4278
NOTES
Analysis of the Dab.-The curves of Figs 1 and 2 indicate that interaction occurs between the ferric chloride solution and the a-glucose solution, and that the presence of the ferric chloride increases the rate of mutarotation of the a-glucose. Our initial experiments, involving spectrophotometric studies of solutions of glucose and ferric ion in equilibrium, and utilizing the limiting logarithiii method described by Bent and French4 and the method of continuous rariattoiis ae modifitd I)\. i ' o i b i i r ~ h ,inti CIiopei indicated the l'ormttioit oi A t IC-iisto m coinples (mitaming iron and gluc 111 cy uii I iolar ai n c ) u n ti: Since tht coiidilions under n hirh (lata for Figs 1 ;mci 2 were itbtauied were t h i b m i t ' (with the cxception of the \yariatiun in the concentration of the ferric chloride solution), only Fig. 2 will be considered in thc following eliscussion. The upper curve of Fig. i! shows the rate of mutarotation of the a-glucose in 0.1 JI hydrochloric acid with no ferric chloride added. The first value shown, obtained five minutes after wlid a-glucose was added to the hydrochloric acid, was M E r n . -1s the curw indicates, no appreciable cliangi. 111 rlic rotation was observed after a period ! t i approsiinately sixty niinutc-s. 'The lower curve i r i Fig. i! shows the rate (tf iii~~arotatiori of u-glucost- 111 0.1 M hydrochloric x i t i in the presence of 0.4 ferric cldoride. Again, the initial reading was made five minutes after the solid a-glucose was added to the solution. This reading (53.50) is appreciably lower tha:i the previous reading. This curve levels ofl much more rapidly than the other, the observee rutatioii becoming constant after J perioc: ( d .tppro.xiitiaLels. twentv-live iuinutes. In m y coniplt-x with 1rontlIL) aiid glucose m e might expect the ferric ion to be attached to the glucose molccule through the carbonyl oxygen. There 1 5 no reasmi tci assume that this cotnplexing shoultl rtestrov the optit:al activity ok the glucose tnnlecult-. I t is known that an equilibriuni exists betweell the (2. and d-forms of glucosr in solution. If the ferric ion were t o coriibiiie with the a-form of glucose, on? would expect the rate of mutarotatioti to decrease. Conversely, if the ferric toil were to combine with the 8-form, the rate should increase, Since, i n the case cited, the addition of the ferric ion results in ,211 incredsed ratc of mutarotatioii, it i b concluded that the ferric ion complexes with the a- rat1ic.r thari tlw n-form of glucose. This ~:onclusion is iupported I)>- thr slighllv lower equilibrium optical rotation of the solutioti containing the ferric chloride. The difference between the two equilibrium values is of the order of the experimental error ; however, approximately the same difference is noted for bath series ofruns. If the complex were forinedwith the a-rather than tlicb $-form of glucose, one would
1'01.72
ured (i. e., total) rotation a t equilibrium to be greater than that for pure glucose solutions, which is precisely the opposite of the observed effect. Thus, assuming that the formation of the cotnplex does not destroy the optical activity ol the glucose molecule, one map say that the iron coniplexes preferably with the $-form. 1)EPARTMENT O F CHEMISTRY
UNIVERSITY OF KANSAS LAWRENCE,KANSAS
RECEIVEDAPRIL20, 1950
,tJ
Calculation of Resonance Energies BY J. 1,
1:KANKI.IV
It has been shown that the heat of formation and free energy of formation of organic molecules are additive functions of characteristic group equivalents. l r 2 Since resonance energy is calculated as the difference between the heat of formation of a molecule and that of the corresponding structure assuming no resonance, it follows that the method of group equivalents can lye used to calculate resonance energies. Further, this method automatically allows for steric effects and hyperconjugation. Iiesoiiarice affects electronic levels primarily and rotatioiial and vibrational levels secondarily. ;issuniing 110 low-lying excited electronic levels, the electronic partition function is simply e-E/RT aiid the electronic entropy is zero. Consequently, change in electronic energy due to resonance will have no effect on entropy. 'The rotational and vibrational entropy of a resonating molecule will be different from that of the corresponding hypothetical non-resonating molecule because of changes in bond distance, symmetry, and force constants. These might be expected to decrease slightly the entropy of the resonating molecule. It is observed, however, that resonance energies calcukted frOrii heats of formation and from free euergies of formation are almost identical and varf but little with temperacure so it might be concluded that the latter effects are small. Consequently, resonance energies may be calculated by the method of group equivalents from T~RLF
r
VALCL'I,ATION OF RESOYANCR E\F.RGI O B REXZKPII, IHp
: ~ I ~ C = C H( C l \ I C6ring correction Symmetry correction ( K T In ( T '
AHfaor AFP for non-resonating structure AHfo or A K 0 for C6HBfrom APIa tables Resonance energy
A1,O
56 (i 71 X
-05 .
