X COXCEPTIOX OF POLARITY DERIVED FROM PHYSICAL MEASUREMESTS AXD I T S RELATIOKS TO T H E ELECTRONIC CONFXG~RATIOKOF AROMATIC ORGANIC COMPOUNDS BP J. F. T. BERLINER
Introduction and Discussion In a recent series of studies on the vapor pressures of a number of highly purified organic compounds several reletions were observed that appear to inerit further consideration. The compounds studied were the isomeric nitroanilines, mononitrotoluenes, toluidines and naphthols. I n this discussion the latter group will be omitted. Many determinations, direct and indirect, have been made of the heats of vaporization and to the present time but two generalizations have been evolved that can be related to chemical phenomena. The first is, that in general the latent heat per gram is less for substances of higher molecular weight, and the second that a relation exists between the entropy and the latent heat of vaporization that involves a consideration of the association of molecules. As the kinetic energy associated with each molecule at the same temperature is the same, it follows that, as there are less molecules in one gram of substance of high molecular weight, there is also less total kinetic energy 1he latent heat is, therefore, also less. The latent heats of vaporization are not simply inversely proportional to the molecular weight; several factors are involved, the principal ones being the mean square velocity of the molecules escaping from the surface of the liquid and the amount of heat necessary to overcome the intramolecular attraction. The velocity may be calculated from a consideration of the assumption that the ratio of the density of the vapor to the density of the liquid is the ratio of the molecules with sufficient speed to escape from the surface of the liquid to the number with insufficient speed. This assumption is justified in that many physical facts may be explained through it and it does not seem to be opposed to any of our present conceptions of vaporization, or its colligative properties. This idea is also applicable to solutions: Thus the lowering of the vapor pressure of a solvent, by the introduction of a non-volatile solute, is explained by considering that the number of molecules with sufficient speed to escape is thereby reduced relatively to the number with insufficient speed. On account of their electrical Berliner and May: Studies in Vapor Pressure. I. The Nitroanilines. J. Am. Chem. SOC., 47,2350 (1925). Berliner and hlay: Studies in Vapor Pressure. 11. The Moninitrotoluenes, J. .\m. Chem. Soc., 48, 2630 (1026). Berliner and May: Studies in Vapor Pressure. 111. The Toluidines, J. .am, Chem. Soc., 49, 1007 (1927). Mav and Berliner and Lynch: Studies in Vapor Pressure. IV. The Saphthols, J-.Am. Chem. SOC., 49, 1012 (1927).
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J. F. T. BERLINER
nature, ions have to overcome a greater cohesive force in order to escape and consequently need a much greater kinetic energy than molecules. They are, therefore, largely non-volatile. I n a similar way the forces of attraction between water molecules and other non-volatile molecules is probably greater than that between the water molecules themselves. Recently MacLeod6 has published a comprehensive treatise on this phase of the subject and has pointed to a possible analogy between the mechanism of evaporation based on differential surface velocities of molecules and osmotic pressure phenomena. (See p. j 4 2 ) . The nitroanilines and the nitrotoluenes have however almost the identical molecular weights, 138.06 and 137.06, respectively. From this Consideration the same heat of vaporization for each compound might be expected. However, it was observed1,*that the heals of vaporization of the isomers of the nitroanilines differ widely, and those of the nitrotoluenes, besides being very much lower than the heats of vaporization of the nitroanilines, also differ from each other to a small extent. As was indicated in the papers referred to above the nitroanilines appear to form highly associated molecules, while the nitrotoluenes are practically non-polar substances. When the heat of vaporization of an associated substance is arrived a t through vapor pressure determinations, a factor is involved that may be called the heat of “deassociation,” that is, the amount of heat energy necessary to decompose an associated molecule into normal non-associated molecules. This heat may be quite high and in the case of para nitroaniline is about 6600 calories (value arrived at from consideration of observed and calculated theoretical heat of vaporization) per mole. Therefore the abnormally high heats of vaporization of the nitroanilines may be readily explained and it is evident that one of the fundamental properties that must be taken into cognizance, in arriving at any conclusions from the experimental and derived data, is that of polarity or association. The heats of vaporization, while in themselves important, are dependent on the degree of molecular association and can not be corrertly interpreted unless the extent of the latter