SELECTIVE ADSORPTION OF NONIONIC ORGANICCOMPOUNDS ON ALUMINA
489
Interactions Responsible for the Selective Adsorption of Nonionic Organic Compounds on Alumina.
Comparisons with Adsorption on Silica’
by L. R. Snyder Union Oil Company of California, Union Research Center, &ea, California 92661
(Received Mav 17, 1067)
It is now well established that selective adsorption (i.e., involving interactions other than dispersion forces) on silica is due to hydrogen bonding of surface hydroxyl groups to adsorbate molecules. This study compares the selective adsorption of several common organic structural groups (e.g., methoxy, nitro, aceto, and amino)’ on water-deactivated alumina and silica. Standard free energies of adsorption from pentane solution were measured for each group on both alumina and silica, within the linear isotherm region. The variation of these group adsorption energies with intramolecular electronic and steric effects was also studied. The resulting experimental relationships are compatible with a hydrogen-bonding adsorption mechanism on silica, but not on alumina. These data suggest that the selective adsorption of a group on alumina is due to a polarization of its p or ?r electrons in a strong positive surface field. The interactions between adsorbed molecules and solid surfaces may be characterized as nonspecific ( i e . , dispersion forces) or specific (e.g., electrostatic forces, hydrogen bonding, charge transfer, etc.). (See ref 2.) Nonspecific interactions make an important contribution to absorption energy in all physical adsorption processes and a predominant contribution in adsorption on so-called nonspecific adsorbents such as graphite. The nature and approximate magnitude of these dispersion interactions in adsorption are now reasonably well understood (e.g., ref 3 and 4). Our present knowledge of the specific interactions between adsorbate molecules and the surfaces of different adsorbents is less well developed. Specific interactions play an important role in the adsorption of many compounds on polar adsorbents (e.g., silica and alumina), and these interactions are largely responsible for the selective adsorption of different solute molecules from dilute solution (as in adsorption chromatography) .6--8 For adsorption on silica, it is now widely agreed that specific adsorption interactions are due largely to hydrogen bonding between surface hydroxyls of the adsorbent and electron-donor groups of the adsorbate (e.g., ref 2,9, and 10). I n the case of alumina as adsorbent, the nature of specific adsorption interactions (and the types of surface sites responsible for these interactions) is still unclear. Various investigators, working generally with y-alumina, have claimed an important role for conventional electrostatic forces, l1 hydrogen bonding, with the alumina surface functioning as either a proton donor*2or acceptor, la charge-transfer complexation, with the alumina surface acting as either an
electron donor14 or acceptor (see review of ref 15), and ionization of the adsorbate upon adsorption. le Pos(1) Presented at the 153rd National Meeting of the American Chemical Society, Miami Beach, Fla., April 1967. (2) A. V. Kiselev in “Gas Chromatography. 1964,” A. Goldup, Ed., Institute of Petroleum, London, 1965,pp 238-265; A.V. Kiselev, Advan. ChTOmatogr., 4, 113 (1967). (3) D.M. Young and A. D. Crowell, “Physical Adsorption of Gases,” Butterworth Inc., Washington, D. C., 1962, Chapter 2. (4) D. H. Everett in “Gas Chromatography. 