2303
BROMINATION OF DIETHYL FUMARATE ON A SILICA SURFACE effective quenching by bonding between the lone pair on the oxygen and the amine proton. In summary, the comparison of the fluorescence behavior of 2-aminopyridine with that for 2-(N,N-dimethy1amino)pyridine indicates that the lowest excited * both molecules. The decrease of the state is a , ~ in fluorescence yield in acid solutions for 2-(N1N-dimethy1amino)pyridine and 2-(N-benzylamino)pyridine is due to electron transfer resulting from a lower ionization potential relative to 2-aminopyridine, The shift of the 0-0 band to a higher value for 2-(N-benzylamino)pyridine when going from acidic to basic media, ie., more basic in the excited state, is expected for an aromatic heterocyclic molecule. The large fluorescence yield for 2-(N-benzylamino)pyridine in 10 N H2S04 may be the result of protonation of the amine lonepair electrons which would inhibit electron transfer to
the ring and tlius lead to an increase of fluorescence yield as observed with 2-aminopyridine.' In fact, the largest yield for this molecule is observed in 10 N H2S04. I n 2-(N-phenylamino)pyridine the fluorescence quenching by ether may be due to bonding between the lone pair electrons of oxygen and the amine proton, which is compatible with increased charge transfer to the heterocyclic ring. The absence of fluorescence in oxygen- and hydrogen-bonding solvents arising from hydrogen-bonding effects is opposite to the observed behavior of simple aminopyridines, but not a generalization for heterocyclic molecules. The results presented in this study illustrate the difficulty and caution in trying to generalize the behavior of closely related molecules in their electronically excited states.
Reactions of Adsorbed Organic Molecules.
I. Bromination of Diethyl
Fumarate on a Silica Surface by M. J. Rosen and C. Eden' Department of Chemistry, Brooklyn College of the City University of New York, New York, New York (Received J u l y 9, 1969)
11810
A study has been made of the bromination of diethyl fumarate adsorbed onto Cab-0-Sill a nonporous hydroxylated silica. The results indicate that the reaction is completely stereospecificand yields exclusively the transbromination product, meso-diethyl 2,&dibromosuccinate. The kinetics of the reaction indicate first-order dependency on the olefin adsorbed close to the surface and on the bromine adsorbed on the Cab-0-Sil. A mechanism for the reaction is proposed.
Very little is known of the effect of surfaces on the bromination of olefins. For the one reaction studied, the bromination of ethylene on glass12it was shown that the presence of a polar surface is essential for the reaction. Preliminary work in this laboratory3showed that silica surfaces exhibit a marked catalytic effect on the bromination of such olefins as oleic and cinnamic acids, both in the presence and absence of a solvent. The rate of bromination, in fact, was so rapid that meaningful kinetic data were difficult to obtain. Since diethyl fumarate is very unreactive to bromine in the absence of st polar solvent or surface and has no allylic hydrogens whose presence might complicate the reaction, this compound was selected for investigating the effect of a polar surface on the bromination reaction. The surface selected for investigation was Cab-O-Sil14a well-defined, nonporous hydroxylated silica.