condition is known. The relations between the entropy of a liquid and its latent heat of vaponzation were soon recognized; though at first the function now denoted as entropy was not recognized. In 1884 Trouton6 announced the rule that is now known by his name-that the quotient from the heat of vaporization per mole divided by the absolute boiling point, is approximately the same for all substances. This law has, in fact, been until fairly recently assumed to be at least approximately correct. The value of the quotient is about 2 0 to 2 2 . The rule as it would now be stated is, that the entropy increase per mole is the same for liquids at their boiling point. It may be said that the validity of this rule would follow from the work of Guldberg? who pointed out that the boiling point on the absolute scale is nearly always about two thirds the absoMacLeod: Trans. Faraday SOC.,20, j z 5-543 (1925 ) . Trouton: Phil. Mag., ( j ) 18, 54 (1884). ‘Gddberg, Z. phyPik. Chem., 5, 374 (1890). 0
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lute critical temperature. A more rigid examination of Trouton’s rule, shows, however, that the ratio is by no means constant, but increases regularly with the temperature, as was pointed out by Nernst* and also by Bingham.9 Trouton’s rule holds very well for non-polar liquids boiling in the neighborhood of 100’to 3 0 0 O C . It has been derived theoretically from the theorem of corresponding states by 1terson;’Ohowever, according to Bingham, it must hold more closely than this theorem. For polar or associated liquids, such as water or alcohol, the Trouton ratio is larger. This is explained, as was previously indicated, by assuming that when a liquid is associated, and its vapor not associated, a certain amount of heat is required to dissociate the molecules of the liquid, hence the normal heat of vaporization is increased. The effect on the boiling point is doubtless not very large, so that when abnormally high values of the ratio are obtained the evidence of dissociation seems very satisfactory.. The quotient from the heat of vaporization divided by the absolute boiling point represents the increase in entropy of the substance during vaporization. A rule essentially the same ~ E Trouton’s J has been given by LeChatelier and Forcrand“ wherein the entropy constant of certain solid compounds at a pressure of one atmosphere is approximately 33 calories per degree. Various modifications have been suggested for Trouton’s rule. Two of these have been proposed by Nernst.’* It is difficult to determine from what Nernst says, whether they have any other than an empirical foundation. They seem chiefly intended to take into account the low-boiling gases. Another formula has been proposed by Bingham13 which is also empirical and has been constructed without much reference to liquid boiling at very high or very low temperatures. More recently F ~ r c r a n d has ’ ~ published a formula, also empirical, but which has attempted to include liquids with very high, as well as those with very low boiling points. The course of this equation at high temperatures was determined by using data for the boiling points of silver and copper. However, Forcrand’s formula is not considered very significant. The most important generalization has been given by Hildebrand,’j derived from the Clapeyron-Clausius equation of state. The ClapeyronClausius equation may be put in the form d log P - J d logT RT Xow IJT is the entropy, so that if log p be plotted against log T, the tangent to the curve at any point is equal to the entropy divided by R, the gas constant. By plotting a number of curves, it was found that the points, where Werust: Gott. Nachr. Heft I , 1906. 9Bingham: J. rim. Chem. Soc., 28, 717 (1906). ‘OIterson: 2. physik. Chem., 53, 633 (1905). ‘I Le Chatelier and Forcrand: Ann. Chim. Phys., 28,384, 531 (1903). Nernst: loc. cit. See also “Theoretical Chemistry,” p. 274 (1911). Bingham: loc. cit. Also see, Sonaglia. Nuovo Cimento, 7, 321 (1914). 14Forcrand: Compt. rend., 156, 1439, n648, 1809 (1913). Hildebrand: J. Am. Chem. Soc., 37, 970 (1915); 40; 45 (1918)
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J . F. T. BERLINER
their tangents had the same slope, were on a straight line which could be represented by log p = log T - constant. If the vapor obeys the gas laws then p = cRT where c denotes the Concentration. This may be converted into the logarithmic equation, p = log T - log Rc; therefore log Rc is the constant of the above equation. Along such a line as represented by this equation, the concentration must be constant, Hildebrand therefore states that : “The entropy of vaporization for normal liquids is the same when evaporated to the same concentration, Le., when the final mean distance between the molecules of vapor is the same.” I t must be emphasized that Trouton’s rule makes a comparison at constant pressure (one atmosphere) and not at constant concentration, and under constant pressure conditions the entropies are not the same. Any concentration may be chosen. Hildebrand arbitrarily chose the concentration of o.oo50; moles of vapor per liter, this concentration having the “desirability of avoiding any extrapolation” of his measured data. The value for the constant is approximatelr 13.7 calories per degree. Applying Trouton’s Rule, the increase of entropy constant varies greatly with substances of widely different boiling points: as, for instance, nitrogen and bromonaphthalene give values of 11.0 and 14.1 calories per degree, respectively, while with Hildebrand’s expression the values for these two substances are the same-13.8 calories per degree. Hildebrand found that associated substances gave abnormal values, for instance, ammonia, water and ethyl alcohol gave 1 6 . 