July, 1943 Sample
NOTES
1433
1 2 3 4 5 Mean 15600 14200 14850 14600 16400 15000 *
decrease acid strengths. Turnbull and Maron have advanced an explanation of this fact on the 476 assumption that the ions of the nitro and aci The average deviation from the mean was there- forms are different, and are not identical with fore about *3%; the square root of the sum of each other as they would have to be if resonance the squared deviations *3.5%. The physical occurs between the two corresponding structures. constants of the monomers and the heats of Since this assumption seems to us to be rather polymerization are given. improbable, we wish to call attention here to the following alternative explanation, which we conMethyl methaVinyl sider to rest upon a more satisfactory theoretical Substance Styrene crylate acetate basis. The ionizable proton in 2-nitropropane, 145-6 100 73 B. p , "C. 0.908 0.937 0.934 d20r for example, is extremely close both to the nitro 1.396 1.543 1.414 na0D group and to the methyl groups in question. 98-99 95 Ext. of polymerization in % 35-85 Consequently, the lines of force between the large 60-90 80 Temp. in "C. 70-140 dipole moments of the nitro group and the proton Time, hours 1/2-2 1/2 1/2 must pass largely through the methyl groups7900 8000 H (mean), cal./mol. 15000 *400 *400 Av. dev. fr. mean *470 that is, through a region of relatively low dielectric constant. As a result, the electrostatic It is a pleasant duty for the authors to express interaction, which is responsible in the main for their sincerest appreciation to Mr. Dan Whyte, the measurable acidities of the substances, must President of the Jay Novelty Company of New be greater for 2-nitropropane than for nitroYork City, whose interest in the subject enabled methane, in which the corresponding lines of us to build the described calorimeter and to carry force pass largely through the solvent of high out the measurements. dielectric constant. Nitroethane is of interPOLYTECHNIC INSTITUTE OF BROOKLYN RECEIVED MARCH4, 1943 mediate acidity, as could have been predicted. BROOKLYN, NEWYORK The fact that the expected order of increasing pK's (nitromethane < nitroethane < 2-nitroAcid Strengths of Aliphatic Nitro Compounds propane) is observed in the aci forms can be reBY G. W. WHELAND AND JOHNFARR lated to the fact that in these aci forms the ionizTurnbull and Maron' have recently reported able protons are farther removed from the nitro the measurement of the aci dissociation constants and methyl groups, so that the lines of force pass of both the nitro and the aci forms of several ali- largely through the solvent in all cases. Conphatic nitro compounds. We had previously sequently, the usual direct effects of the methyl been making these same measurements by the groups in decreasing acid strengths have a chance same experimental method. Our provisional to operate, as in the analogous carboxylic acids. values of the PK's of the nitro forms a t 25' are: Furthermore, in 1-nitropropane, the terminal nitromethane, 10.24; nitroethane, 8.60; l-nitro- methyl group is farther removed from the proton, propane, 8.98; and 2-nitropropane, 7.7-7.8. and so it it exerts only its direct effect in making All values are corrected for salt-effect and for the substance less acidic than nitroethane. It is not clear to us how the explanation of Turnbull hydrolysis, and have probable errors of *0.04 as judged by the internal consistency of the data. and Maron would apply to this substance. The foregoing discussion is based upon a closely These figures are mostly in satisfactory agreement with those of Turnbull and Maron, but the similar one given by Westheimer and Shookhoff discrepancy for nitroethane is somewhat larger in regard to the acid constants of substituted than the probable error in either set of measure- malonic acids. We wish to thank the Commercial Solvents ments. Under the circumstances, we have disCorporation for furnishing the 1-nitropropane and continued work on the problem. The pK's of the nitro forms decrease in the 2-nitropropane used in our work. unexpected order: nitromethane > nitroethane > DEPARTMENT OF CHEMISTRY OF CHICAGO 2-nitropropane. The opposite order might have UNIVERSITY RECEIVED MAY 10, 1943 CHICAGO, ILLINOIS been anticipated, since methyl groups ordinarily
H
(1) D. Turnbull and S. H. Maron, THISJOURNAL, 66, 212 (1943).
(2) F. H. Westheimer and M. W. Shookhoff, iqid.. 61, 555 (1939)
