IODINE COMPLEXES OF
THE
DINEOPENTYLTETRAMETHYLBENZENES
from solids to solution and vary in polymorphic forms of the same compounds.26 A comparison of detailed infrared structure of three different crystalline materials would therefore not seem to be valid. Moreover, the shifts found in infrared maxima in the present investigation are surprisingly small and contrary to the earlier report6 do not allow for generalizations with respect to direction and magnitude Of shifts Of bands due to various structural types.
2553
Acknowledgment. This study was supported by Grant C-5946 from the National Cancer Institute, National Institute of Health, U. S. Public Health Service, and by Grant L-46 of the American Cancer Society, New York, N. Y.
(26) L. J. Bellamy, "The Infrared Spectra of Complex Molecules," 2nd Ed., John Wiley and Sons, Inc., New York, N. Y., 1958, p. 379.
Iodine Complexes of the Dineopentyltetramethylbenzenes and Other Potentially Hindered Donors
by R. E. Lovins, L. J. Andrews, and R. M. Keefer Department of Chemistry, University of California, Davis, California (Received A p r i l 8, 1964)
Equilibrium constants and A H o and A S o values for formation of iodine complexes of the three isomeric dineopentyltetramethylbenzenes, 3,5-diisopropyltoluene, and 1,2,4,5-tetraisopropylbenzene in carbon tetrachloride have been evaluated by spectrophotometric methods. Dineopentylprehnitene is classed as a hindered donor with respect to coordination with iodine, and the interactions of dineopentylisodurene and 1,2,4,5-tetraisopropylbenzene with this halogen also appear to be subject to some steric retardation. In contrast to iodine complexes of other hexaalkylbenzenes the dineopentyldurene adduct (in CCl,) has no observable ultraviolet absorption maximum in the 380 mp region.
Equilibrium constants for interaction of hexamethylbenzene with inorganic acceptors such as iodine and iodine monochloride' and with organic acceptors such as tetracyanoethylene* are considerably larger than those for formation of the corresponding hexaethylbenzene complexes. The relatively low donor strength of hexaethylbenzene is attributed to a steric barrier created by the combined bulk of the six ethyl groups, which prevents the close approach of an acceptor molecule to the ?r-orbital of the aroinatic nucleus. The fact that 1,3,5-tri-t-butylbenzeneis a considerably weaker donor than mesitylene is also explained in terms of steric effects.l b z C Recently, Newman, LeBlanc, Karnes, and Axelrad3
completed the synthesis of the isomeric dineopentyltetramethylbenzenes. Professor Newman has called these new hydrocarbons to the attention of the authors and has suggested that a study of the relative stabilities of their halogen complexes might prove interesting. This report presents the results of an investigation of the interaction of the three dineopentyltetramethyl(1) (a) L. J. Andrews and R. M.Keefer, J. Am. Chem. SOC.,74,4500 (1952); (b) R. & Keefer I. and 1,. J. Andrews, ibid., 77, 2164 (1955); (c) N. Ogimachi, L. J. Andrews, and R. M.Keefer, ibid., 77, 4202 (1955). (2) R. E. Merrifield and W. D. Phillips, ibid., 80, 2778 (1958). (3) M. S. Newman, J. R. LeBlanc, H. A. Karnes, and G . Axelradi ibid., 86, 868 (1964).
Volume 68, Number 9
September, 19G4
R. E. LOVING,L. J. ANDREWS, AND R. M. KEEFER
2554
benzenes and two other poteiitially hindered polyalkylbenzene donors with iodine.
