PROTON TRANSFER IN SOLID LONG-CHAIN AMINES
Nov., 1949
to an ionic reaction, may take the form km
- Zoe-&/RT
(12)
Use of li, and Eoshould be most nearly comparable with a reaction between uncharged gas molecules, if calculation of the collision frequency is to take the simplest form. This frequency, Zo,in terms of moles, liters and minutes, is given by (13)
1000
where u is the average collision diameter and p is the reduced molecular weight, taken in this case to equal 19 X 121/140. On combining equations (12) and (13) there is obtained log u = log k, - 13.92 Eo/2730 (14) Table I V gives log K , (equation ( 5 ) , using ( r 4~ r ~ =) 1.79 A.) and the corresponding values of u. It is evident that there is reasonable agreement with the collision theory in its simplest form.
+
TABLEIV COLLISION DIAMETERS FOR D = 9i Dioxane 0 10
20 30
m,
log k m
25” c,
0.65 .63 .65 .64
A.
3.72 2.09 1.66 1.25
Similar calculations were made by Amis and LaMer12 for the reaction of brom phenol blue and hydroxyl ion, and similar results were obtained.
Summary The rate of reaction of potassium ethyl xanthate [CONTRIBUTION FROM
THE
3591
in dilute acetic acid-sodium acetate buffers has been measured, a t five temperatures from 15 to 35O, in dioxane-water mixtures from 0 to 60% dioxane. The effect of added dioxane, or changed dielectric constant, on the over-all reaction, has been shown to be small. Also, in agreement with previous work, the effect of low concentrations of inert salt has been found negligibly small. The possible mechanism of the reaction has been discussed, and the formation of a critical complex by bimolecular reaction of xanthate and hydrogen ions has been proposed as most probable. Since the ionization constant of acetic acid in dioxane-water mixtures is known, it was possible to calculate the bimolecular rate constants for the postulated rate determining step. The variation of rate constants with dielectric constant was found to follow the predictions of electrostatic theory (the ChristiansenScatchard equation) over the range found usable in other cases (D = 80 to 50), and reasonable values for ra) the radius of the critical complex or (YA were obtained. The experimental energy, free energy and entropy of activation have been calculated, and the electrostatic contribution to these quantities has been evaluated. Constancy of the values calculated for D = m has been found satisfactory. It has been shown that reasonably good agreement with the simple collision theory is obtained when rate constants and activation energies extrapolated to D = m are used in the calculations. It is believed that these factors support the suggested reaction mechanism.
+
NEWYORK,N. Y.
RECEIVED JUNE11, 1949
FRICKCHEMICAL LABORATORY, PRINCETON UNIVERSITY ]
Molecular Freedom and Proton Transfer in Solid Long-chain Amines1 BY JOHN D. HOFFMAN AND CHARLES P. SMYTH
In the course of investigations of solid longchain alcoholsla a direct current conductivity effect probably attributable to proton transfer2 was found almost uniquely associated with the solid rotator state, often called the “CY phase.” The term “rotator” was not meant to imply actual dynamic rotation of all the molecules, but merely their possession of sufficient energy of rotational vibration to permit of frequent passage over restricting potential barriers. The effect also appeared to a smaller extent below the transition point. No such phenomenon appeared in long-chain bromides,S ketones4 or esters.6 It was (1) This research was carried out with the support of the Office of Naval Research. (la) Hoffman and Smyth, THIS JOURNAL, 71, 431 (1949). (2) Stearn and Byring, J . Chcm. Phys., 6, 113 (1937); Bernal and Fowler, i b i d . , 1, 515 (1933). (3) Hoffman and Smyth, to be published. (4) Maler, Proc. Roy. Soc. (London). Al68, 403 (1937). ( 5 ) Baker and Smyth, TIXIS JOVRNAI., 60, 122 (1938).
thus thought advisable to look for proton transfer and transitions in the amines. If the molecules rotated about their long axes, direct current proton transfer conductivity giving rise to MaxwellWagner polarization would be expected to appear. Purification of Substances The samples of n-dodecyl, n-tetradecyl, n-hexadecyl and n-octadecyl amines obtained from the Paragon Chernical Company were purified by a single vacuum distillation. TABLEI Amine
-
n-Octyl n-Dodecyl n-Tetradecyl n-Hexadecyl n-Octadecyl
M. p.,OC.
F. p., O C .
?ID
0.0
27.5 37.6 45.6 52.2
288 28.32’ 37‘ 38. lQ7 46.77’ 53,067
1.4377 (30 ”) 1.4382 (40 ”) 1.4389 (50 ”)
(6) “International Critical Tables.” (7) Ralston, Hoerr, Pool and Harwood. J . OTK. Chcm., 9. 102 (1944).