2 , 16.0 and 16.7 calories per degree, respectively. He also found a small though significant difference for the metals which were OD the average about 0.6 calories per degree lower than the normal liquids. I t is suggested that this deviation is due to a difference in molecular complexity. When a molecule escapes from the liquid to the vapor it is relieved of a very high internal pressure which exists in the liquid, and may, conceivably, expand with an absorption of energ?.. The amount of energy so absorbed would be expected to be greater in the case of molecules containing many atoms than with those containing but afew. The expression for the entropy of vaporization may therefore be written (L - e)/RT. where ((e’’denotes the energy absorbed within the molecules on expansion from the high pressures existing within the liquid to the low pressures existing in the vapor. The quantity of “e” is doubtless small compared to “L”, that is most of the energy is required to overcome the attraction between the molecules, and but little in the expansion of the molecule itself. It may be stated that, in all probability, in so far as it concerns the overcoming of the attraction between molecules, the entropy of vaporization at the same concentration is the same for all normal liquids. If a liquid is associated, a third, and much larger quantity of energy is involved in the dissociation of the complex molecules into simpler ones. I n such cases the total entropy of vaporization is distinctly greater than the normal values for a given concentration. Hildebrand considers that the low values of metals and also of gases boiling in the region of absolute zero are due to effects of the abnormally rapid changes of the specific heats.
A CONCEPTIO?; O F POLARITY
297
The entropies of vaporization of the nitroanilines, toluidines and nitrotoluenes a t temperatures a t which the concentration of the vapor is .00507 moles per liter are given in Table I.
TABLE I Substance
Tern erature-absolute at u hich = W j o ? moles’liter
Ortho nitroaniline Meta nitroaniline Para nitroaniline Ortho toluidine N e t a toluidine Para toluidine Ortho nitrotoluene Meta nitrotoluene Para nitrotoluene
8
‘$0
500
9 4
530 4 401 6 405
1
Entropy of J-apoi mation
Calories, degree 16.0 1j.j 17.5
16.4 15.6
402 1 416 3
1j.8
327
I+O
2
431 5
13.6 73.9
It will be noticed that the toluidines and nitroanilines give values for the entropy of vaporization which are much higher than 13.6, which Hildebrand considered the normal value. Therefore, in all probability these compounds form associate or polar liquids. This i s not the case with the nitrotoluenes which may be considered to be practically non-polar liquids. It may not be too much to assume that there could be a relationship between the entropies of substances and their electrical structure, since, as is known, the heat of vaporization and association, dependent upon the polarity of the molecule, are related to its electronic structure. From a thermodynamic consideration it is evident that the entropy, association and heat of vaporization are very closely related. Therefore, a consideration of the electronic structure should, qualitatively, at least, interpret the relationship of the entropies of polar and non-polar compounds h t the present time there exists no absolute means, or even quantitative relative means of expressing association. The order of association of various compounds can be arrived a t through a consideration of their entropies of vaporization (also from the dielectric properties), but a compound may be twice as much associated as another and yet only be a few calory per degree higher in entropy. E-et, while not a measure of the association, the entropy of vaporization does offer a valuable means for comparing the association of various compounds. Hildebrand has shown that all non-associated compounds exhibit the same entropy of vaporization when the concentration of their vapors are the same. This rule is valid even to the critical temperature, for at this temperature the heat of vaporization is zero and the entrop? of vaporization for all substances is zero and the entropy of vaporization for all substances is zero at their critical temperature. If the entropies of a series of compounds be compared at the same vapor phase concentration their relative association will be apparent. This is shown in Table I for the nine compounds under discussion.