1964,” A. Goldup, Ed., Institute of Petroleum, London, 1965,pp 219-236. (5)Thus adsorption of a molecule of sample requires desorption of an approximately equivalent volume of solvent molecules from the adsorbed monolayer, and the net adsorption energy is determined by the differences in solution and adsorption interactions of sample and solvent molecules, respectively (see ref 6). Since dispersion contributions t o adsorption are approximately proportional t o molecular polarizability (see ref 4 and 7) and the latter is roughly proportional to molecular volume, the corresponding dispersion energies of adsorption for solute and solvent are about the same and, hence, cancel, so far as their combined effect on the net adsorption energy of the solute. Similarly, it is now apparent8 that solution interactions of solute and solvent molecules are generally unimportant in affecting the net energy of adsorption of a solute molecule from solution, particularly in the case of nonpolar solvents. (6) L. R. Snyder, Advan. Anal. Chem. Instrum., 3 , 251 (1964). (7) A. V. Kiselev, Discussions Faraday Soc., 40, 228 (1965). (8) L. R. Snyder and E. R. Fett, J. Chromatogr., 18, 465 (1965). (9)A. V. Kiselev in “Structure and Properties of Porous Materials,” D. H. Everett and F. 5. Stone, Ed., Academic Press Inc., New York, N. Y.,1958,pp 195-226. (10) L. R. Snyder and J. W. Ward, J. Phys. Chem., 70, 3941 (1966). (11)J. King, Jr., and 8. W. Benson, Anal. Chem., 38, 261 (1966); J. Chem. Phys., 44, 1007 (1966). (12) L. 5. Bark and R. J. T. Graham, Talanta, 11,839 (1964). (13) C. H. Giles in “Chromatography,” E. Heftmann, Ed., 1st ed, Reinhold Publishing Corp., New York, N. Y., 1961,Chapter 4. (14) C. Naccache, Y. Kodratoff, R. C. Pink, and B. I. Imelik, J. Chim. Phys., 63, 341 (1966). (15) L. R. Snyder, J. Phys. Chem., 67,234 (1963). (16) L. R. Snyder, J . Chromatogr., 23, 388 (1966). Volume 76, Number 2 February 1988
490
L. R. SNYDER
sible surface sites for adsorption on y-alumina include hydroxyls, oxide ions, exposed aluminum ions or protonated vacancies, or strong electrostatic fields (for a discussion of the surface structure of y-alumina or partially hydrated y-alumina, see ref 11, 17, and 18). Previous work16has shown that the adsorption energy (from solution) Qoi of an adsorbate group i (e.g., phenyl, methoxy, nitro, and hydroxy) is generally similar for adsorption on either silica or alumina, and these group adsorption energies correlate with the so-called hard basicity of the group. That is, most organic molecules appear to adsorb on alumina or silica as hard bases interacting with hard-acid groups on the adsorbent surface. This observation is supported by the effects of electron donor (e.g., methyl) or acceptor (e.g., chloro) substituents on the adsorption energies of other groups within the adsorbate molecule, for adsorption on either a l u m i r ~ a ~or~ Jsilica.21*22 ~ Electron-donor groups generally increase the basicity and adsorption energy of other groups in the adsorbate molecule (after correcting for their own adsorption energies) and vice versa for electron-withdrawing groups (but see ref 23-25). The present study was intended to define further the nature of selective adsorption interactions on alumina and the types of adsorbent sites on the alumina surface.
Experimental Section Linear isotherm retention volume values Bo (ml/g), equal to equilibrium adsorption distribution coefficients, were determined as previously (see Discussion of ref 15 and 25). The repeatability of individual $0 measurements was *1.6% (std dev). The alumina used has been described (e.g., ref 25 for a similar preparation). It is a commercial sample of impure y-alumina (Alcoa F-20; Aluminum Co. of America) with surface area equal to 200 10 m2/g and average pore diameter of 70 A. I t was first calcined in air for 16 hr at 400", following which 0.037 ml/g of water was added. The silica (Davison Code 62; W. R. Grace Co.) has been studied in detail (sample I1 of ref 10). Its surface area is 319 m2/g and its average pore diameter is 241 A. The silica was first dried in air for 4 hr at 170°, and 0.04 ml/g of water was added. The relative surface coverage by added water was estimated at 35-50% of a monolayer for each adsorbent. All retention volume values were corrected to a pentane solvent basis as previously (e.g., ref 25)
*
log R,
=
log go
+ aeoA.
