, whatever such a term may mean, is certainly vastly smaller than the radius of even a hydrogen atom. In fact it is probably this singular smallness and corresponding mobility which make the hydrogen ion so unique among chemical entities. The older models represent all atoms by balls of the same size. We emphasize the one important divergence from the general rule by using much smaller objects to represent protons. Even so, these objects are not scaled down as much as they should be to represent protons for two reasons: k s i , if they had be& so &shed, either they would not be visible or else we should require a derrick and scaffolding to assemble the other parts of the model; and second, although the hydrogen atoms in, for example, water may be considered as protons to a k t approximation, it is quite true that the electrons are not a l w y s between the H and the 0 atoms but spend some of their time, if only a small fraction of the total. in the oooosite o o s i t i o n a f a d which causes the H "atom" to ;tend somewhat farther out than a mere proton would. Finally, and most important, an effort has been made to rrprcscnt correctly the angles between valence bonds. The investigations of Linus Pauling and others havc established certain general rules about these angles, and these rules havc been fohowcd in constructing the present models. Older models naturally fail
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In a series of o a ~ e r s(1) . , Pauline showed that the application of the quantum mechanics to atoms in chemical combination leads, with a. few plausible assumptions, to verifiable conclusions about bond angles. A second and more important alteratidh of previous No attempt is made at present to assign to an electron models is suggested by recalling that the most recent in an atom a definite place at a definite instant. The investigations of atomic structure reveal that perfect most that can be said is that the electron spends a spherical symmetry is confined to the atoms of rare fraction x of its time near the point P. It is permissible gases and to the ions of similar electronic configuration. in this sense to speak of "electron density" along any Atoms which enter into combination with other atoms direction about the nucleus of an atom, meaning the through the medium of non-polar bonds are never of relative chance of finding the electron somewhere along spherical symmetry. The bond direction is inevitably that line. Pauling shows that the presence of another differentiated from other directions. The actual sym- atom capable of forming a chemical bond with the atom metry is determined by the electronic arrangement, and in question causes a mutual intensification of electron this factor also determines the valence number itself. density (already at a maximum in that direction) along Hence we choose blocks of approximately the appropri- the axis of the bond. A bond in such a compound has a ate symmetry instead of spheres to represent the indi- certain objective existence in the form of a region of strong electrical density extending from the nucleus or vidual atoms. A third difference lies in the relative sizes of the core of each atom toward that of the other. The atomic models. It is true that we have no exact analogy of this to the static electron-pair bond of G. N. knowledge of atomic diameters and that even the Lewis is striking. Pauling's conclusion is that the usual non-polar bonds significance of such a term is in doubt. Nevertheless approach one of two limiting cases, enclosing angles of we are accustomed to speaking of atomic, ionic, and molecular radii, and we all have some notion of what is 90' and 109.5' (the tetrahedral angle), respectively. It Y
FIGURE1
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AIR
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( a ) Methane and water-assembled and in parts. ( b ) Ethane, ethylene, and acetylene. (6) Octane in 1-3 zigzag. ( d ) Octane in 1-5 zigzag. (e) Ethyl alcohol, acetone, and acetic acid.
WATER
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(f) Cohalt-ammonia ion and sulfate ion. (g) Sodium stearate (soap). ( h ) Cellulose. (i) Orientation of toluene, benzene, and phenol at the air water interface.
is true that the presence of other groups distorts these angles, but for the purpose of representation with a static model they may be considered as preserved. Thus oxygen has its two valence bonds a t right angles; the three bonds of nitrogen extend along the three Cartesian axes; the four carbon bonds point to the apices of a regular tetrahedron; the six equivalent bonds in a complex ion (of coordination number six) point to the vertices of a regular octahedron and are consequently at right angles to one another. Hence, in order to produce a model with the maximum utility, each atom (with the exception of hydrogen) is represented by a cube with truncated comers, making a fourteen-sided figure with eight faces of equilateral triangles and six faces of squares as shown in Figure 1. This makes possible the construction of models having tetrahedral, cubic, square, and octahedral symmetqeach of which may be exemplified by typical compounds. The actual material used in constructmg the models is a matter of convenience. For hydrogen atoms, halfinch cork balls are used. The other atoms are cut from two-inch wooden cubes. Holes are bored one-half inch deep and one-fourth inch in diameter to accommodate the valence bonds which are made of short pieces of quarter-inch dowel rod. Double and triple bonds are represented by bent pieces of soldering wire. The different kinds of atoms are painted in contrasting colorsfor example, black for carbon, red for hydrogen, and light blue for oxygen. I t is suggested that these models may be useful adjuncts in teaching and research in the major branches of chemistry--organic, inorganic, physical, and colloidal. The organic chemist may use such models to portray the tetrahedral nature of the carbon atom; to demonstrate that "straight-chain" compounds are always crooked, being spiral in character if permitted to bend freely in three dimensions, or, if confined to a plane, being one or the other of two possible zigiags-one in which every third, and one in which every fifthcarbon atom is a repetition of the h t * ; to illustrate the nature of side chams and stereoisomerism; to show the structure of aromatic compoundst; to demonstrate the
