Uec., 1934
DIPOLE MOMENTS AND MOLECULAR STRUCTURE OF AMIDES
above the precipitate was quite clgsr. Sometimes the precipitate was filtered off at once, and sometimes after twenty-€our hours. The clear liquid was poured through a Neubauer crucible (to amid thk use 6f a filter paper) and the precipitate transferred to the mcible d t h a *himum m o u n t of a dilute solhtion of eilver nitrate (9 g. of the salt in 1000 cc. of &O). It was washed seven to eight times by letting from 4-5 cc. of the wash-solution flow from a pipet around the upper rim of the crucible and then drawing it Off with the pump. The $Mall amount of silver nitrate reniaining ifi the p&cipitate Was washed out with 15 cc. of 96% alcohol, 5 cc. being used each time. Finally, after removing the alcohol as thoroughly as possible with the pump, the precipitate was dried to constant weight a t 110'. At this temperature it still contained trace9 of water (0.0003 g., average of many determinations), and a further heating to 250' was necessary, This was carried out as follows. A large porcelain crucible of about 75-cc. capacity was adjusted 11 cm. above the top of a Bunsen burner and a 550' thermometer clamped upright with its bulb in contact with the bottom of the crucible. By regulating the length of the burner flame the desired temperature could be obtained roughly. When the temperatdre was reached and remained constant the thermometer was removed and the Neubauer crucible introduced. During the heating the precipitate became pink and then purple in color, which slowly disappeared above 250" and was completely discharged at 350°.z The disappearance of the purple color was not accompanied _. by any perceptible change in weight of the precipitate, During all the operations care was taken to protect the precipitate from direct sunlight. (2) Smith aad Bradbury (loc. oit.) noticed this purple color. Their statement that the salt when moderately hented melts to a clear, yellow liquid is not in accord with the experience of the writer, who finds that the liquid is purple and on cooling solidifies to a yellowish-wkite mass.
2549
1. Seven samples of Merck blue label ammbnium molybdate ranging between 0.2642 and 0.5104 g. were analyzed and, when corrected for traces of water in the AgzMo04 dried at llOo, gave an average value of 54.40% Mo, with ad average deviation of 0.02% and a maximum deviation of O.oS%. Calcd.: Mo, 54.36%. Found by the ignition method: Mo, 54.43%. 2. Three determinations next were made In another sample of Merck blue label salt, which by the ignition method gave 54.48% Mo. Found: Mo (average), 54.47%. The course of procedure was exactly that given above. 3. Some sodium molybdate which had lost a part of its watet of crystallization was tocrystalliwd and carefully dried. Found in i t by ignitioh 14.82% H20; calculated, 14.87% H20. Five determinations of the molybdenum in this recrystallized salt averaged 30.69%, the lowest result being 39.65% and the highest 39.75%. The theoretical percentage is 39.67. Nothing is gained by dissolving the precipitate in nitric acid and titrating the silver according to Volhard. The solution goes on too slowly-indeed, is only complete when the acid has been heated for some time. The resdlts, however, are satisfactory. For the determination of molybdenum in alkali molybdates the method is easier and more expeditious than are those of Berzelius-Rose (precipitation as HgzMoO,) and Chatard (precipitation as PbMoOd).
Summary It has been shown that the molybdenum in alkali molybdates can be determined easily and accurately by converting them into silver molybdate, which while slightly soluble in water is practically insoluble in watercontaining silver nitrate. PRINCETON,NEWJERBEY
RECEIVEDJULY 12, 1934
[CONTRIBUTION FROM THE CHE~ICAL LABORATORY OF THE UNIVERSITY b~ C A L ~ P ~ R ] NIA
Dipole Momeilts and Molecular Structure of Amides BY W. D. KUMLER AND C. W. PORTER The fact that certain groups of atoms have different moments when attached to the benzene ring than when attached to aliphatic radicals has been ascribed to the inductive and electromeric effects of the benzene ring. Resonance invofving essentially the same idea as electromerism gives perhaps a more precise picture of this phenomenon. In many cases the structure of a molecule is not one definite form, but a combination of several electromeric configurations. The large moments of most simple aromatic compounds as compared with the corresponding simple aliphatic compounds are due, in part, to the contributions 1) Sutton, Proc. Roy. SOC.(London), lSSA, 668 (1931).