(1)
The term R, is the value of gofor pentane solvent, (Y is an adsorbent activity parameter, e0 is a solvent strength parameter, and A , is a function of solute structure. For adsorption onto 4% H 2 0 S i 0 2 , aeO was measured equal to 0.041 for 10 vol % methylene chloride-pentane and 0.086 for 30 vol yo methylene chloride (methyl benzoate and methyl p-methylbenzoate as solutes). The values of A , for the compounds acetophenone, The Journal of Physical Chemistry
p-methylacetophenone, aniline, p-methylaniline, omethylaniline, phenol, p-methylphenol, and o-methylphenol were calculated as previously:s 15.2, 16.2, 13.2, 14.2, 14.2, 13.5, 14.5, and 14.5 for adsorption on silica, Free energies of adsorption from pentane on either adsorbent could be calculated as previ0usly2~
AF, = 2.3RT log (RpVa) where R is the gas constant, T the absolute temperature, and Va is the so-called adsorbent surface volme.^^*^^ It will prove convenient to define a corresponding quantity, proportional to AFp at a given temperature, the dimensionless free energy of adsorption AE
AE
=
AFp/2.3RT
The contribution of a group i in a substituted benzene C6H5-i to the dimensionless free energy of adsorption of CaH5-i can be calculated (assuming a flat or parallel with all molecular configuration of adsorbed CGHS-~, groups adjacent to the adsorbent surface; see ref 26, 27) as Ei =
AECsHs-i
Ei = (log R p ) c a H s - i
- AEc~H~--H - (1% R p ) ~ o ~ , - ~
The dimensionless free energy of adsorption, Ei, of the group i is equal to the difference in log R, values of the compounds CBH5-i and benzene. The change in Ei as a result of electronic activation of i by a methyl group para t o i (Ae) can be calculated as previously:'9 L\, =
(log Rp)Me-CaHs-i
- (log RpICsHs-i (1% Rp)Me-CaHs
-
+ (1%
Rp)CsHs--H
(17) J. B. Peri, J. Phye. Chem., 69, 211, 220 (1965). (18) J. H. DeBoer, J. M. H. Fortuin, B. C. Lippens, and W. H. Meijs, J. Catalysis, 2, 1 (1963). (19) L. R. Snyder, J. Chromatogr., 17, 73 (1965). (20) L. R. Snyder, ibid., 20, 463 (1965). (21) L. R.Snyder, ibid., 11, 195 (1963); 25, 274 (1966). (22) L. R. Snyder, Advan. Chromatogr., 4, 3 (1967). (23) Two minor exceptions to this normal adsorption pattern should be noted in the case of alumina. Acidic molecules (pK, < 13) are preferentially adsorbed on alumina relative to silica.16 The increased adsorption energy (on alumina) correlates with adsorbate pK, in such a way as to suggest an ionization mechanism, with transfer of an adsorbate proton to a surface oxide or hydroxyl group (see also Discussion of ref 24 and 25 for adsorption of substituted phenols and indoles on alumina). Aromatic molecules with ionization potentials less than 6.8 eV (e.g., naphthacene, pentacene) appear to adsorb on alumina as charge-transfer complexes, with donation of an electron to a surface site." The above discussion is concerned with specific adsorption interactions on alumina apart from these special cases. (24) L. R. Snyder, J. Chromatogr., 16, 56 (1964). (25) L. R. Snyder, J. Phye. Chem., 67,2344 (1963). (26) This is definitely true for each of the adsorption systems cited in Table I (e.g., ref 6, 16, 21, 25, and 27). Among the different observations which support the latter conclusion, the free energies of adsorption of these compounds show that all groups within the solute molecule are interacting with the adsorbent surface. This is only possible when all groups are adjacent to the surface &e., flat adsorption). (27) L. R. Snyder, J . Chromatogr., 8,319 (1962).
SELECTIVE ADSORPTION OF NONIONIC ORGANIC COMPOUNDS ON ALUMINA
491
Table I : Sample Group Adsorption Energies and Their Change with Steric or Electronic Activation for Adsorption on Alumina or Silica a t 24' 4% HBO-SiOz
Sample oompound
i
Benzene Toluene (1) Bromobenzene (2)Thioanisole p-M ethyl(3)Anisole p-Methylo-Methyl(4) Nitrobenzene p-Methylo-Methyl(5)Benzonitrile p-Methylo-Methyl(6)Methylbenzoate p-Methylo-Methyl(7)Acetophenone p-Met hyl(8)Aniline p-Methylo-Methyl(9)Phenol p-Methylo-Methyl-
Log
RQ
-0.18 -0.11
-Br -SCHI -0CHs
-NO* - k N -CO%CHs
-COCHa
-NH, -OH
O.4lb 0. 4Qb 0.68b 0.81b 0.52b 1. 15b 1.25b 1.06b 1.63b 1.73b 1. 56b 1.64b 1.76b 1.4Qb 1 .45d 1.59 1.13' 1-27' 1.03# 1.41' 1. 27# 1.03'
Ei
3.7% Ha0-AlaOs
4
...