Experimental Section Materials and Apparatus. The diethyl fumarate (DEF) and diethyl maleate (DEM) used were both Eastman Kodak Co. White Label grade, redistilled before use. The bromine (Baker Analyzed) was kept over P205. The Cab-0-Si1 used in the experiment was Type M-5. It had a surface area of 200 li: 25 m2/g and a surface OH concentration of 0.9 mmol/g.6 Before use, it was (1).On leave from the Physical Chemistry Department, The Hebrew University, Jerusalem, Israel. (2) (a) T.D. Stewart and K. P. Edlund, J. Amer. Chem. Soc., 45, 1014 (1923);R. G.W.Norrish, J . Chem. Soc., 123, 3006 (1923); (b) G.Williams, J . Chem. Soc., 1747,1758 (1932). (3) M. J. Rosen and A. Silverstein; M. J. Rosen and C. Eden, unpublished results. (4) Cabot Corp., Boston, Mass. (5) R. Z. Naar, Cabot Corp., private communication. T h e Journal of Phy&cal Chemistry, Vol. 74,No. 11, 1070
2304
M. J. ROSENAND C. EDEN serted into R until it was beneath the gas stream and the treated Cab-O-Si1 poured into R, the argon again serving as a blanket to prevent contact with water vapor or air. The stoppered empty flask was weighed again and the difference in weight was recorded as the weight of impregnated adsorbent used. The introduction of the sample was done while the barrel of stopcock R was out and no grease present, and only after the sample had been added was the greased barrel inserted. (The vacuum grease used was Kel-F No. 90.) At this stage liquid bromine was introduced into arm B and outgassed by repeated freezings and thawings, using liquid Nz. The whole apparatus was evacuated to 10-6 Torr through joint V. Arm R was covered with opaque black material to exclude light. The apparatus was put into a Gilford-modified Beckman DU spectrophotometer equipped with a specially constructed light-proof cover and a jacketed spectral cell holder connected to a water bath maintained at 25 0.05". With stopcocks V and R closed, bromine vapors were introduced from arm B into arm S. With stopcocks B, V, and R closed, the optical density of the bromine at X = 416 nm was measured, and the initial amount of bromine was calculated, using E 170.6 Stopcock R was then opened, and the rate of bromination was followed by the change in the optical density of the bromine in the gas phase as a function of time. Initial concentrations of D E F (A"/g) used varied from 0.14 mmol/g of Cab-O-Si1 to 0.8 mmol/g; initial bromine concentrations (B'lg) used varied from 0.21 mmol/g of Cab-O-Si1 to 1.26 mmol/g. Concentrations beyond these ranges could not be used because of limitations imposed by the size of the apparatus and the accuracy of optical density measurements. Measurements of adsorption of bromine on Cab-O-Si1 covered with brominated diethyl fumarate (BDEF) showed that 10-40 min, depending on the bromine pressure, was required to reach adsorption equilibrium. As a result, only kinetic data taken after the first 30-40 min were used. Since 5-6 hr was required to complete a kinetic study, the data recorded for the rates of reaction are considered to be at adsorption equilibrium. The amount of unreacted bromine, B o - x, in the system at any time (where x = the amount reacted at that time) was obtained by adding the amount of bromine adsorbed on the surface to that in the gas phase. Thus B o - 2 = Ba [B]V (1)
*
S
Figure 1. Apparatus for measurements of adsorption isotherms of bromine on Cab-O-Sil.
dried for 3 days a t 110" and kept stoppered in a desiccator over Pz06. The apparatus used (Figure 1)consisted of three parts connected by stopcocks. Arm B is a reservoir for dry, out-gassed liquid bromine. Arm S-the central partcontains at the bottom a l-cm spectrophotometer cell; arm R is a water-jacketed reaction cell connected to a water bath maintained at 25 i 0.05'. The apparatus is of such dimensions that the spectrophotometer cell fits into the cell holder of a Beckman DU spectrophotometer. Measurements were made in a room maintained a t 25 f 1". Preparation of the Xurface. The desired quantities of diethyl fumarate (DEF) or diethyl maleate (DEM) in Spectral grade CC1, solution were added under argon to the Cab-O-Si1 in a long-necked Erlenmeyer flask. The CCL was evaporated under reduced pressure in a rotary evaporator and then on a vacuum system to a constant weight. Before removal from the vacuum system, the Erlenmeyer flask containing the treated Cab-O-Si1 was filled with dry argon to prevent contact with air or moisture. Kinetic Studies. While argon was flowing into the reaction apparatus through joint V and out the top of arm R, the long neck of the Erlenmeyer flask was inThe Journal of Phg8ical Chemistry, VoL 74, No. 11, 1970
+
where V = total free volume of sections R and S of the apparatus. (The experimental error in Bo - 2, is estimated at f5%.) The amount of bromine, Ba, on the surface at any bromine vapor concentration, [ B ] was , taken from the (6) A. A. Passchiev, J. D. Christian, and N. W. Gregory, J. Phys. Chew., 71, 937 (1967).