298
J. F. T. UERLIPI'ER
The Molecule and its Electronic Structure There are several electric conceptions of the structure of molecules, many of which have had to be radically modified or entirely discarded. The theory of chemical union as is now generally accepted is based on the Bohr conception of the atom under conditions defined by the quantum theory. The chemical aspect of the atom has been amplified by Kossel, Lewis, Langmuir, Stieglitz, Lowry, and Kharasch. However, as applied, there is the polar conception and the partial polar conception. In the polar conception, which is now rather limited to the field of inorganic chemistry, the bonding pair of electrons in a molecule are held by one atom or the other, depending upon which is the negative. In the partial polar conception the atoms comprising the molecule share the bonding pair (or pairs in the case of unsaturated compounds) of electrons between them, the position of the shared electrons shifting toward one side or the other dependent to the relative negativity or positivity of the atoms or groups. Thus it may be seen that there is really no fundamental difference between the polar, non-polar, and partial polar bond, but merely one of degree. If the bonding pair of electrons be held midway between two atoms the bond is of the typical co-valent or non-polar type; if the bonding pair is shifted so that one atom obtains exclusive possession of the electron pair the valency is of the electrovalent or polar type; while if the electron pair is not held by one or the other atom and not exactly midway between, the valence is partially polar in character. According to Dr. Rideal the gradation between polar and non-polar linkage due to a shift in the orbits of bonding or shared electron pairs takes place in quantumised stages. In the case of the double bonding pair of electrons in the unsaturated compounds there is quite a distinction between the two pairs of electrons. One of these pairs of electrons is assumed of the non-polar type, that is, takes a position midway between two atoms, while the other pair of electrons is a partial polar type which can take up any position between the two atoms and its position will be determined by the relative negativity of the two atoms. This conception was developed by Lowly,*6and Lapworth and R o b i n ~ o n . ' ~ If one should consider a saturated hydrocarbon, for instance, ethane: being a non-polar compound the bonding pair of electrons would be midway between the carbon atoms Now if one should introduce a negative group like chlorine into the molecule, the bonding pair of electrons of the carbon and chlorine would be displaced towards the chlorine atom; and the electron pair between the two carbon atoms would be shifted away from the carbon atom attached to the halogen. That this is the direction in which the displacement takes place may be made evident by considering ethylene; in this compound the electrons forming the two bonding pairs are somewhere midway between the two carbon atoms. If the chlorine derivative of this compound-vinyl chloride-be considered, the position of the extra bonding pair of electrons between the two carbon atoms can be easily ascertained by treatLowry: J. Chem. SOC., 123, 8 2 2 (1923). Lapworth and Robinson: Trans. Faraday S O C . , 19, 505 (1923).
A CONCEPTION OF POLARITY
299
ing this compound with a halogen acid. Upon treating vinyl chloride with hydrochloric acid, ethylidene chloride (unsymmetrical dichloroethane) is formed, showing that the bonding electrons shifted away from the carbon atom t,o which the chlorine is attached, and caused the other carbon atom to assume a relatively electro-negative conditon since the positive hydrogen of the hydrochloric acid became attached to this carbon atom. This conception has been used by Kharasch, Stieglitz and others to explain many organic reactions. A survey of the existing literature on electronic structure revealed but, little on the electronic configuration of the benzene molecule. There have been several presentations on the electrical structure of benzene considering it from an electrostatic conception. Lowry,‘6 Holleman,’8 V ~ r l a n d e r ,Ker~~ mack and Robinson,2O Lapworth21 and others, however may have had an electronic conception in mind, but did not publish anything that could be so considered. However, there a few investigators who consider benzene and its derivatives from the electronic concept.ion of valence. Notable amongst these are HugginsZ2and C r ~ c k e r . ? ~ Huggins considers t,he benzene grouping in the light of K ~ r n e r ’ scentroid *~ structure and briefly reviews the present evidence for andagainst thisstructure which is based, t o a large extent, on the conjugation hypothesis introduced by Erlenmeyer, Jr.