3232
THEODORE L. BROWN
Vol. 81
[.4CONTRIBUTION FROM NOYES CHEMICAL LABORATORY, UNIVERSITY O F
ILLINOIS]
The Electronic Properties of Alkyl Groups. 11. The Dipole Moments of Alkyl Benzenes and Derivatives BY THEODORE L. BROWN RECEIVED DECEMBER 3, 1958 T h e dipole moments of some p-alkylbenzene derivatives have been determined in cyclohexane, dioxane a n d chloroforin solutions. Along with d a t a on other series of p-alkyl derivatives taken from t h e literature t h e results are analyzed t o determine t h e nature a n d magnitude of t h e contributions made by t h e alkyl groups t o the over-all moments. I n considering t h e data for non-polar solvents, i t is found t h a t when a correction is made to account for t h e electrostatically induced moment in t h e alkyl groups, t h e variation in moment in a series of p-alkyl compounds is approximately independent of t h e functional group located in the para-position. These results do not appear consistent with the idea t h a t hyperconjugation plays a n important role in affecting the electron-releasing ability of alkyl groups in this class of compounds. T h e dipolc moment d a t a for t h e p-alkylbenzonitriles in dioxane and chloroform are in the Baker-Xathan order after t h e correction for induced moment. This result is in agreement with the proposal t h a t the solvent plays a n important role in affecting the apparent relative electron-releasing abilities of alkyl groups. as just described for the ethyl compound, except t h a t tlic acids were used as starting materials. Vapor phase chromatograms of each compound failed t o reveal any impurity. I n Table I are listed the refractive indices and the results of elemental analyses of t h e pure materials. Benzonitriie was obtained commercially, dried and carcfully fractionated; T Z ~ ~1.5261. D Nitrobenzene was fractionally crystallized, dried and tlieii fractionated; +SD 1.5301. p-Methylnitrobenzene was Eastman material, recrystallized from metlianol six times and then dried. After initial drying the material was ground t o a fine powder and dried in a vacuum desiccator; m.p. E12.0O.7 p-t-Butylnitrobenzene was fractionally crystallized from a mixture of the mononitro isomers. The fractional crystallization was repeated twice, followed by recrystallization from methanol twice and finally vacuum drying; m.p. 28.4O.' Apparatus and Procedure .-The apparatus employed in the measurements of dielectric constant has been described.8 Experimental T h e resonance frequency settings on t h e converted BC-221 frequency meter were calibrated with a General Radio Materials.-Cyclohexane mas purified by passage through 722-D precision capacitor. T h e dielectric constants of n 60 cm. column of acid-washed alumina, followed by carccyclohexane and chloroform a t the bath temperature of ful fractional distillation from phosphorus pentoxide. 23.97' LTere taken as 2.017 and 1.744, respectively-. T h e Storage was over sodium wire, d?, = 0.77410. dielectric constant of dioxane was determined with reference Dioxane was dried with anhydrous magnesium sulfate, refluxed with sodium and then fractionally distilled. Stor- t o a sample of purified benzene8 t o be 2.227. A t least four separately prepared solutions of each compound were emage was over sodium wire, dl: = 1.0292. Chloroform rr-as refluxed with pliospliorus pentoxitlc and ployed in evaluating each dipole moment. Weight frxthen simply distilled. Storage \vas under dry nitrogen gas tions Xvere in the range 0.0007 t o 0.008. It was not found possible to obtain refractive index measurements with in the dark, d24 = 1.4754. pTolunitri1e was Eastmail IVhite I ~ b c X l t t e r i ~ t l ,dried enough precision to permit a reliable evaluation of the clecivith magnesium sulfate, fractionally crystallized once (n1.p. tronic polarizations, so t h e values calculated froin bnnrl 28') and then fractionally distilled under reduced pressure refractions were employed. T h e values obtaincd cxperiinentally from the dioxane solutions of the p-alkylberizr~in a six inch spinning band column. Ccriter fractions of nitriles were most nearly acceptable and are reported for coilstant refractive index were collected for use. comparisons. Densities were obtained with a 5-nil. pycp-Ethylbenzonitrile was prepared from p-cthylacetophenone b y oxidation t o the acid with sodium h y p o ~ h l o r i t e , ~ nometer with calibrated side-arms. this in turn was converted t o the acid chloride with thionyl Results chloride.6 T h e acid chloride was converted to the amide, which when purified was treated with excess phosphorus From the experimental quantities, values of inychloride and sodium metahisulfiteg added. Tlie reaction O( = bE/bwz and p = dv/dw2, where E is the dielectric inisturc \ - a s poured carefully onto ice, anti after completion rlrolysis t h e iioti-aquenuS layer ivas separated :tiid constant, v is the specific volume and w2 is weight tilled. The iiitrile ci>llccted in this \ m y was dried fraction of solute, were obtained for each c o n \\it11 mihydrous magnesium sulfate anti fractionated under pound. The method of Halverstadt and I(umler$ reduced pressure :is described aho\-c for t h e methyl comwas employed for calculation of the molar polarizapiiund. tion P2, with the exception that the data on the Tlie pisopropy1 and p-t-butyl compounds Ivere prepared
The operation of hyperconjugative interactions in affecting the electric dipole moments of alkylcontaining compounds has been discussed in a number of p l a ~ e s . l - ~I n general there does not appear to be any unambiguous evidence from these studies t h a t hyperconjugation does contribute significantly to variations in dipole moment. Except for the alkylbenzenes, however, aromatic alkyl-containing compounds have not received much consideration in connection with this problem. I n the present paper the moments of the fi-alkylbenzonitriles and two nitrobenzene derivatives are reported and interpreted along with literature values for the moments of analogous series of aromatic compounds.
pure solvent were weighted equally with the solutions. I n Table TI are listed the results for the pdkylbenzonitriles in the three solvents, 31011g with the values of R2 obtained froni the refractive iiickx measurements on the dioxane solutions, listed as R2 (obsd., Table 111 lists the results for the 17') \XT
h? S r h i i h r r t ,
I R i i i i i r i i ,iwd 1 I
l 3 n > \ v n , I ) . 5-
TT.tiiri.
C u r i i n ;mil I