2305
BROMINATION OF DIETHYL FUMARATE ON A SILICA SURFACE
II
Figure 2. Nmr spectrum of BDEF in CHCL. 2.0
I "
' ~ " " I3.0" " ~ " " I " 4.0 " ~ ' " ' l " ' 5.0 " " ' 'PPMITI I " ' ' ~6.0
" " I 7.0 " " ~
8.0 I "
. '
" I "
3w
r
I c
')
7.0
6.0
5.0
PPMId)
4.0
3.0
,
,
,
,
I
I . . . 2.0
, , , . I , , , , , , , I, I _ . . . I . , . . , . . . I . . , ' ,
1.0
0
Figure 3. Nmr spectrum of a mixture of DEM and DEF in CC14 solution (19% DEF).
adsorption isotherm of bromine on a BDEF-covered surface with the same coverage (moles/gram) as the DEF used. The error in using the isotherm for a BDEF-covered surface rather than that for a DEFcovered surface (which was impossible to obtain because of the simultaneous bromination reaction) is believed to be insignificant. Since the DEF is essentially nonvolatile under the conditions of our experiments, its initial concentration on the surface was taken as equal to that originally used in treating the surface.
Adsorption Isotherms of Bromine on Cab-0-Xi1 Covered with Brominated Diethyl Fumarate (BDEF). The same apparatus and procedures for preparation of the surface and introduction of the sample were used in the determination of these adsorption isotherms. However, after measuring the initial optical density of the bromine in arm S, with stopcock R closed, the latter was opened and the bromine vapors were allowed to reach equilibrium with the adsorbent. The optical density was again recorded, and from the predetermined volumes of arms S and R the amount of bromine The Journal of Physical Chemistry, Vol. ?a, No. 11, 1070
2306
t
M. J. ROSENAND C. EDEN
-1
Figure 4. Nmr spectrum of a mixture of BDEM and BDEF in CCl, solution (only methylene quartet part): 1, BDEM; 2, BDEF.
adsorbed per gram of Cab-O-Si1 was calculated. Further additions of bromine from arm B permitted the recording of the full adsorption isotherm.
Results and Discussion Nature of the Bromination Product (BDEF). The nmr spectrum and other physical properties of the bromination product show it to be the product of trans-bromination of diethyl fumarate, meso-diethyl 2,3-dibromosuccinate. The nmr spectrum of diethyl fumarate has a peak a t 398 cps (6.63 ppm) corresponding to the transolefinic hydrogens; in the brominated product (Figure 2), extracted directly from the surface by CHCIS, without further purification, these hydrogens are a t 280 cps (4.67 ppm). The relative intensity of these hydrogens to the quartet of the methylene group is 1: 2, as expected for a completely brominated product. The spectrum in the Vicinity of the methylene peaks (inset Figure 2) shows no trace of the isomeric cis-bromination product (see below). To check the stereospecificity of the bromination, a diethyl maleate-diethyl fumarate mixture was adsorbed onto Cab-O-Si1 and brominated by the same procedure used for DEF. The original mixture contained approximately 19% DEF, as indicated by the relative intensities of the D E F and DEM olefinic hydrogen peaks at 398 (6.63 ppm) and 364 cps (6.07 ppm), respectively (Figure 3). After bromination, the product contained The Journal of Physical Chemistry, Vol. 74, No. 11, lQ7O
approximately 20% BDEF, as indicated by the relative intensities of the peaks in the methylene quartets for BDEF and BDEM (Figure 4). The bromination product is a solid, crystallizing in needles, mp 58”, in agreement with the literature.’ By contrast, the cis-bromination product, dl-diethyl 2,3-dibromosuccinate, is a liquid.’ The possibility of isomerization of D E F to DER1 on the Cab-O-Si1 surface was eliminated by a run in which DEF was adsorbed onto the surface, but no bromine added. Extraction of the DEF from the surface and examination of its nmr spectrum showed it to be identical with the original. No trace of a peak corresponding to cis-olefinic hydrogen (at 364 cps) was found. It is concluded therefore that the bromination of both D E F and DER1 on this surface proceeds via an exclusively trans-bromination mechanism. Mobility of the Adsorbed D E F and BDEF. Bromination experiments using 0.8 mmol DEF/g, equivalent to more than four monolayers (see below), showed that a stoichiometric amount of Br2 was consumed in the reaction and that the bromination product had no unbrominated material (as evidenced by the absence of olefinic hydrogen adsorption peaks at 398 cps in the nrnr spectrum). Since Brz in the gas phase does not react significantly, under the conditions of our experiments, with DEF which is not adsorbed onto a surface, this implies both lateral and vertical diffusion of the DEF and its bromination product on the surface under these conditions, in the absence of any solvent. Adsorption Studies. Adsorption isotherms of D E F and BDEF from hexane onto Cab-O-Si1 are given in Figure 5 . From these curves, the monolayer values of DEF and BDEF are 0.183 and 0.150 mmol/g, respectively. These values are in good agreement with the inverse ratio of their molecular cross-sectional areas as determined by models (98 iz for DEF and 125 i2 for BDEF). Adsorption isotherms of bromine onto bare Cab-O-Si1 and Cab-O-Si1 covered with various amounts of BDEF and 25” are illustrated in Figure 6. These isotherms were taken after the termination of the bromination process. Only the uncovered surface of Cab-O-Si1 shows a Type I1 BET isotherm; the other isotherms are of Type 111. All isotherms were found to be reversible. The amounts of adsorbed bromine per gram of Cab-O-Si1 are very close to being independent of the coverage of the surface by BDEF at low bromine pressures and low BDEF concentrations. At higher bromine pressures, there is dependency on the extent of coverage by BDEF. Contrary to what one might expect, the amount of bromine on the surface per gram of Cab-0-Si1 at high BDEF coverages increases with (7). “Dictionary of Organic Compounds,” Vol. 11, 4th ed, Oxford University Press, New York, N. Y., 1965, p 946.