3592
JOHN
D. HOFFMAN AND CHARLES P. SMYTH
n-Octylamine, also from Par$gon, was distilled at atmosCarbon dioxide, which pheric pressure, b. p. 179 forms the carbamates, was carefully excluded in the distillations and handling. The melting points of the purified compounds are given in the second column of Table I with literature values for comparison. Refractive indices for the D sodium line are given in the last column.
.
Experimental Results The experimental method used to make dielectric constant-temperature runs a t 0.5, 5.0 and 50 kc. has been described previously.1a In addition to the usual precautions regarding moisture entering the dielectric cell, a tube containing Ascarite was attached to the air vent on the cell to prevent contamination by carbon dioxide. The dielectric constants for the amines studied are given in Table I1 as a function of temperature. Only the cooling curves are given since the warming curves were very similar. The melting points were higher than the freezing points by 0.2 to 1.3'. The dielectric data are plotted in Figs. 1-5. nDodecylamine was cooled from 100 to 0' as rapidly as possible in the dielectric cell, and the dielectric constant a t 0.5, 5.0 and 50 kc. plotted as a 3.2
- 50
-30 - 10 10 Temperature, "C. Fig. 1.-Temperature dependence of dielectric constant of n-octylamine. Hollow circles represent values a t 0.5 kc., half-filled circles values a t 5.0 kc., and filled circles values a t 50 kc.
J 3.4
F:
1 . i
0
8 16 24 32 40 Temperature, "C. Fig. 2.-Temperature dependence of dielectric constant of n-dodecylamine. Hollow circles represent values at 0.5 kc., half-filled circles values a t 5.0 kc., and filled circles values a t 50 kc.
Vol. 71
TABLE I1 DIELECTRICCONSTANTS OF AMINES MEASUREDWITH . FALLING TEMPERATURE t, o c .
12.3 2.0 0.2 0.3 - 0.4 0.4 - 0.4 - 1.3 - 8.8 -15.7 -18.7 -26.0 -29.7 -38.7 -49.7 -61.7
-
35.0 32.4 29.5 26.7 26.0 26.2 26.3 26.0 25.8 24.5 21.9 16.7 9.6 3.1 - 1.4 39.4 36.3 36.8 36.9 36.8 36.4 35.4 32.4 28.5
0.5
6'kc.
Octylamine 3.90 3.90 .. 4.05
.. .. ..
.. 3.10 3.05 3.12 3.40 3.48 3.20 2.95 2.80 2.63 2.55
..
3.90 3.55 2.70 2.65 2.58 2.55 2.63 2.63 2.56 2.52 2.48 2.44 2.41
Dodecylamine (Cooling) .. 3.10 .. 3.12 .. 3.13 .. 3.14 .. 3.12 .. 3.10 3.10 2.97 2.92 2.74 2.82 2.63 2.72 2.48 2.74 2.36 2.91 2.38 3.10 2.50 3.14 2.53 3.04 2.50 Tetradecylamine (Cooling) .. 2.90 .. 2.94 2.78 2.81 2.60 2.57 2.39 2.41 2.32 2.32 2.30 2.28 2.24 2.24 2.23 2.23
Hexadecylamine (5 kc.) 55.2 2.71 2.75 46.3 2.53 45.4 2.40 45.3 2.32 44.2 2.29 42.3 2.25 34.4 2.25 28.4 2.24 19.7
3.90 4.05 4.13 3.90 3.53 2.70 2.60 2.50 2.45 2.47 2.47 2.43 2.42 2.42 2.41 2.40
..
*.
.. ..
.. 3.09 2.94 2.74 2.63 2.47 2.32 2.28 2.30 2.29 2.29
..
.. 2.82 2.54 2.41 2.32 2.28 2.24 2.23
Octadecylarnine (5 kc.) 58.2 2.64 53.2 2.67 50.6 2.66 51.8 2.60 51.8 2.55 51.7 2.46 51.5 2.40 49.8 2.33 43.7 2.30 38.3 2.30 35.5 2.29 25.4 2.27
PROTON TRANSFER IN SOLIDLONG-CHAIN AMINES
Nov., 1949
function of time as shown in Fig. 5. No values are given for the loss factor since they were negligibly small except for n-octylamine and n-dodecylamine in the region below the freezing point where direct current conductance was observed. The omitted curve for n-hexadecylamine is similar to those in Figures 3 and 4.
