JAMESE. CASSIDYAND WENDELL J. WHITCHER
1824
Vol. 63
THE POLAROGRAPHIC CHARACTERISTICS OF FURFURYLIDENEACETOPHENONE AND SOME OF ITS para DERIVATIVES BY JAMESE. CASSIDY] AND WENDELL J. WHITCHER Contribution from the Department of Chemistry, University of Vermont, Burlington, Vermont Received March 17,1969
Plots of the half-wave potentials of the furfurylideneacetophenones in a buffered acetate solution us. Hammett's u constants give essentially linear relationships. The slopes of these plots are similar to that of benzophenones, and indicate that an increase in the positivity of the carbonyl carbon facilitates the reduction.
Sigma values obtained from the Hammett function have 'been used to correlate quantitatively half-wave potentials of aromatic carbonyl compounds with the type of substitution present in the rings. I n attempting any such comparison all substances and all operating conditions must be very similar due to the fact that organic reactions for the most part are irreversible and therefore the half-wave potentials are not directly related to the change of the free energy for the electrode process.2 Benzophenone, benzalacetophenone and furfurylideneacetophenone are similar in nature with the exception that the first compound does not have a double bond in conjugation with the carbonyl. 0
0
benzophenone
benzalacet~ophenone 0
apparatus, the half-wave potential of zinc was found to be in good agreement with the values in the literature. No correction was made for the I R drop or the junction potential. All polarographic analyses were executed with the H cell placed in a bathat25.0 & 0.5". The capillary, when in the stock solution, had these characteristics a t zero potentiaI: drop time = 3.70 sec., m = 2.46 mg./sec., and m%t1/6 = 1.73. A stock solution ( I M , 3: 7 ratio of acid to salt) was used as buffer, solvent and supporting electrolyte throughout this investigation. It was prepared by dissolving 57.40 g. of sodium acetate and 17.2 ml. of glacial acetic acid in an alcohol-water mixture and then diluting to a liter. The alcohol-water mixture was prepared by mixing equal volumes of distilled water and isopropyl alcohol. The apparent p H of the stock solution was 5.90 using a Beckman pH meter, Model G. The p-bromo-, p-methyl-, p-methoxy- and the p-hydroxyfurfurylideneacetophenones were prepared by the conventional condensation' of furfural and the substituted acetophenone in alcoholic sodium hydroxide (see Table I). With the exception of the para-hydroxy derivative, a yield of about 50% for each product was obtained when the reaction mixture was cooled in an ice-salk-water bath. For precipitation of the para-hydroxy compound, acetic acid had to be added to the cooled reaction mixture.
TABLE I DERIVATIVES OF FURFURYLIDENEACETOPHENONE
--
Each product was recrystallized from hot ethyl alcohol furfurylideneacetophenone
Compound
Color
Lit.
M.p., ('2.Found
Brockman and Pearson3 have completed a study of the derivatives of the benzophenones and Geissman and Friess4 have studied some of the benzalacetophenones. The reductions of these compounds yield dimeric compounds at a low pH, whereas a t higher values the saturated ketone is mainly founde5 However, investigations6 have shown that a sharp delineation between the two processes does not exist. The purpose of this paper is to present polarographic characteristics of some of the furfurylideneacetophenones, to plot the half-wave potentials against the proper sigma values, and t o show the similarity of the reduction of these compounds to those of benzalacetophenones and benzophenones. Experimental
p-Bromo Yellow 80-81" 78 67b 63-64 pMethyl Yellow 75-7gc 75-76 p-Methoxy Yellow 160-161 Yellow None p-Hydroxy N . Maxim and J. Angelesco, BuEl. SOC. chim., 51, 1365 (1932). N . Maxim and I. Popescu, zbzd., 58, 89 (1939). R. Robinson and W. M. Todd, J. Chem. Soc., 1743 (1939).
(1) American Cyanamid Go., Stamford, Conn., to whom inquiries should be directed. (2) P. Delahay and W. Vielstioh, J . Am. Chem. Soc., 77, 4955 (1985). (3) R. W. Brocknian and D. E. Pearson, ibid., 74, 4128 (1953). (4) T. .4.Geissman and S. L. Friess, ibid., 71, 3893 (1849). (5) R. Pasternak, Helv. Cliim. Acta, 81, 753 (1948). (6) H.J. Gardner, Chemistill & I n d u b t r y , 819 (1948).