2307
BROMINATION OF DIETHYL FUMARATE ON A SILICASURFACE
%I .2
-25-
rrmdIgr
A
-
c
I
,20
-
.15 *
.10 I
I
I
I
1
0
5
10
15
20
C.\$M
Figure 5 . Adsorpt,ion isotherm of D E F (0)and BDEF ( X ) from hexane onto Cab-0-Sil.
-
.05-
0
.1
.2
.3
.4
.6
.5
.7 A l g
Figure 7 . Kinetics of bromination of DEF (notations correspond to run no. in Table I): D',1; 0, 2; 8,3; U, 4; 0 , 5 ; x, 6; +, 7 and 8.
Figure 6. Adsorption isotherms of bromine on Cab-0-Si1 a t different coverages by BDEF. Ba/g = millimoles of bromine adsorbed per gram of Cab-0-Sil; [B] = equilibrium concentration of bromine in the gas phase. Amount of BDEF in moles per gram: 0 , O ; V, 0.39; A,O.O86; 0,0.088, Q0.183; +,0.200; 0,0.364; a, 0.532; 0 , 0.736.
g. For kinetic measurement it is important to distinguish between the two. Kinetics of Bromination. The rate of bromination of D E F was followed by determining the amount of unreacted bromine per gram of Cab-0-Sil, Bo - x / g , as a function of time. During the first 0.5 hr or so a sharp drop in the concentration of bromine in the gas phase was observed, due mainly to the establishment of adsorption equilibrium with the surface. Therefore, only data taken after the first 30-40 min were used for kinetic studies. The rate of bromination a t any instant [d(B" - x)/g]/dt =S was determined from slopes of plots of the above results. Plots of S/[B]vs. A/g, where A is the amount of unreacted diethyl fumarate at time t, are given in Figure 7. The initial concentrations ( AO / g and Bo/g) and the rate constants are given in Table I. The data indicate that the bromination is first order with respect to the gaseous bromine and ester on the surface, i.e.
Table I
coverage instead of decreasing. This effect begins to become apparent at a BDEF coverage of more than 0.2 mmol/g, which is slightly above the monolayer value for BDEF. This increased surface concentration can be explained &s soIubility of the bromine in the adsorbed multilayer of BDEF. The adsorption isotherms, therefore, illustrate two phenomena: (1) adsorption on Cab-0-Sil, which is linear with [ B ] up to a bromine vapor concentration of about 3 X loF3134, for low coverages by BDEF (0.2 mmol/g or less), and (2) dissolution in, or adsorption on, BDEF which is dependent on BDEF coverage and becomes significant at a BDEF coverage of more than 0.2 mmol/
S Run no.