.4;
8.2
-
3593
: \
I
3.0 26
30 34 38 42 46 Temperature, OC. Fig. 3.-Temperature dependence of dielectric constant of n-tetradecylamine. Hollow circles represent values at 0.5 kc., half-filled circles values a t 5.0 kc., and filled circles values at 50 kc.
j 2.8 U
-
i! 8
.%
-
2.6 -
5
8
2.4
-
42 46 50 54 Temperature, “C. Fig. 4.-Temperature dependence of dielectric constant of n-hexadecylamine a t 5.0 kc. 38
Discussion of Results No transitions in the solid are evident in any of the compounds studied, and all of the materials appear to freeze predominantly into the stable non-rotator form. Thus the large direct current conductivity effect originally sought in this research cannot appear in full force since no a phase producing sheets of rotating -NHz groups appears. Proton transfer demands molecular rotation and a convenient path such as a sheet of -NH2 or -OH groups. A small conductivity effect appears in the two shorter amines even though few of the molecules are rotating. The effect is thus analogous to the conductivity observed in n-tetradecyl, n-octadecyl and n-docosyl alcohol below the transition point. The conductance is manifested as Maxwell-Wagner polarization as shown in Figs. I , 2 and 5. The dispersion should not be confused with anomalous dispersion, since, a t low frequency, the dielectric constant exceeds the extrap-
2.8
I
1
L 80 100 Time in minutes.
240
Fig. 5.-Dependence of dielectric constant of n-dodecylamine a t 0’ on time. Sample was cooled rapidly from 100 to 0’. Hollow circles represent values a t 0.5 kc., half-filled circles values a t 5.0 kc., and filled circles values a t 50 kc.
olated liquid value (Fig. 5 ) and the loss factor, e”, was observed to exceed (EO - e , ) / 2 , where EO is the static dielectric constant and e,, the so-called optical dielectric constant. It is thus clear that, as in the alcohols, the high dielectric constant is not due to orientation polarization. When E ” 2 e’ and when two phases of different conductivity are present, large dielectric constants exceeding those possible by orientation polarization are commonly observed.la I n n-octylamine and n-dodecylamine the high conductivity observed for some distance below the freezing point makes e” 2 e’, and causes the dielectric constant to be abnormally high, suggesting the possible coexistence of two phases with proton transfer occurring in one of them. I n Fig. 5 i t is shown that the dielectric constant of n-dodecylamine a t 0” a t low frequency drops with time, just as was found for n-octadecyl alcohol. This indicates that a slow change of condition perhaps, a trace of vertical phase transforming to tilted, often called a &-pZ transition, is taking place, wherein molecules capable of molecular freedom and proton transfer are reorienting into more stable positions in the lattice. It should be emphasized that the combination of proton transfer and Maxwell-Wagner effect greatly magnifies the effect of what is undoubtedly a rat-her small amount of molecular freedom in the solid state. The pronounced maximum in the low frequency dielectric constant which occurs a t about 20” below the freezing point in n-octylamine and ndodecylamine has no anal5g in the alcohols. The maximum would seem to suggest a second order transition, but this view must be regarded with
3594
JOHN
D. HOFFMAN AND
CHARLES
P.SMYTH
Vol. 71
caution. It has been shown that the observed di- amines is weaker than in alcohols or acids. The electric constant due to Maxwell-Wagner effect differing degree of hydrogen-bonding may be seen depends on the ratios of the conductivities of the by comparison of the boiling points of the eighttwo phases.'*r8 It is much more likely that the carbon acid, alcohol and amine. n-Octanoic acid ratio of the conductivities of the two phases varies boils a t 237O, n-octyl alcohol a t 1 9 5 O , and n-octylin such a way as to produce a maximum of the amine a t 179'. An unassociated long-chain moletype shown in Figs. 1 and 2. cule will generally exhibit rotation about the long Molecular freedom in the solid state may have axis, as manifested by solid transitions, if i t is a t several origins. Impurities may cause the forma- least eighteen to twenty-four carbon atoms long. tion of small amounts of liquid with correspond- Thus, n-docosyl bromide and n-docosane show ing molecular freedom. Premelting9 has been at- monotropic transitions, while n-octadecyl brotributed by Oldham and Ubbelohde to the effect mide and n-hexadecane do not. Since hydrogenof a network of cooperative flaws which breaks up bonding will change the effective chain length, we a crystal into a mosaic producing a sort of unsharp