'
Experimental Materials. A commercial sample of 1,2,4,5-tetraisopropylbenzene (Aldrich Chemical Co.) was recrystallized from absolute ethanol before use, m.p. 116-117'. A sample of Aldrich Chemical Co. 3,5-diisopropyltoluene, which showed no detectable impurity when analyzed by gas chromatography, was used in the equilibrium studies without further treatment. Professor Newman was kind enough to supply generous amounts of pure samples of the isomeric dineopentyldurenes. Eastman Organic Chemicals Spectro grade carbon tetrachloride was dried over calciuin sulfate before use as a solvent. Spectrophotometric Studies. A series of solutions of varying mole fraction (0.04-0.005) of each donor was prepared which also contained known amounts of iodine in the concentration range 10-d to 10-4 M . The molar concentrations of donor in these solutions were always in large excess of those of the halogen. The optical densities of the mixtures were measured at several wave lengths in the vicinity of the near-ultraviolet absorption maxima of the complexes. One-centimeter absorption cells were used with carbon tetrachloride in the blanks. The measured optical densities were corrected to eliminate the small contributions of the hydrocarbons to the measured absorptions. The temperatures of the mixtures were altered during the course of the measurements by changing the cell housing temperature. The accompanying changes in iodine concentrations of the cell contents were accounted for on the assumption that the percentage variations in solution volumes with temperature were the same as those for pure carbon tetrachloride. Further details of the instrumental method and of temperature control procedures have been described previously. l b The equilibrium constants K , (eq. 1) which are reported below were determined by graphical interpretation of the data according to the equation developed by Ketelaar and his associates (eq. 2). ArH
+ IZ
ArH.12 K,
=
rection for absorption of the hydrocarbon, 1 is the cell path length, and [I2Itis the sum of the concentrations of free and complexed iodine. Further details of the method of evaluating K , and AHN' and ASNO for the interactions have been given in the earlier publication.lb The data recorded a t each wave length were used in calculating individual values of K,, AH", and AS'. The constants which are reported are the averages of these individual values. For the dineopentylisodurene-iodine solutions, for example, optical densities were recorded a t 360, 366, 370, 374, 376, 378, 380, and ,390 mnp; the correspondihg KN values a t 24.7' are (l/mole fraction units) 8.85, 8.42, 8.80, 8.89, 8.99, 8.08, 8.94, 8.81.
Results I n Table I K , values and thermodynamic constants for formation of iodine complexes of the dineopentyltetramethylbenzenes and of 3,5-diisopropyltoluene and of 1,2,4,5-tetraisopropylbenzeneare compared with those reported previouslylh for durene, mesitylene, and hexamethyl- and hexaethylbenzene. I n interpreting the variations in such constants with changes in the numbers 'and locations of donor alkyl substituents it is generally assumed that the strength of an alkylbenzenetype donor does not vary appreciably as the structures Table I: Values of K,, AHN', AS,', and of X and Charge-Transfer Absorption Maximum for ArH-I, Complexes in CCl4
Donor
Hexamethylbenzene" Hexaethylbenzenea Dineopent yldureneb Dineopent ylisodureneb Dineopentylprehniteneb 1,2,4,5-Tetraisopropylbenzeneb 3,5-Diisopr~pyltoluene~ Durene" Mesitylene0
KN (24.7'). mole fraction-1
eo
a t the
- AHNO (24.7'1, -A#,' kcal./ (24.7'1, mole e.u.
X,
mp
€ E , 1. mole,-l cn-1
15.7 3.78 20.6 8.72 5.82 4.50
3.73 1.79 4.15 3.64 3.42 2.30
7.1 3.4 7.9 7.9 7.8 5.7
375 378 .,. 376 382 344
8200 16700
5.31 6.49 5.98
2.66 2.78 2.86
6.1 5.6 6.0
336 332 332
7600 9000 8800
... 6200 5800 4400
[A~H*I~]/[I~]NA~H (1) a Values
reported are from ref. l a and l b ; the constants listed for these compounds were actually measured a t 25.0' rather than 24.7". b The measured values of K , (24.7') are precise to within ~k5'3'~;the error in -AH," ranges from izO.06 to 10.22 kcal. and the error in -ASN' ranges from f 0 . 2 e a . to 1 0 . 7 e.u.