1 2 3 4 5 6 7 8
AO/o, mmol/g
Bo/o, mmol/g
0.144 0.212 0.183 0.629 0.200 0.396 0.200 0.617 0.364 0.658 0.532 1.045 0.736 0.568 0.492 1.260 (+0.244 BDEF)
S -
A/o[Bl k, I./mol min
A*/QIBI = k,,
0.72 0.65 0.72 0.70 0.53 0.42 0.27 0.25
0.72 0.65 0.72 0.70 0.67 0.77 0.69 0.64
I./mol min
The Journal of Phgsical Chemistry, Vol. 74, No. 11, 1070
2308
However, the results of previous investigators2 and the accepted mechanism for bromination of olefins require a polar medium, which in the present case can only be the Cab-O-Si1 surface. It must be concluded therefore that only bromine adsorbed on the Cab-O-Si1 surface can be activated and react with the olefin. The adsorption studies of bromine on Cab-O-Si1 covered with BDEF (Figure 6) show that at low coverages and bromine pressures the amount adsorbed is linear with the pressure in the gas phase. Under these conditions, therefore, the rate of bromination is also proportional to the concentration of adsorbed bromine. At high bromine pressure and/or high BDEF coverages, however, the bromine concentration on the surface is not proportional t o the bromine concentration in the gas phase and under these conditions, consequently, the rate of bromination is not proportional to the bromine on the surface. The explanation is that the adsorption of bromine directly on the Cab-OSi1 surface must be distinguished from the second effect causing disappearance of bromine from the gas phase, namely, dissolution of bromine in, or adsorption on, BDEF. This effect, which is superimposed on the adsorption isotherm, becomes significant only at high bromine pressures and at coverages higher than 0.2 mmol of BDEF per gram of Cab-O-Sil. Only the bromine adsorbed onto the Cab-O-Si1 surface, not that dissolved in or adsorbed onto BDEF, is “active” bromine and can act as a brominating agent. For concentrations of A”/g up to approximately 1.5 monolayers (0.29 mmol/g), the value of the rate constant, k , in eq I1 is 0.69 & 0.04 l./mol min. As A ” / g is increased above this value, the rate constant drops. Since all of the ester used in our experiments was eventually brominated, the drop in rate constant with increase in A”/g above the value equivalent to about 1.5 monolayers implies that above this concentration only a fraction of the unreacted ester is “active” at any instant. The change in rate constant in the neighborhood of the equivalent of 1.5 monolayers of adsorbed ester may therefore be a reflection of the collision diameter of the “active” bromine, signifying that the DEF in the upper layers can react only after diffusion to the surface of the Cab-O-Sil. Hence, the values of k , for the experiments where A”/g is higher than 0.29 mmol/g have to be corrected accordingly. The amount of “active” ester, A*, at any instant is A (0.29/A ”) for experiments where A ” exceeds 0.29 mmol/g, and therefore the corrected rate constant, k,, equals kA0/0.29. These corrected values are given in the last column of Table I. When corrected in this manner, the rate constants for all runs fall within the range 0.69 0.08 l./mol min. The concentration of “active” bromine, Ba*, abThe Journal of Physical Chemistry, Vol. 74,No. 11, 1970
M. J, ROSENAND C. EDEN sorbed directly on the Cab-O-Si1 surface can be introduced into the rate equation by using the linear correlation between the concentration of bromine in the gas phase and the adsorbed bromine at low bromine pressure and low coverages. Since the slope of the linear portion of the adsorption isotherm is about 0.065 l./g, this yields a rate constant for the bromination reaction (k2) of about 10 g/mol min. On the basis of the foregoing, a possible mechanism for the bromination reaction is SiOH*
+ Br2(g)
ki
SiOHBr2* (fast, reversible)
(1)
k-1
SiOHBr2*
+
-%
SiOH*Br-.
\
\/ C / SiOH*Br-- +Br + Br,(g) ””, \
SiOHBr2*
+
Br-C-
I
1
-C-Br
(3)
(fast)
I where SiOH” represents an active OH group on the Cab-O-Si1 surface. According t o this mechanism, the effective brominating agent is a complex formed by adsorption of a molecule of bromine onto an active OH group on the Cab-OSi1 surface. This is consistent with infrared absorption studies on Cab-O-Si1 which indicate that there are no Lewis acid sites on this surface* and that the hydrogen of the free OH groups on the surface is used to hydrogen bond polar and polarizable molecules to the surface.s This complex reacts with a molecule of olefin adsorbed close to the surface in a rate-determining step which is first order in both adsorbed olefin and bromine adsorbed on active sites on the Cab-O-Si1 to yield an ion pair on the surface. Rapid reaction of this ion
(8) N. W.Cant and L. H. Little, Can. J. Chem., 43, 1252 (1985). 79,850 (1957); J. Phys. (9) R.S. McDonald, J. Amer. Chem. SOC., Chem., 62, 1168 (1958).