made by excited forms h which certain atms are positively charged and oth&rs are negatively charged. In nitrobenzene the excited forms
+ contribute appreciably to the structure of the molecule and these excited forms raise the moment of tlie aromatic compound considerably above that of an aliphatic nitro compound in
W.D. RUMLER AND C. W. PORTER
2550
which corresponding excited states do not occur. Likewise in the case of aniline the forms
raise its moment above that of methylamine. In some cases the moment of the aromatic compound is lower than the moment of the corresponding aliphatic compound. Chlorobenzene is an example, but here the moments in the excited forms are opposite in direction to the moment in the normal form, thus making the moment of chlorobenzene less than that of methyl chloride.
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Vol. 56
plain the discrepancy by assuming induction, We are convinced that in many cases the discrepancy is due in part to resonance. The difference between the observed and the calculated values, in molecules that present the possibility of resonance, is usually in the direction one would expect from the contributions made by the excited forms. Moreover, many of these discrepancies are too large to be explained by induction. The difference of about 1.8 in p-nitroaniline is an example.2 Pauting and Sherman* have pointed out from thermochemical considerations that unsubstituted amides resonate. The excited form R-C /O-
%HZ
has a moment of the order of 10. Since the moment in the excited form is in approximately the same direction as the resultant moment in the normal form, R - d
0
the"moments of amides
\"I
should be greater than the values calculated The group moment calculated from one com- from bond moments derived from ketones and pound cannot be applied accurately to the calcu- amines. The problem is complicated, however, lation of the moment of another compound unless by several factors. First, there is some uncerthe two have practically the same amount of tainty in regard to bond angles. Second, the resonance or the change in moment due to reso- NH, group may be either rotating or fixed.' nance can be evaluated. p-Nitroaniline long Third, the compound may be partly iu the taupresented an anomaly because aniline has a moment of 1.56, nitrobenzene a moment of 4.0 and tomeric form R-C /OH \NH p-nitroaniline a moment between 6 and 7. Even If resonance occurs in unsubstituted amides it is if we assuine the moment of the NH2 group to be highly probable that it occurs also in substituted in the plane of the ring (which is not correct) the By using substituted amides we elimimaximum calculated moment for p-nitroaniline amides. nate the possibility of complications arising from is 5.56, which is considerably less than any of the OR measured values reported in the literature (7.1, the tautomeric form R-< since such a NR 6.4 and 6.1).2 We measured the moment of this compound, using dioxane as a solvent, and ob- tautomeric shift of a radical is ruled out on tained a value of 6.68. The great tendency of the chemical evidence. We measured diethyl acetnitrogen in the amino group to acquire a positive amide and dimethyl acetamide and found that charge and of the oxygen atoms in the nitro group their moments are 3.72 and 3.79, respectively. Now if we calculate the moment of these comto acquire negative charges causes the form pounds, using the generally accepted bond moments (C=O) = 2.3, (C-N) = 0.4, (H-C) H = 0.4 (the positive end of the dipoIe is written to contribute much more to the moment of p- first), an angle of 110' for the carbon and nitrogen nitroaniline than the excited forms given above valence angles, and if we assume the radicals of contribute to nitrobenzene and aniline, for in p - the amido group are fixed in such a position as to nitroaniline each group enhances the tendency of give the largest possible moment to the molecule, we get a value of 3.14 for these compounds, which the other group to pass into the excited state. is considerably below the measured moments of When the measured and calculated values do 3.79 and 3.72. (Changing the nitrogen valence not agree it has been the general practice to ex-
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Hy=v