The polarograms of furfurylideneacetophenones and benzalacetophenones3 are very similar in that they all have three distinct waves. Of the two series, the furfurylideneacetophenones are the easier
Q
Freshly prepared solutions in the range of 2 X 10-4 to 6 X 10-4 M gave polarograms which had total diffusion currents that were directly proportional to the concentrations. These colorless solutions, which became yellow on standing, showed an 8% decrease in the total diffusion currents after standing two days. I n comparison no such changes were noted in benzalacetophenone solutions. The procedure for each polarographic run was standardized as follows: a 20-ml. sample of fresh solution was flushed with oxygen-free and solvent saturated nitrogen, and the voltage-current relationship immediately recorded. From the resulting plots, the half-wave potentials (Ei/,'s) and the diffusion currents (id's) A manually operated polarograph, Sargent Model 111, for each step were determined graphically. These results are was used with a polarographic H cell with a saturated calo- given in Table 11. mel cell as a reference. The calibration factor of the galvanometer was 0.0058 microampere per millimeter. With this Discussion of Results
(7) N. L. Drake and H. W. Gilbert, J . Am. Chem. Soc., 62, 4965 (1930).
POLAROGRAPHIC CHARACTERISTICS OF FURFURYLIDENEACETOPHENONE 1825
Nov., 1959
TABLEI1 POLAROGRAPHIC REDUCTIONS OF FURFURYLIDENEACETOPHENONE Substituents
Waves
Unsubstituted
a
Ellava. (S.C.E.),
1st 2nd 3rd p-Bromo 1st 2nd 3rd p-Methyl 1st 2nd 3rd p-Methoxy 1st 2nd 3rd p-H ydroxy 1st 2nd 3rd H. H. Jaffe, Chem. Revs., 53, 191 (1953).
Concn., mmoles
V.
id,
*amp.
0.522
0.845 1.18 1.35 0.780 1.12 1.31 0.851 1.23 1.38 0.887 1.23 1.42 0.96 1.30 None
.500
.500
.518
.487
to reduce. The data for both series show that electron releasing groups decrease the ease of reduction a t the electrode whereas the electron withdrawing groups increase it. The id/c values for both series are of the same order of magnitude; but when these values and those for benzophenone are compared, the latter are greater by a factor of about 2. The most interesting results are those in Fig. 1 which relates Hammett’s a-values to half-wave potentials. That the plots of the furfurylideneacetophenones and benzophenones are not only linear but parallel indicates these waves are subject to the same type of structural influences and suggests that the rate-controlling electron transfer may occur a t the same site for all steps. The plot of the three benzalacetophenones though linear is not parallel to the others. The reason for this difference may
id/C
u4
0
2.11 2.39 1.11 2.34 2.49 1.64 2.18 2.18 1.04 2.13 2.44 1.01 2.08 2.42
1.10 1.25 0.580 1.09 0.957 0.943 1.09 1.07 0.552 1.10 1.26 0.582 0.914 1.28
,232
-
.170
-
,268
-
,475
probably a nucleophilic attack on the carbonyl by an electron resulting in the formation of a free radical. \c=o .++ -Lij e + H + + \C-OH / I / In the case of the benzophenones, which give only one w a ~ e ,the ~ , free ~ radical is not stable and thus is reduced immediat.ely to the carbinol; whereas the furfurylideneacetophenones can form a stable free radical which can undergo a subsequent reduction at the electrode. A possible explanation for this difference is that the double bond causes a decrease in the electron density a t the carbonyl and results in a relatively stable free radical. The reduction sequence of the furfurylideneacetophenones may be represented as
+
H
H
Tautomerism
0
U
I
IV
I1
I11 H
VI
V
U
U
VI1
VI11
be the higher pH and the change in buffer systern; however, due to the h ~ i t e ddata available, little significancecan be attached to this slope. In the case of the furfurylideneacetophenones each wave apparently corresponds to an one electron reduction process.4.6 Consistent with the views of 0thers,3J,6,~ the initial step in each case is
The mechanism as outlined above accounts for the three waves and for the parallelism of the slopes in Fig. 1. The latter observation may be due to the electron and proton transfers occurring a t the site for each reduction step. Another tautomeric form of
(8) J. W. Baker, W. V. Davies and M. L. Hemming, J . Chern. SOL,
(9) R . .4.Day, Jr., and J. J. Kirkland, J . Am. Chew. Soc., 1 2 , 2766 (1950).