...
0.59
0.01
...
0.86
0.06
-0.29
1.33
0.03
-0.19
...
(log Rp)orm
4
0.26" 0.91
-0.01" 0.08
-0.07a
1. 13c
0.11
-0.20
1.66 1.770
0.15
-0.25
-0.35 -0.31 0.56b 0.56b
1.31b 1.50b 1.25b
0.200
0.03
-0.17
2.080
0. 23c
-0.150
1.82
0.05
-0.27
2.120
0,200
-0.2Qe
2.25
0.11
..*
2.3QC
0.24c
...
2.48
0.19
-0.24
2.82O
0.276
-0.36c
2.73
0.03
-0.33
...I
i.e., the sum of log Rp values for the compounds pmethyl-i-benzene and benzene minus the values of log R, for the coinpounds i-benzene and toluene. The change in Ei as a result of steric interference to the adsorption of i by a methyl group ortho to i (As) can also be calculated as beforel9 E=
a,
Ei
1.81
a Data of ref 20,corrected for differing adsorbent activity. b Pentane solvent. solvent, a 8 = 0.041. e 30% methylene chloride-pentane solvent, a d o = 0.086. tion of phenol on alumina.
AB
Log RQ
A9
- (log Rp),uru
Le., the value of log Rp for the compound o-methyl-ibenzene minus the value of log Rp for the corresponding para isomer. It should be noted that electronic interactions cancel approximately in the ortho and para isomers, so that As represents the change in Ei as a result of steric interaction only. Discussion The derived quantities Ei, Ae, and As are proportional to differences in standard free energies of adsorption. Since these 1att)er quantities are defined for adsorption from pentane, contributions of nonspecific adsorption interactions and solution interactions cancel (see ref 4), and Ei, A,, and As are determined largely by specific interactions between solute and adsorbent (specific adsorption interactions for the solvent pentane can be ignored). In the following discussion, Ei may be regarded as equivalent to the specific interactional
...I
*
10% methylene chloride-pentane D a t a of ref 20. Experimental data not determined because of ioniza0
f
...I
energy of the group i with the adsorbent surface, A, is the change in this group adsorption energy as a result of electronic interaction between i and a p-methyl group on the same benzene ring, and As is the change in Ei as a result of steric interaction between i and an omethyl group on the same ring (note that the effect of i on the adsorption energy of the methyl group is small, since Ei for a methyl group is quite small; for a further discussion see ref 20). Table I summarizes experimental values of Ei, Ae, and A, for adsorption of nine different organic groups i on alumina and silica. Data from the present study are supplemented with previously reported data. Values of A, are generally positive, and values of A. are negative. The dependence of A, and As on Ei is shown in Figures 1 and 2 for adsorption on both alumina and silica. In Figure 1, it is seen that A, is generally larger for adsorption on alumina relative to silica (for the same value of Ei). For adsorption on alumina, A, is proportional to Ei, but the plot of Ae us. Ei scatters badly for adsorption on silica. In Figure 2, it is seen that values of A. for the two adsorbents are generally of similar magnitude, but again the alumina plot is linear and the silica plot is scattered. As we will see, the interpretation of the data of Figures 1 and 2 appears reasonably straightforward.2s-30 Volume 78,Number 8 February 1968
492
A;::y, b, 0. I
0.0
0
2
El
4
0
2
Ei
4
Figure 1. As us. Ei, data of Table I.
Figure 2.
As us. Ei,data of Table I.