*j This st,ructure agrees well with many properties of benzene especially its X-ray crystal structure. The principal objection to this structure is that it necessarily postulates that ortho and meta disubstituted benzene derivatives should be optically active and as yet no indication of this has been noted though much investigation has been instituted on this problem. Several investigators attempted a separation of the optically active forms by means of bacteriological means, but all their results were negative. Some of the workers on this phase were LeBel,26Lewkowitz,2’, Meyer and Luhn.** However, Huggins does not consider this sufficient proof against this conception and makes the rather interesting statement that “The objections raised to it are invalid or inconclusive,” basing this on the consideration that the optically active isomers could be separated if the proper conditions were known and on the assumption that if they were separated they would rotate the plane of polarized light, very little or not at all. Severtheless the Korner centroid structure,?‘ which consists essentially of six carbon tetrahedra having Holleman: “Die direkte Einfuhrung von Substituenten in der Beneo:kern” (191n). Vorlander: Ber., 52, 263-233 (1919). Kermack and Robinson, J. Chem. Soc., 121, 427 (1922). ?‘Lapworth: SIemoirs Manch. Phil. SOC.,64, I (1920); J. Chem. Soc., 121, 416(1922). ?* Huggins: Science, 55, 674 (1922) J. Am. Chem. SOC., 44, 1607 (1922). 23 Crocker: J. .Im. Chem. Soc., 44, 1618 (1922). ? 4 Korner: Gam., 4, 444 (1874). 26 Erlenmeyer: J. .Inn. 316, 71 (1901). 26LeBel:Bull., 12) 38, 98 (1922). “Lewkowitz: J. Chem. Soe., 53, 781 (1888). Meyer and Luhn: Ber., 88, 2795 (189j). Is
ID
3 00
J . F. T. BERLINER
their bases all in one plane and their apexes alternately above and below thib plane, does give quite a clear interpretation of the st,ructure and it has been shown that the dimensions are almost exactly those corresponding to graphite. Huggins devotes very little of his discussion to the consideration of the electronic structure, which he considers to consist of six electrons around the center of each tet'rahedron and two at each corner of the six hexagon formed by the six tetrahedrons making up the benzene nucleus. However, no evidence is given for this electronic structure and it is not discussed or applied. Crocker'~?~ conception deals with the benzene configurat,ion from A strictly electronic viewpoint. His views agree extremely well with thosr of Kharasch and Stieglitz and brings the views of Lewis, Langmuir, Conant. and Parson into close agreement. His view may be briefly summarized in this manner-there is a ring of six carbon atoms, each singly bonded, by means of pairs of electrons, to its neighbor on either side, and to hydrogen. The remaining six electrons are placed between the carbons in the plane of the ring thus forming an octet for each carbon atom. Substitutents of the hydrogen would cause a shifting in the position of these latter six "aromatic" electrons. The direction and degree of t,his shift, depends on the electrical nature of the substituent and ('rocker considers this shift of the aromatic electrons to be mechanically the same as the shift of the electrons in the unsaturated hydrocarbons as in the illustrative example (ethylene) in the preliminary consideration; Le., if any one of the aromatic electrons moves from its position midway between the carbon atoms, the electrical equilibrium of the systeni is disturbed and the others must move in such a manner as to restore it. Thus if a positive group, such as the amino group, which repels the paired electrons. to be introduced into the nucleus to form aniline, the aromat,ic electrons would be attracted towards the 3 and j posit,ions and repelled at the z and 4 positions. If a negative group such as the nitro group, that strongly attracts the elect,ron pair be introduced into the nucleus to form nitro benzene thr electrons will be attracted towards positions 2 , 4 and 6 and repelled at the 3 and 5 positions. Crocker considers that substitution is possible only at thosr positions where the hydrogens are lightly held, that is, the posit'ion where the electrons are repelled. It is seen how well this conception explains the reason why benzene derivatives containing negative group-which causes repulsion of electrons a t the meta positions are readily substituted in this position while in the case of compounds like aniline, substitution takes place at the orth and para positions, since it is at these positions that the positive aniino group causes the electrons to be shifted away from the nucleus. HollemanZ9 has very ably discussed the relation of the various groups to their directive powers in the benzene derivat'ives and Schwalbe"" has made quite a complete compilation of the data pertaining to this. This conception seems to give thc clearest view of the operation of the forces about the molecule and is in very fine agreement with the known facts. *Q
Holleman: "Substituenten in den Benzolkern", (1910). Schwalbe: "Benzol Tabellen" (1903).