I n evaluating ICN the aromatic hydrocarbon concentrations were expressed in mole fraction units ( N A ~ H ) . The term ea represents the apparent extinction coeffiof alkyl groups a t fixed ring positions are changed (e.g., cient of iodine in a psrticular solution; ea. = d,/Z[I,]t from -CH3 to -C(CH3)8). In this regard it is notewhere d, is the optical density of the solution after corThe Journal of Physical Chemistry
IODINE COMPLEXES OF THE DINEOPENTYLTETRAMETHYLBENZENES
worthy that the equilibrium constants for formation of iodine complexes of toluene, ethylbenzene, isopropylbenzene, and t-but,ylbenzene in carbon tetrachloride a t 25' are the same within experimental error.'* Also the 1,3,5-triethylbenzene-iodinecomplex1b [ K , (25 ") = 5.25 mole fraction-', AHNO = -2.64 kcal./mole, and ASNO = -5.6 e.u. in CCl,] is only slightly less stable than the mesitylene-iodine adduct. A comparison of the constants for mesitylene and 3,5-diisopropyltoluene complexes leads to the conclusion that the isopropyl groups of the latter donor do not offer appreciable steric hindrance to coordination of the halogen. The iodine adduct of 1,2,4,5-tetraisopropylbenzene is noticeably less stable than that of durene. The replacement of the four methyl groups of durene by isopropyl substituents does not, however, have nearly so drastic a steric effect on donor strength as occurs when 1,he methyls of hexamethylbenzene are replaced by ethyl groups. The alkyl groups in hexaethylbenzene are sufficiently crowded by neighboring substituents so that rotation about the bonds which join them to the rings must be highly restricted. The conformation which provides for least steric interference between adjacent ethyl groups (with alternate substituents overlapping opposite faces of the ring) also provides for a high degree of protection of both ring faces against the attack of a bulky acceptor. Even if one discounts the screening effects of a bulky neopentyl group, it is surprising (in view of the insensitivity of alkylbenzene donor strength to changes in alkyl group structure) that dineopentyldurene (p-dineopentyltetramethylbeinzene) forms a more stable complex than does hexamethylbenzene. It is possible that ring strain resulting from interference of adjacent methyls with the two neopentyl groups may distort the ring and the a-orbital of the former hydrocarbon sufficiently to contribute substantially to an enhancement of donor strength. A consideration of molecular models leads to no definitive conclusion, pro or con, that such strain exists. Newman, LeBlanc, Kames, and Axelrad have observed3 that the bulky parts of the neopentyl substituents (the t-butyl groups) can lie on the same side of the ring of dineopentyldurene without coming into physical contact with each other. When that conformation is assumed, one face of the ring is rather well protected from attack by an acceptor, but the opposite face is unhindered. Because of their large size the two ortho located neopentyl groups of dineopentylprehnitene (0.dineopentyltetramethylbeneene) must be preferentially oriented so that they shield opposite sides of the ring. Both of the ring faces of this donor are a t least partially screened against coorldination with iodine. In the case
2555
of dineopentylisodurene (m-dineopentyltetramethylbenzene) a conformational arrangement in which both bulky groups lie on the same side of the ring may constitute a more strained situation than when the two groups are oppositely oriented, though less so than for a comparable arrangement of the ortho isomer. Tentatively, then, the order of donor strengths of the dineopentyltetrarnethylbenzenes with respect to iodine coordination ( p > m > 0) is ascribed to increasing protection of both ring faces by the two bulky groups in the order p < m < 0. Qualitative evidence3 has been presented that with the relatively large acceptor, tetracyanoethylene (which almost certainly is oriented in the complex so that the plane of its carbon skeleton is parallel to the donor ring), both the p- and m-dineopentyl compounds form complexes, but the ortho isomer does not. Rather surprisingly it appears