For adsorption on silica, where hydrogen bonding between the group i and a surface hydroxyl can be assumed, ABis expected to be positive; Le., an electrondonating p-methyl group is expected to stabilize the hydrogen-bonded complex I because of the positive charge ai-
6-
- i s . . , H . . . .&&
I developed on i. The extent of this stabilization (and A.) is expected to be proportional to the net charge qi developed on i during adsorption and to the extent of electron donation by the p-methyl group &e., its Hammett u value).a1 The values of qi and A. should tend to increase with increasing strength of the hydrogen bond between i and a surface hydroxyl (and, therefore, with increasing Ei). An exact proportionality between A. and Ei is not expected for adsorption on silica, however, for two reasons. First, the hydrogen bond has both covalent and electrostatic character (see ref 32), and the relative importance of each of these two contributions to bond energy will vary with the group i involved. Therefore qi will not be exactly proportional to Ei. Second, the energy of interaction of the charge on i with the p-methyl group is determined not only by the magnitude of qi but also (in the case of a polyatomic group i) by the distribution of this net charge over the various atoms which constitute i. This charge distribution is expected to be a marked function of the relative position of attachment of the bonding proton to i, which will of course vary among the different groups of Table I, An o-methyl substituent is electronically equivalent to a p-methyl (see Discussion of ref 24) but sterically interferes with the formation of a hydrogen The Journal of Phyeical Chemietry
L. R. SNYDER
bond (or any bond) to i. Consequently ABis expected to be negative in adsorption on silica (and alumina). Because of differencesin the relative position of attachment of the hydroxyl proton to i for different groups i, no correlation between Asand Ei is expected. The data of Table I (and Figures 1 and 2) for adsorption on silica are thus fully compatible with an adsorption mechanism based on hydrogen bonding. The differences between adsorption on alumina and silica, which are summarized in Figures 1 and 2, seem to preclude hydrogen bonding as a major contribution to group adsorption energies on alumina. The preceding discussion and the correlations of Figures 1 and 2 for adsorption on alumina also seem to rule out any adsorption mechanism based on the formation of a chemical bond (of any type) between the group i and an atom or group on the alumina surface. The only alternative specific adsorption interactions possible are those arising from purely electrostatic interaction of the group i with a positive field perpendicular to the alumina surface. King and Benson" have provided a detailed discussion of these alumina surface fields, emphasizing their strength and probably contribution t o adsorption energy. Aromatic solutes generally adsorb on alumina and silica in a flat or parallel configuration, as in Figure 3.2s Interaction of a group i with a positive surface field of strength (at the position occupied by i) should result in the polarization of i as indicated in Figure 3. An induced dipole is formed, of moment pi equal to aiF. The term ai is the polarizability of i in the direction of the field. The effective charge on i ( i e . , qi) is proportional to pi, and hence to ai. The energy of this inductive interaction of i with the surface is given as 'lzCyiF2. Since A. is proportional to pi, qi is proportional to ai, and the inductive interaction energy (Le., Ei) is proportional to ai, A. is predicted to be proportional to E*. This is observed for adsorption on alumina (Figure 1). Since F varies with the distance r above the surface," the preceding relationship assumes that the distance ri (Figure 3) is constant for the various groups i of Table I. This is not unreasonable, since the van der Waals radii of most of these groups are quite similar (see ref 33), and the
(28) Several possible reasons exist for the scatter of data points in adsorption on silica but not alumina, in addition to the explanation offered a t the close of this paper. A number of these hypotheses have been considered in detail and rejected on various grounds. For example, one of the reviewers has raised the possibility of greater heterogeneity in the silica surface, relative to alumina. However, the greater linear capacity of this silica sample (see ref 29 and Discussion of 30) suggests that it is in fact more homogeneous than the alumina sample described here. (29) L. R. Snyder, Separation Sci., 1 , 191 (1966). (30) L. R. Snyder, J . Chromatogr., 5 , 430 (1961). (31) In agreement with theory, A, for the pyridines substituted by different groups in the 3 and/or 4 position correlates well with the u values of these groups in adsorption upon either alumina*' or silica.$* (32) R. 5. Drago and B. B. Wayland, J . Am. Chem. SOC.,87, 3671 (1966).