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* The second type of zigzag can be carried t o completion only if an even number of carbon atoms is in the chain. This may or may not have some hearing upon the alternation of properties in many homologous series of hydrocarbons or their derivatives which gives rise to add and even sub-series. While it is doubtless true that, exceot ~ossiblvin the crystalline state, free rotation about each sinile bond permits a ~ o l e c u l et o assume not only either zigzag shape but also many intermediate spirals, yet the possibility of "rotational isomers" has been pointed out by K. I,. WOLF( 2 ) and it mav verv well be that some one confieuration is favored by the mol&ule Ff not exclusively present. t The shape of the benzene ring and the arrangement of the valence bonds seem t o constitute a perennial moot question. Using the KekuK formula with alternating single and double bonds, thew models lead inevitably t o a configuration in which all the C atoms lie in a plane and the bonds t o the exterior also lie in that plane. Indeed it is obvious that, if two carbon atoms are joined by a double bond, the four remaining honds lie in one plane, and that plane also contains the centers of the two atoms. The KekulO formula makes the benzene ring consist of three such pairs. ADAM(3) notes that the only available piece of experi-
optically active isomers associated with an asymmetric carbon atom. The inorganic chemist will find these models particularly applicable to the study of complex ions and their st&eoisomerism, and in interpreting the behavior of the ammonium ion which is the only simple ion having a positive outer shell (of protons). In physical chemistry the models may be used to construct unit cells of crystals and to study the angles between valence bonds-an important factor in the specific beats of gases and in dielectric constants. Colloid chemistry is primarily concemed with the behavior of molecules at the boundaries between any two phases. This behavior, involving adsorption, orientation, and resultmg phenomena, is conditioned by the structure and polarity of the molecules concemed. For example, molecules such as those of the soaps, sulfated fatty alcohols, and sulfonated oils, consisting of a long non-polar organic portion combined with an active polar group, are found to have the greatest d e c t in lowering the surface tension of water, the ability to stabilize foams or films and to a d as the most &dent emulsifying agents, wetting agents, and detergents. These "surface-active" molecules possess polarity in a limited portion of their structure, the remainder being practically free of this property. Such molecules are frequently referred to as polar-homopolar or polar-nonpolar molecules. The more polar regions of molecules are often known as "active" portions. They have a marked effect upon surface tension, causing it to rise considerably, as might be expected. Molecules orient themselves in the surface layer so as to have their active parts within the body of the liquid and their least adiv2 portions exposed. To be specific, the free surface energy (f.s.e., numerically equal to surface tension) is approximately the same from hexane to molten paraffin: hexane, 21; octane, 23; petroleum, 25. The f.s.e. of the alcohols, acids, aldehydes, and ketones of the paraffin series is practically the same as that of the hydrocarbons because the surface is still covered with CHs groups, the active OH or CO group being oriented toward the body of the liquid: CH80H, 23; C%HsOH,21.7; CHaCOOH, 23.5; (CHa)&O, 23.3. In benzene the ring lies flat upon the surface, thus keeping the active double bonds as near the interior as may be: f.s.e. 284. If an OH or other active group is substituted in benzene, this group is drawn inward and the ring is tilted on edge. Two ortho groups have no greater effect than a single group. -
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mental evidence points to a plane ring with suhstituents also in that plane. I t is found, of course, that the bonds in the benzene model must he flexible, because it is not feasible t o make a hexagon with six angles of 10g028', which is what the angles would he if the honds were all single, nor yet with six angles of 125' 16'. which is what they would be if the bonds were alternately single and double. The sum of the angles of a hexagon is 720'. while in the *st ca? the sum of the available angles is 656" 48' and in the second 751 6'. If the principle of the Baeyer strain theory (4) is correct. the second structure, which necessitates a distortion of 5' 19' per angle ismore stable than the first with its distortion of 10'38' per angle. A zigzag formula for benzene is possible using single honds, but for most purposes the KekulC formula seems preferable.
Two para groups cannot both go below the surface so the f.s.e. increases. Substitution of methyl groups in benzene lowers the f.s.e. unless the presence of an active group in the ortho position draws it beneath the surface: phenol, 40.2; nitrobenzene, 41.8; pnitrophenol, 55; toluene, 28.2; cymene, 28.2. Substances differing widely in polarity are usually only slightly miscible, e. g., oil and water. Such systems may become colloidally dispersed by the addition of an emulsifying agent, which is usually a long molecule the two ends of which differ widely in polarity so that one end is potentially miscible with the one constituent of the mixture and the other end potentially miscible with the other constituent. The specific example of soap occurs to mind a t once as a demonstration useful in explaining detergency . to beginners in chemistry. One most curious instance of the importance of polaritv mav be well ~ortravedwith these models. Consider two series of compounds: (1) water, methyl alcohol, dimethyl ether; (2) methane, ethane, propane. In these two series the molecular weights of corresponding members are nearly the same and the increments of molecular weights in the series exactly the same, but in the one case the boiling point rises with increasing molecular weight and in the other it falls, as indicated in the table. The apparent reason for the paradox is that in series (1) the increase in molecular weight is overbalanced by the great decrease in polarity (with its accompanying decrease in intermolecular attraction),
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while in (2) polarity is not a factor. The polarity is most evident with a model, which produces far more &ect upon the student, and, indeed, upon the instructor, than a written formula and a book of words. ,*Bra
M d . *I. 18 32 48 Ma. wt. 16
30 44
The value of "visual education" is so widely recognized as to require no discussion here. It is essential that the models used as educational aids should approach as closely as pmsible the ideal case of identity with the thing modeled. It is equally essential to avoid creating in the mind of the student the impression that the picture and the original are one and the same. The models here presented for consideration serve to emphasize in the minds of student and teacher alike certain important structural principles and properties of molecules. LITERATURE CITED
(1) PAULING, L., 3. Am. Ckem. Sac.. 53, 1367, 3225 (1931); 54, 988, 3750 (1932). (2) DEBYE, P., ''Stm~ctureof mdecules," Blackie & Sons, London, 1932. (3) ADAM,N. K.. "Physics and chemistry of surfaces," p. 97 ff Oldord, 1930. (4) v. BAEYER, J. F. W. A,. Be?., 18,2277 (1885).
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