that these three donors coordinate about equally well with tetranitromethane, an acceptor which might be expected to present a t least as many geometric problems in forming complexes as does iodine. As is generally the case for polyalkylbenzene-halogen adducts,' variations in the enthalpies of formation of the complexes studied in the present investigation with changes in donor are roughly parallel to the changes in the free energies of formation. It is interesting to note that in a plot of log K , us. AHofor a group of polyalkylbenzene-iodine complexes, Fig. 1, the point for the dineopentyldurene adduct falls close to the straight line which accommodates the points for complexes of a group of unhindered donors. The points for dineopentylisodurene and dineopentylprehnitene complexes, like those for iodine adducts of hexaethylbenzene, 1,3,5tri-t-butylbenzene, and 1,2,4,5-tetraisopropylbenzene, fall off that line. Ordinarily the entropy changes accompanying complex formation are also directionally parallel to the enthalpy changes. In this sense the - ASNO values for the 0- and m-dineopentyltetramethylbenzenes appear to be abnormally high. The position of the charge-transfer absorption maximum of an alkylbenzene-halogen complex tends to shift toward longer wave lengths as the donor ring is increasingly substituted with alkyl groups.' This is illustrated in Fig. 2 in which €,-values for iodine (the apparent extinction coefficients of iodine) in carbon tetrachloride solutions approximately 0.2 M in aromatic hydrocarbon are plotted us. wave length. The absorption maxiina of the complexes of hexaniethyland hexaethylbenzene and of dineopentylisodurene and dineopentylprehnitene all are near 380 mp. Those for the 1,2,4,5-tetraisopropylbenzeneand 3,5-diisopropyltoluene adducts appear a t 344 and 336 mp, respectively. Volume 68, ATumber 9 September, 1964
2556
R. E. LOVINS,L. J. ANDREWS,AXD R. 31. KEEFER
1.3 1000
1.1 800
0.9 600
i
Q 2 0.7
400
0.5
200
0.3
I 0.1
L
I
I
0
1.0
310
350 A,
I
I
2.0 - A H o , kcal.
3.0
I 4.0
Figure 1. A plot of log KN us. A H o for formation in carbon tetrachloride of iodine complexes of (1) dineopentyldurene, ( 2 ) hexamethylbenzene, (3) dineopentylisodurene, (4)durene, ( 5j mesitylene, (6) dineopentylprehnitene, ( 7 ) 1,3,5-triethylbenzene, (8) 3,5-diisopropyltoluene, (9) 1,2,4,5-tetraisopropylbenzene, (10) hexaethylbenzene, (11) 1,3,5-tri-tbutylbenzene, (12) benzene.
Interestingly enough, no charge-transfer maximum appears in the spectrum of the dineopentyldurene complex in the region lying above 345 nip. Measurements a t lower wave lengths could not be made because of intense absorption of the free donor. Most alkylbenzene-tetracyanoethylene complexes have characteristic single broad visible absorption maxima. The p-xylene complex actually has two absorption bands which have been associated with a splitting of the doubly degenerate donor r-orbital as a consequence of para disubstitution of the ring.2 It is conceivable that such a splitting may also occur in the case of dineo-
The Journal of”Physical Chemistry
330
370
390
w.
Figure 2. The apparent extinction coefficient of iodine in carbon tetrachloride solutions of polyalkylbenxenes. The various hydrocarbons and their concentrations in the solutions on which curves 1-7 are based are, respectively: (1) 3,5-diisopropyltoluene (0.193 M), (2) 1,2,4,5-tetraisopropylbenzene(0.168 M ) , (3) hexamethylbenzene (0.185 M ) , (4)dineopentyldurene (0.156 M j , (5) dineopentylisodurene (0.172 MI,(6) dineopentylprehnitene (0.179 M ) , (7) hexaethylbenzene (0.165 M).
pentyldurene, which has two para-oriented neopentyl groups. The iodine complex of this donor may actually have two charge-transfer peaks so arranged that the one which normally might appear in the 380 mp region is masked by the tail of the other. This second peak presumably would lie a t a shorter wave length than the first.
Acknowledgment. The authors are indebted to the National Science Foundation for a grant in support of this research. L. J. A. is indebted to Dr. Eva Voigt for an interesting discussion of the spectrum of the dineopentyldurene-iodine complex.