might expect the compounds with the strongest melting and molecular freedom. The flaws may hydrogen-bonding (e. g., acid dimers) to have stem from impurities or thermal fluctuations. transitions a t shorter formula weight chain length Prenielting is generally considered to be operative than weakly bonded substances. In accordance only near the melting point. Another cause of mo- with this, acids show transitions when the chain lecular freedom in the solid state is the rotation of length is eleven carbons," and alcohols a t thirteen a molecule from one equilibrium position to an- carbons," while amines do not show transitions a t other due to cooperative loosening of the lattice. eighteen or shorter. Thus, the eleven-carbon The term "prerotation" has been used to describe acid dimer, which has an effective chain length of this effect below the transition point.1° This ef- about twenty-four atoms behaves like a hydrocarfect will also occur if no transition point is present. bon of that length. It should be pointed out that, Operationally, prerotation differs from premelting in addition to increasing the effective length of a in that i t covers a much wider temperature range, molecule, hydrogen-bonding may stabilize the and its onset is more gradual. Prerotation will be tilted form, which does not rotate, and thus alter used henceforth in this paper to describe the occa- the chain length a t which transitions are observed. sional rotation or reorientation of molecules in a It is likely that a solid rotator state will be found solid a t any temperature, probably due to coopera- in amines of twenty-two to twenty-six carbon tive loosening of the lattice. Finally, molecular atoms in length. The evidence for proton transfer rests mainly on freedom may result in a solid from the presence of a stable or unstable rotator phase, or a substance the appearance of direct current conductivity acwhich contains a portion of such a rotator phase. companied by strong Maxwell-Wagner polatizaA rotator phase will yield a liquid-like dielectric tion below the freezing point in n-octylamine and n-dodecylamine, and the similarity of this pheconstant. In the present case, impurities and premelting nomenon to that found below the transition points may be ruled out as the cause of the molecular in the alcohols. The dielectric constant showed freedom in the lattice 20' below the freezing point time dependence in these materials, which in the two shorter amines, so the effect must be strengthened the correlation with the alcohols. due to prerotation or the presence of a small Since molecular rotation is a requirement for amount of a rotator phase which is unstable. In- proton transfer, some molecular freedom was indiasmuch as prerotation is an equilibrium phenome- cated. It has been pointed out that the failure of non, it would appear, in view of the drop in dielec- any rotational transitions to appear in the amines tric constant with time observed in n-dodecyla- may be due to the different degree of hydrogen mine, that the major part of the molecular freedom bonding in the amines as compared to the alcomay be due to the presence of a small amount of hols, Prerotation was observed in the higher unstable rotator phase. The longer amines show a members of the series. definite decrease of dielectric constant with temSummary perature far below the freezing point with no MaxThe dielectric constants of n-octyl, n-dodecyl, well-Wagner effect. This is characteristic of pre- n-tetradecyl, n-hexadecyl, and n-octadecyl amines rotation. The decrease of conductivity in the have been investigated in the vicinity of the m d t longer amines may be ascribed, in part, to the fact ing point. No solid phase corresponding to all the that fewer dipoles are present per cc. and, in part, molecules rotating about their long axes wm deto a lower cooling rate used in the measurements. tected, so that the large apparent direct current The amines differ from the corresponding alco- conductivity due to proton transfer associated hols in that they have no rotational transitions in with the rotator state could not be investigated. the solid in the range studied. This may be con- I n n-octylamine and n-dodecylamine the dielecnected with the fact that hydrogen-bonding in the tric data indicated the presence of a small con( 8 ) Frosch, A n n . Physik, 43, 264 (1942). ductivity in the solid state attributable to proton (9) Oldham and Ubbelohde, Proc. Roy. SOC.(London), 15178, 5 0 transfer. The dielectric data for the longer mem(1940). (10) Smyth, Trans. Faraday Soc.,
42A, 175 (1946).
(11) Meyer and Reid, THIS JOURNAL, 66, 1574 (1933).