2309
CONFIRMATION OF SPONTANEOUS SPREADING BY WATERON PURE GOLD pair with a molecule of gaseous bromine yields the dibromide product and regenerates the active bromine complex. The rate equations corresponding to the proposed mechanism are
'
K=-= k-1
'Ba*'sl (adsorption equilibrium) [SiOH] [ B ]
(4)
dA'g-
dt
-
k2[Ba*/g][A*/g]= k& [SiOHI [BI [ A*/g I
(5)
where A* = value of A corrected for the collision diameter of "active" bromine, Ba*. The calculated corrected rate constant k, = k,K [SiOH]; the [SiOH] is a constant typical of the silica surface used.
Confirmation of Spontaneous Spreading by Water on Pure Gold by Marianne K. Bernett and W. A. Zisman Laboratory for Chemical Physics, Naval Research Laboratory, Washington, D. C. ,90890
(Received October 7 , 1989)
A drop of pure water was found to spread spontaneously and remain spread (Le., contact angle remained zero) on a smooth, clean, pure gold surface provided that strict precautions are observed to avoid any hydrophobic contamination. Throughout these experiments emphasis was placed on the care necessary, even when initially preparing a pure gold surface, to prevent the presence or accidental introduction of trace organics in the atmosphere or in the water from adsorbing on the gold as hydrophobic contaminants. A description is given of the methods used to eliminate possible sources of hydrophilic artifacts on the polished gold surface, such as residual polishing agent or traces of base metal oxides. These experiments confirm the hydrophilic nature of pure gold at ordinary temperatures.
Introduction Within the last five years renewed attention has been paid by a small group of investigators to the question whether a clean gold surface is wettable by mater. The problem in relation to the wetting properties of highenergy surfaces had been investigated and summarized in more general terms as early as 1955 by Fox, Hare, and Zisman,l who concluded then that all pure liquids spread spontaneously on high-energy surfaces unless they are organic liquids which are either autophobic or are hydrolyzed on contact with the solid surface. Renewed interest in the water wettability of gold resulted from a paper by White,2who reported experiments with condensed water vapor on gold in a Pyrex and metal system and concluded that water would not spread in the absence of a layer of gold oxide. Erb,3 subsequently working with a closed cyclic distillation system where pure steam was continuously condensed, reported a steady state and large water contact angles on each metal. I n the case of gold, he reported values from 55 to 85" even after several thousand hours of continuous still operation, when he expected the gold would have been thoroughly cleaned from physically or chemically adsorbed contamination; he explained that these results were caused by the presence of an oxide film on the gold. Using a different method, Bewig and
Zisman4 reported that water on pure gold always exhibited a zero contact angle if the system was maintained free from traces of organic contamination; they came to this conclusion from heating and cooling gold in carefully purified streams of hydrogen, nitrogen, neon, argon, and krypton, and thus were in agreement with the conclusions reached earlier by Fox, Hare, and Zisman. White and Drobek5 then dropped their claims that a gold oxide caused water wetting of gold and stated that the presence of an inorganic impurity, such as residual alumina polishing agent, made the gold surface more hydrophilic. Therefore, they used diamond paste suspended in kerosene as their polishing agent and followed that by heating the specimen in oxygen at 1000". By this procedure, they obtained water contact angles on gold of 61". Erb, whose earlier results were used for theoretical calculations by Thelen,o reported more recently7 that the values of 60(1) H. W.Fox, E. F. Hare, and W. A. Zisman, J. Phys. Chem., 59, 1097 (1955). (2) M. L. White, ibid., 68, 3083 (1964). (3) R.A. Erb, ibid., 69, 1306 (1965). (4) K.Bewig and W. A. Zisman, ibid., 69, 4238 (1965). (5) M.L.White and J. Drobek, ibid., 70, 3432 (1966). (6) E.Thelen, ibid., 71, 1946 (1967). (7)R.A. Erb, ibid., 72, 2412 (1968). T h e Journal of Physical Chemiatry, Vol. 74, No. 11, 1070