5 CONCEPTION OF POLARITY
301
If these deductions are now expanded upon and the conception of a type of co-valence introduced this view of the molecule of benzene and its derivatives may be applied to the interpretation of facts with which it appears, at first, to have no connections As was previously mentioned, it seems quite reasonable to postulate that entropy, molecular association and electronic structure may be interrelated. The association of molecules takes place only in the case of polar molecules, that is in those molecules in which the electrical charges are not balanced or the electric moment is not zero. It is seen that this is also one of the conditions under which compounds exhibit co-ordination values. Thus, it has been suggested by Huggins and by Lewis3‘that the reason for the association of water, ammonia, hydrofluoric acids and similar compounds can be attributed to hydrogen exhibiting a secondary valence or co-valence under the conditions of the great electric or magnetic moments that exist in the molecules of these substances. This suggestion of bivalent hydrogen has been applied by Latimer and R o d e b u ~ hin~ the ~ interpretation of the association and structure of ammonium hydroxide. As will be indicated in the succeeding treatment on this subject, the co-valency of the hydrogen may be the factor causing the association of the organic molecules, although it is not necessary to postulate that this bivalent form of hydrogen, if it does really exist, is essential to the explanation of the observed phenomena. It is merely noted as a possible factor in the future development. Electronic Structure of the Compounds studied In the substances considered in this discussion there are three groups of atoms, the nitro, the methyl, and the amino group, that enter into all the isomeric relations t o each other that are possible in their disubstituted benzene derivatives. These may be considered in terms of relative basicity. I n the following treatment, a group will be considered as negative if, when joined to the hydroxyl group, it allows the hydrogen of the hydroxyl to be reactive, such as the nitro or sulphonic acid group, N02.0H, S03H.0H. Conversely, positive groups such as amino when joined to the hydroxyl allow the whole hydroxyl group to react. It is apparent that from this consideration the methyl group is very weakly negative, since, although in methyl alcohol the hydroxyl may be readily split off, as by the action of hydriodic acid, yet it canin addition have the hydrogen of the hydroxyl replaced by much more positive substances such as sodium and potassium. From the present knowledge of the electronic structure of benzene it may be represented as in -1,Fig. I . This is essentially that proposed by Crocker13. Here the small dots represent the normal bonding pair of electrons between the carbon atoms and are considered to be stationary; the large dots the aromatic or polar electrons, which are the ones that shift under the influence of various substituents; and the small circles represent the bonding pair of electrons between the hydrogen and carbon atoms. These latter electrons 31 32
Lewis: “Valence and the Structure of Atoms and ~lolecules,”109. (1923) Latimer and Rodebush: J. Am.Chem. Sac., 42, 1419(1920).
J. F. T. BERLINER
302
shift in their positions relative to the character of the substituent and the position of the aromatic electrons. For convenience of illustration the benzene nucleus will be employed as shown by (B) in Fig. I , where the large black dots represent the aromatic electrons and the position of the hydrogenH
:C,.