SELECTIVE ADSORPTION OF NONIONIC ORGANIC COMPOUNDS ON ALUMINA
F
\n
\\\\\'
Figure 3. Electrostatic model for adsorption on alumina.
separation of i from the surface is determined largely by the (constant) configuration of the attached benzene ring. The exclusively electrostatic nature of these interactions between adsorbate groups i and the alumina surface serves to explain the generally larger A, values for adsorption on alumina relative to silica (Figure 1). The introduction of an o-methyl group into the compound CBH6-iof Figure 3 will tend to push the group i some distance, AT, away from the surface, depending upon the geometry of the surrounding surface. Because of the steepness of the intermolecular repulsion potential, this displacement, AT, will be relatively independent of the attraction of i to the surface (Le., independent of Ei). The effect of this displacement of i on its adsorption energy (Ei or 1 / 2 a i F 2 )is given as (bEi/&)Ar or '/,ai(dF2/bT)ArlSO that
AB =
'/zai [ ( b F 2 / b r ) , , ] A r
Since T , AT, and (bFz/&) are approximately constant for each group i, A. is predicted to be proportional to ai and hence to E i . This is observed for adsorption on alumina (Figure 2), with the exception of the much less sterically hindered o-methylbenzonitrile. A simple electrostatic model thus provides an adequate description of adsorption on alumina, so far as the data of Table I are concerned. It is reasonable to assume that these same electrostatic interactions account for the specific adsorption energies of other organic groups on alumina (and of organic compounds in general, noting only the exceptions of ref 23). However the present model should not be confused with the, commonly invoked, classical interaction of a molecule or group with a uniform electrostatic field. I n the latter case, the electrostatic interaction energy is determined largely by the dipole moment and polarizability of the molecule (or group) as a whole. No such correlations exist for adsorption on alumina or other polar adsorbents (e.g.; ref 2 and 16). The reason for this difference in the two cases (classical electrostatic interaction us. adsorption on polar adsorbents) arises from the special character of the electrostatic field over the alumina surface. For an idealized model of the y-alumina surface, King and Bensonll have shown that F is quite large at a point directly below i and diminishes sharply in moving away from this point upward or laterally. Consequently the major effect of this field will be felt by the electrons in a single p or ?r orbital (on i) oriented perpendicular to the surface, or nearly so, and situated in the most intense part of the field at the
493
distance Ti from the surface. These electrons will be pulled toward the surface, creating the dipole p i , while the remaining electrons in the group i will be much less affected. Thus ai represents the polarizability of a particular pair of electrons (the most easily polarized) in the group i, along a definite axis. Similar electrostatic contributions make a dominant contribution to the interactions of hard acids with hard bases (see ref 32), which explains the observed correlation of group adsorption energy (and hence ai) with the hard-basicity parameter EB for the group i.l$ The failure of the permanent dipole moment of a group i to make a dominant contribution to its adsorption energy on alumina may seem strange, in view of the normally greater importance of dipole orientation forces relative to induction forces. The answer to this paradox is found in the rapid decrease in field strength upon moving away from the adsorbent surface. The permanent dipole moment of a group is centered at the nuclei of the atoms which compose i (or are attached t o i), and these nuclei lie in a weaker part of the field than the underlying p or ?r electrons of i. These observations emphasize the danger of assuming classical electrostatic relationships (ie., those based on uniform surface fields) for interactions at surfaces, a point which has apparently been overlooked by many previous workers in the field of adsorption thermodynamics. The present electrostatic model for adsorption on alumina is in agreement with other experimental observations which suggest that surface hydroxyl groups are not important sites for adsorption on alumina (as opposed to adsorption on silica). Thus, thermal dehydration of alumina generally increases the energy of selective adsorption interactions, whereas similar dehydration of silica reduces the energy of selective adsorption interactions. Surface hydroxyls are lost in each case, but with opposite effects on adsorption. This is illustrated by gas-solid chromatography studies using alumina34or silicaaswhich has been preactivated at different temperatures T,. As T. is varied from 200 to 1OOO" (with a corresponding variation in surface hydroxyl coverage from nearly complete to essentially zero), the preferential adsorption of ethylene relative to ethane (which is due to specific adsorption interactions) continuously decreases with increasing T, for adsorption on silica and continuously increases with increasing T. for adsorption on alumina. Similarly, the heat of adsorption of nitrogen (which has a quadrupole moment) on alumina increases with increasing T,,a6 while on silica the opposite is observed.2 Nonpolar adsorbates (33) L. Pauling, "The Nature of the Chemical Bond," Cornel1 University Press, Ithaca, N. Y., 1940, p 189. (34) C. G. Scott, J. Znst. Petrol., 45, 118 (1959). (35) G. R. Schultze and W. J. Schmid-Kuster, Z . Anal. Chern., 170, 232 (1959). (38) J. King, Jr., private communication.