Nov., 1949
A SYNTHESIS OF VINYLCYCLOPROPANE
bers indicate some molecular freedom below the freezing point, but no Maxwell-Wagner polarization due to conductivity was encountered in the cases of the three longer molecules. The dielectric
[CONTRIBUTION FROM THE
DEPARTMENT OF
3595
properties of the alcohols and amines have been compared, and tentative reasons advanced for the absence of the rotator state in the amines. PRINCETON,
NEW JERSEY
RECEIVED MARCH 29, 1949
CHEMISTRY O F THE OHIO STATE UNIVERSITY ]
A Synthesis of Vinylcyclopropanel BY Ross VANVOLKENBURGH, K. W. GREENLEE, J. M. DERFERAND C. E. BOORD
The hydrocarbon vinylcyclopropane is interest- the failure of previous workers to observe it. The ing from both the synthetic and theoretical points structure of the hydrocarbon was proved by an of view. I n 1922, Demjanov and Dojarenko2 ozonolysis experiment from which both formaldeprepared i t by exhaustive methylation of the am- hyde and cyclopropanecarboxaldehyde were idenine obtained from the oxime of methyl cyclopropyl tified. The physical properties listed in Table I ketone, and they stated that this hydrocarbon were determined on a sample of vinylcycloprocould not be prepared by the dehydration of pane, estimated from a time-temperature freezmethylcyclopropylcarbinol. It has sometimes ing curve to be 96 * 2 mole % pure. Evidence for conjugation between the double been assumed that vinylcyclopropane is "incapable of existence."a Recentlv there has been specula- bond and the cyclopropane ring of vinylcycloprodon concerning the possible utility of the hydrocarbon in the manufacture of synthetic r ~ b b e r . I~n the present 80 - C h a r g e - 38 7grornr D i a l i l l o l e - 28 9 grams work methylcyclopropylcarbiriol was Residue - 6 5 grams ( = I 4380) successfully dehydrated to vinylcyclopropane, and some of the hydrocar- e 70 2 bon's properties were observed. 0 The methylcycloprop ylcarbinol was prepared by reduction of methyl g 6 0 cyclopropyl ketone; four different P methods were tried. Catalytic hy- d 50 drogenation gave the desired carbinol Boillnp Point along with a nearly equal amount of Fi 40 the close-boiling 2-pentanol. ReducJ -1 Taken for Redlslillollon 1 tion by sodium and ammonium sulfate in liquid ammonia gave ringopening products exclusively. The Meerwein-Ponndorf method gave the desired carbinol in low yield along with large amounts of condensation products, The only method which gave methyl- pane is seen in the boiling point which is 4.5" cyclopropylcarbinol exclusively and in high yield higher than that of ethylcyclopropane; by conwas reduction with lithium aluminum hydride. trast, the double bond produces a 7.8' lowering The dehydration of methylcyclopropylcarbinol when the related 2-methylbutane is converted to was accomplished by refluxing i t with a catalytic 3-methyl-1-butene. Similarly, the refractive inamount of sulfuric acid, giving 3970 yield of vin- dex of vinylcyclopropane shows an elevation of ylcyclopropane with 0.8' boiling range (Fig. 1). 0.0264 (at 20") which approaches the elevation of The dehydration was extremely slow, apparently 0.0332 exhibited by 2-methyl-l,3-butadiene. because of the influence of the cyclopropane ring TABLE I adjacent to the carbinol group; this may explain
-
(1) This paper forms p a r t of the dissertation submitted in 1949 hy Ross Van Volkenburgh to the Graduate School of The Ohio State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. It was presented before the Organic Division a t the 116th meeting of the American Chemical Society. The investigation was sponsored by the American Petroleum Institute Research Project 45 in cooperation with The Ohio State Universit!, Research Foundation. ( 2 ) Demjanov and Dojarenko, Bcr., 65B,2718 (1822). (3) Whitmore, "Organic Chemistry," D. Van Nostrand Company, tnc., New York, N. Y.. 1937, p. 632. (4) Jones, Chcm. Ene. Ncws, 27, No. 7. 454 (1949).
VINYLCYCLOPROPANE This work
F. p., "C. E.p., 'C.(760mm.) d '0, ?a%
Literature2
-112.6 .... 40.41 40.0to40.2(755mm.) 0.7160 0.723 a t 18" 1.4156 1.4172at 15"
A small portion of the hydrocarbon was hydrogenated over Raney nickel. The product, insufficient for purification by distillation, was identi-