. '3'
.,C:
I
!J( ! Il H
'
H
FIG.I
-
6,
A CONCEPTION O F POLARITY
3 03
ducing the electronic arrangement presented above. In toluene the methyl group is practically neutral or slightly negative and the differences between it and the other configurations are but of degree. I n the compounds chosen for this investigation, all the combinations of the above structures are involved. Besides having a definite effect on the aromatic electrons, each group has a profound influence on the effects of the other group present, which depends both on the type of group and its position in the benzene nucleus. The nine compounds studied are represented in Fig. 3 on the basis of the interpolated conception of the electronic structure introduced in this treatment. An attempt has been made to illustrate the degree of repulsion and attraction of the bonding pairs of electrons by means of the variations in the distances from the nuclei of the bars employed to represent the relative positions of the hydrogen-carbon bonds, and of the aromatic electrons. These are to be considered strictly of a qualitative nature and are to some degree exaggerated, for purposes of illustration the relative effects on the electronic configuration caused by the complex inter-effects of the two substituent groups. I n the nitroanilines there is a strongly positive group and a strongly negative group present. -4s will be evident from the illustrations in Figs. z and 3, the amino group in the ortho and para position to the nitro group would act in such a mode a s t o greatly exaggerate the conditions already present, that is the electrons would be attracted very powerfully to the 2 , 4 and 6 position and very strongly repelled at the 3 and 5 positions. However, in the case of the meta isomer, it will be apparent that the effect of the amino group is opposed to that of the nitro group because the nitro group is attracting the paired electrons away from the nucleus while the amino group is forcing them towards the nucelus. Since this condition exists, the effects of the nitro group will be materially diminished and the configuration altered to one that more nearly approaches that of benzene than that of nitro-benzene or aniline. There are some differencesbetween the ortho and para configuration that are of importance to note. While the amino group in either the ortho or para position to the nitro group tends to greatly increase the effects caused by the aniino group there are differences in degree of this effect between them. The amino group in the para position has a more profound influence in exaggerating the nitrobenzene structure than the ortho amino group. This difference may be accounted for by two considerations. The first, is that the ortho amino group, due to its stereochemical position has a greater effect on the reactive repelled electrons in the 3 position than it has on those in the 5 position and in its effect on the 5 position it is to some extent hindered through having the nitro group in an intermediate position, on one side, between the amino group and the j position. The second consideration is a purely chemical one. It has been shown by several investigators, notably Baly, Edwards, and Stewart,33Baly, Tuck and LMarsden,34H a n t ~ c hand ~ ~ Kharasch, LomBaly, Edwards and Stewart: J. Chern. SOC.,89, 517 (1906). Baly, Tuck and Marsden: J. Chem. SOC , 97, 581 (1910). 36 Hantmch: Ber., 43, 1668 (1910). 33 34
304
J. F. T. BERLINER
men, and J a c ~ b s o h nthat , ~ ~ there is marked tendency for the nitro group and amino group, when in the ortho or para positions to each other or t o other reactive groups, to react to form quinone derivatives of the nitronic acid and quinone-dioxime types. This effect will be subsequently discussed. Xow it is apparent that in a reaction of this nature, which can be considered as forms of neutralization, the effects of the substituents on the electronic configuration would be such as to cause a change of the electronic arrangement in the direction of the non-polar type of compound; also it is apparent that in an intramolecular rearrangement or reaction, (may also be considered as an intermolecular reaction), the reacting groups would have a much greater tendency t o react if they are adjacent to each other as in the orthonitroaniline, than if they were relatively greatly displaced as in the para derivative. These two considerations are believed to be sufficient to account for the difference in electronic configuration and therefore the relative association between the ortho and para nitroaniline. In fact it is easily conceived that the rearrangement producing a partial electrical neutralization in the ortho derivative, mag be so great as to cause this compound to, electronically, approach quite close to the meta arrangement. Electronic Structure and Molecular Association From the consideration of the previously discussed hypothesis of Huggins and Lewis, it is conceivable that the greater the electronic moment of a molecule, that is, the more labile its electronic charges, the more polar it becomes and therefore the more associated. In the nitroanilines, as is evident from this postulate, the para isomer is very much more polar than the ortho or meta, and the ortho nitroaniline is more polar than the meta. Since the polarity is the cause of association of molecules, the higher the polarity thc greater the association. As was explained in the preceding section, the association of a molecule is related to the entropy of vaporization of that substance. Therefore, it may be stated, that the polarity and entropy of a molecule are directly related. Reference t o Table I indicates that the entropy of vaporization of the isomeric nitroanilines are in fine agreement with the conclusions of the preceding discussion of their relative polarities. As is required by the electronic conception that is advanced in this treatment, the para nitroaniline should have a far greater entropy of vaporization than the ortho and meta isomers since its polarity is so much greater; likewise since the polarities of the ortho and meta nitroanilines are not greatly dissimilar the values for the entropies of vaporization are relatively close together for these two compounds. This agreement gives very substantial indication that the premises assumed for the structure of these compounds are correct. I n the toluidines, there are present a strongly basic or positive group and a very weak negative group. As would be supposed the methyl group has a relatively small though appreciable effect. Considering the electronic structure as that of aniline, it is readily noted that a negative group would tend to 38
Kharasch, Lommen and Jacobsohn. J. Am Chem. SOC.,44, 793 11933).