Volume 72, Number 8 Febrwry 1968
494
W. J. CHENGAND MICHAELSZWARC
(e.g., oxygen, argon, etc.) show heats of adsorption on both silica and alumina which are essentially independent of T, (see Discussion of ref 2). The difference between adsorption on alumina and silica in this respect is also shown by Parry’s studiesa’ of the infrared absorption spectrum of pyridine adsorbed on alumina and silica. Hydrogen-bonding interactions between adsorbed pyridine and the adsorbent surf ace are indicated in the case of silica but not alumina. Borisova, et ~ l . , and Uvarovae have shown that some interaction (ie., hydrogen bonding) occurs between surface hydroxyls on alumina and adsorbing molecules, but this interaction is much weaker than corresponding interactions on
silica (infrared absorption studies of the change in vibrational frequency of surf ace hydroxyls during adsorptiona8). Furthermore, these hydroxyl-adsorbate interactions on alumina disappear when the most weakly adsorbed material is removed from the adsorbent. Presumably hydroxyl-adsorbate interactions on alumina play at most a minor role in determining the total adsorption energy of typical organic compounds. ~ ~ (37) E.P.Parry, J . Catalysis, 2 , 371 (1963). (38) M. 9. Borisova, V. A. Dzis’ko, L. A. Ignat’eva, and L. N. Timofeeva, Kinetika i Kataliz, 4, 461 (1963). (39) A. V. Uvarov, Lakokraeochnye Materialy i ikh Primenenie, 3,7 (1966).
Studies of Gas-Phase Reactions of CF, Radical with Methylchlorosilanes by W. J. Cheng and M. Szwarc Department of Chemistry, State University College of Forestry, Syracuse University, Syracuse, New York 13910 (Received May M,1067)
The abstraction of hydrogen by CF3 radicals from the following series of substrates: Si(CH&, ClSi(CH&, C12Si(CHa)2,and ClaSi(CHa), was investigated in the gas phase at 180”. The reactivities of the CH bonds decreased with the successive substitution of CHs’s by C1 atoms. The absolute rate constants of these reactions were calculated and compared with the reported rate constant of the analogous reaction of neopentane. The lat’ter is less reactive toward CFa radical than Si(CHJ4. An interesting correlation was observed between the CH reactivities and the deshielding of the respective methyl protons in the silanes. This again demonstrates the importance of polar factors in CF3 radical reactions. The differences in chemical behavior of silicon and carbon atoms may be attributed to the following factors: (1) the larger size of the former than that of the latter (Rsi = 1.17 8, whereas RG = 0.77 8); (2) the lower ionization potential of Si atom (8.15 eV) as compared with that of carbon (11.1 eV); (3) and the availability of empty d orbitals in Si, which are absent in C atoms. The electron negativity of this element is lower than that of carbon, being 1.8 and 2.5, respectively, in Pauling’s scale. Nevertheless, it has been claimed that carbon atom forms the positive center of a C-Si dipole.’ This conclusion is based on comparison of dipole moments of Et,SiClr-, and H,SiC14,, and it is rationalized by attributing an electron sink character to the Si atom caused by the presence of empty d orbitals.2 Such polarity should be reflected in the reactiviI
ties of 4Si-C-H bonds when compared with those of I
The Journal of PhysiLal Chemistry
I
3 C-C-H. I
For example, in a series of radical reactions
described by the general equation
I I
+Si-C-H
+ CF3. -+ -Si-C. I + CF3H
I
the silanes should be less reactive than their carbon analogs. The work reported in this communication showed the opposite, casting doubt on the conclusions drawn from the studies of dipole moments. Studies of radical chemistry of organosilicon compounds are scanty. Most of the reported investiga(1) (a) E.L. Reilly, C. Curran, and P. A. MoCusker, J . A m . Chem. Soc., 72,4471 (1950); (b) K.Sohaaraohmidt, 2.Anorg. AZZgem. Chem., 310,78 (1961). (2) B. J. Aylett, H. J. Emel6uus, and A. G. Maddook, J. Inorg. Nucl. Chem., 1, 187 (1955).