692 (1940).
1826
ARNULF MUANAND W. C. HAHN,JR.
Vol. 63
1.40 1.30 1.20 1.10 1.00 3,3 d l - b
0.80 I
'
-0.400 -0.200 0 0.200 0.400 0.600 0.800 p-OH p-OCHs p-CH3 p-Br Sigma. Fig. 1.-Half-wave potentials vs. U-values: (1) 1st step, furfurylideneacetophenone; (2) 2nd step, furfurylideneacetophenone; (3) 3rd step, furfurylideneacetophenone; (4) benzophenone, 0.1 N acetate bufferpH 5.2 (R. W. Brockman and D. E. Pearson, J. Am. Chem. Soc., 74 4128 (1952)); 15) 1st step, chalcone, ( CH3)aNOH-acetk acid buffer T. A. Geissman and S. L. Fries, J. Am. Chem. Soc., 71, 3893 (1949)).
However, the electron attack and protonation in this case would not be a t the carbonyl site. Though this reduction mechanism is not conclusive, the second wave does represent the formation of the saturated ketone VI1 a t the electrode surface. The first two waves are simple diffusion controlled reductions representing one electron transfers, whereas the third wave does not represent such a simple reduction. The id/C values for this wave are about half those for the first and second waves. In addition, a change in the concentration of the buffer used in this investigation results in decided differences in the heights of the third wave. The polarographic characteristics of this third wave as a function of the buffer are being investigated.
SOME ENERGY RELATIONS IN SOLID STATE REACTIONS INVOLVING CRYSTALLINE PHASES OF VARIABLE COMPOSITIONS BYARNULF MUANAND W. C. HA",
JR.
Contribution No. 68-101 from College of Mineral Industries, The Pennsylvania State University, University Park, Pennsylvania Received March 19,1969
In a recent experimental study of the equilibrium between hausmannite and manganosite according to the approximate equation 2MnaOd = 6Mn0 021, it was observed that manganosite used as starting material reacted with oxygen of the atmosphere to form hausmannite as a metastable phase a t conditions of temperature and 02 artial pressure within the stability field of mangrtnosite but close to the manganosite-hausmannite boundary curve. $his apparently anomalous reaction is explained in terms of a free energy-composition diagram for the system Mn-0 Implications of this observation on criteria used for judging equilibrium in some solid state reactions are indicated.
+
Introduction The equilibrium between hausmannite and manganosite according to the approximate equation'
reducing conditions (pO2 = atm. a t llOOo), and hausmannite had been prepared by thermal decomposition of MnOz in air a t 1100". After equilibrium was attained among gas and crystalline 2MnsO4 = 6Mn0 + 02 phases, the samples were quenched rapidly to was studied recently by Hahn and Muan,2 using room temperature and the phases present deteran open system in which the desired 0 2 partial mined by using X-ray diffraction. In some cases, pressures of the gas phase were attained by mixing supplementary data were obtained by using only COzand O2or COZ and HZ. one starting material, either manganosite or hausThe method consisted in equilibrating two mannite, and following the progress of the reaction samples side by side a t constant temperature and by frequent weighing of the sample in a simple chosen constant O2 partial pressure, the starting thermal balance setup. material in one sample being manganosite and in The results of the investigation are summarized the other hausmannite. The manganosite had in Fig. 1, where the solid line represents conditions been prepared in advance from Mn02under strongly for coexistence of the two phases hausmannite (1) Hausmannite has approximately stoichiometric MnrOi com- and manganosite in stable equilibrium. position and occurs in two modifications, a low temperature ( c l l 6 0 " ) In preliminary experiments for determination of tetragonal and a high temperature ( > l l 6 O o ) cubic form. Manganosite, with sodium chloride structure, has variable composition, the equilibrium between the two crystalline phases, Mnl-.O.~~' the transformations of manganosite to hausmannite (2) W. C. Hahn, Jr., and A. Muan, Am. J. Sci. (in press). vice versa were studied under conditions of and (3) M. LeBlano and G. Wehner, Z. phyaik. Chem., A168,59 (1934). constant temperature and partial pressures of Oa (4) T. E. Moore, M. Ellis and P. W. Selwood, J. Arner. Chem. which were later found to be well within the regions Soc., 18, 856 (1950).