A CONCEPTION OF POLARITY
305
intensify the polarity of the initial electronic configuration if it were in the ortho or para position. Therefore, the ortho and para toluidines should be more polar, hence more associated, and should have a higher entropy of vaporization than the meta. I n the ortho toluidine the effect of the methyl group on the electrons of the 6 position, is transmitted and increased by the amino group thus causing the electrons of the 6 position to be somewhat more displaced than those of the 4 position. I n the para toluidine the two electron pairs ortho to the amino should be equally repelled from the nucleus; thus it is evident that the ortho toluidine should be more polar than the para compound and therefore have a higher entropy of vaporization. As may be ascertained from the table of the entropies of vaporization, the above conclusions are justified. The precepts, as presented for the toluidines and nitroanilines, should apply to the nitrotoluenes as well. The validity of this is observable from the following consideration and from the representation of these compounds in Fig. 3. Since both the nitro and the methyl group are negative it will be at once evident that when they are in a meta position to each other the electronic configuration originally present in the nitrobenzene will be somewhat intensified and therefore the meta nitrotoluene will be the most polar mononitrotoluene. Since the effect on the electron displacement in the nitrotoluenes is due almost entirely to the nitro group the methyl group in the para position will have little or no effect on the electron displacement of the positions meta to the nitro group and therefore the para nitrotoluene is but slightly less polar than the meta isomer. I n the ortho nitrotoluene the effect of the nitro group is diminished by having t o be exerted through the methyl group which is in a position such as to oppose the forces of the nitro group, at least for one position (ortho to the methyl). As in the case of the ortho nitroaniline there may be some intramolecular rearrangement or reaction. This mill subsequently be referred to. The nitroluenes which, it is to be recalled, are polarized but slightly if at all, are nevertheless in accord with the principles that are applicable to the other compounds. Thus it is evldent that the electronic structures of these compounds are related to their association and polarity and the sequence and relative degree of this association is in agreement with the values derived from experimental evidence. Effect of Intramolecular Reactions From a consideration of the electronic configuration alone, one would deduct that the ortho nitroaniline and the ortho nitrotoluene would be more highly associated and have a higher entropy than they actually exhibit. As has been indicated the lower degree of polarity of these compounds is thought to be due to some intramolecular reaction. There is a t the present time much speculation on this type of reaction and the constitution of these intramolecularly condensed compounds that are known as “meriquinoids.” These were first observed by Wurste?’ in 1879 and then studied by RernthsenPY Wurster: Ber , 12, 1803, 1807, 2071 (1879); 13, 3195, 3217 (1880) J8Bernthsen Ann, 230, 162 (1885); 251, 1 1 , 49, 82, (1889). 3i
J. F. T. BERLINER
3 06
Willstatter, and P f a n n e n ~ t i e land , ~ ~Willstatter and Pic~ard,~O who concluded that these compounds were a class of quinone derivatives of a semiquinoneimide type. It has been suggested by K e h r m ~ n nK , ~a ~ f m a n n , and ‘~ M e ~ e r ~ ~ that the two reactive components exist in a condition of dynamic equilibrium with reference to each other which they call “isorropesis” or the make-andbreak in the linkages of the residual valencies. While this term is usually applied to an intramolecular condition, it may be extended to include an intermolecular condition such as is represented in the recurrent making and breaking of linkages between different molecules. It may therefore be safely assumed that in ortho nitroaniline and probably ortho nitro-toluene there is a great tendency for an isorropesic condition to exist and from a consideration of their electronic arrangements and their entropies of vaporization one may reasonably presuppose that such condition does exist.
summary A relationship between the electronic configuration and association as derived from vapor pressure measurements is shown to exist. This has been applied in the interpretation of the variations in the entropies of vaporization of the isomeric introanilines, monoitrotoluenes and toluidines. The significance of the relations has been considered. Washington, L). C. 1997.
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Willstatter and Pfannenstiel: Ber., 38, 2244 (190j). W i k t a t t e r and Pfannenstiel: Ber., 38,2244 (1905). 4‘ Kehrmann: 41 2340 (1908);see also p. 1458. 42Kaufmann: Die V6enzlehre,” p. j ~ oalso ; Ber., 42,4324 (1909). 43 Meyer: Ber., 41,2568 (1908); 42, 1149(1909); 43. I 5 7 (1910). 39
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