-n4 I 5 _ I
56 1 19 P
66 9 0
36 8
35 9
__ _- 31
(1) Rlnklin, I d Blig C h c n , 41, 1070 (1949) 2 ) Biernner dud Thomas, Trans Faraday Sor , 4 4 , 3 t B (1948) ( 3 ) Selected Value\ of Properties of thc Hydrocarlmtis, Ah'[ t ' i o i r r i 4 1 V:itiwid H l i r t LII of S t m i l r r i l s Circular C-101 Yo\ IU47"
Sept., 1950 either heats of formation or free energies of formation, as illustrated for benzene in Table I. The results are in excellent agreement with the value calculated from heats of hydrogenation. Table I1 compares resonance energies of several compounds computed from group equivalents and heats of formation with values obtained from bond energies, heats of hydrogenation, and heats of combustion. TABLEI1 RESOSANCE EXERGIES Pauling4 Whelandb Bond Hydro- Cornevergenabusgies tion tion
Benzene Toluene Ethylbenzene o-Xylene Mesitylene Styrene Phenylacetylene Phenol Aniline Acetophenone Benzoic acid Benzaldehyde Diphenyl hTaphthalene Anthracene Phthalic anhydride 1,3-Butadiene
39
36.0
41
...
41
35.2 35.2 33.1 36.7
43 44 50
105 7" 6"
... ...
52
7P
... ...
.. .. .. .. 7"
4b 4" 8"
75 105
.. ..
1,4-Diphenyl-1,3-butadiene . .
, . .
... ,.. , . .
51
50 51 54 67' 47 91
77
... ...
116
3.5
..
10.7" 79
.. ..
This study group equivalents
36 35
35 36 34 37 35 37 39 37 2b
32 75 62 87 52
3.2
5.2" 5.9" 1 101 Stilbene .. ... 16 7.0 Benzoquinone 17.2 2 1 28 23 Furan . . . 29 31 31 Thiophen ... 27 24 31 Pyrrole ... 41 38 37 Urea . . , 25 22 28 Acetic acid (monomer) 41 29 . . .. Acetic anhydride ... 25 24 18 Ethyl acetate ... 17 21 20 Formamide ... 21 25 21 Acetamide ... 41 38 37 Urea a Resonance energy in excess of that of the aromatic nucleus. * Resonance energy in excess of that of the aroCorrected to the matic nucleus and COOH Group.
monomer.
It will be observed that there is excellent agreement between resonance energies calculated from heats of hydrogenation and those calculated from group equivalents and heats of formation even when the heats of formation a r e obtained from heats of combustion. We may conclude therefore that the method of group equivalents gives resonance energies of satisfactory accuracy. There are, however, quite large discrepancies in many instances between resonance energies Pauling, "Nature of the Chemical Bond," Cornell University Press, Itheca, N. Y ,1940. ( 5 ) Wheland, "The Theory of Resonance," John Wiley and Sons, Inc., New York, N. Y . , 1944. (4)
4279
NOTES
calculated 'from group equivalents or heats of hydrogenation and those based entirely upon heats of combustion or bond energies. Since the latter do not in most instances include allowances for the effect of neighboring groups, they may be considered to be less accurate than the former. It is of interest to note that resonance energies calculated by the method of group equivalents for such compounds as phenol, aniline, benzoic acid, benzaldehyde and phenylacetylene show resonance energies that are very little different from those of benzene. Although t.he resonance energies of these compounds are in general slightly higher than that of benzene, they are in most cases smaller than the values given by Pauling4 and Wheland.6 Small resonance energy between the benzene ring and substituent does not necessarily indicate small resonance interaction since it can be shown that the resonance energy may be quite insensitive to the extent of interaction. I n contrast, the spectra, dipole moments and chemical activity of monosubstituted benzenes can be shown to be quite sensitive to the extent of interaction! Another example is the quite large effect on chemical activity resulting from hyperconjugation in mono-olefins even though the heats of hydrogenation may differ by only a few kilocalories. As a consequence of the fact that the entropy of a molecule is not affected by resonance as discussed above, the resonance energy of a compound calculated from heats of formation a t 298'K. can be employed with the aid of group equivalents a t various temperatures t o calculate the free energy of formation a t those temperatures. Table I11 compares the free energies of several compounds calculated by this method with measured values. TABLE111 FREEENERGIES CALCULATED WITH THE AIDOF RESONANCE ENERGIES 7
~----298~K.--, MeasCalcuured lated
Benzene Toluene Mesitylene Styrene p-Methylstyrene Naphthalene Anthracene 1,3-Butadiene Isoprene Thiophene Phenol Aniline Urea Acetic acid Ethyl acetate
31.0 29.2 28.2 51.1 50.2 50.4 72.4 36.4 34.9 27.6 6.3 39.5 -42.5 -91.2
-
-77.8
AFp -1OOO'KMeasured
30.6 62 3 29.0 76.3 28.8 113 0 50.0 93.9 49.6 111 0 52.0 .... 74.1 62 8 35 6 33.7 78.9 25.7 - 4.9 39.5 ,... -46.0 -92.4 -58.5 -74.6 .. .
Calculated
63.4 77.7 115.6 93.1 111.0
60.3 76 0
.... -61.9 .. .
The agreement i,s quite good in most instances. The discrepancies with urea and ethyl acetate, (6) Matscn. Tms
JUUKN.II,. i i i
prrs?.
NOTES
4280
although rather large, are probably well within the accuracy of the data, I t should be borne in mind that certain of the "measured" free energy values given actually represent results that were obtained on materials in the solid or liquid phase, and free energies of vaporization of considerable magnitude had to be estimated in order to convert the values to the gas phase where resonance energies are applicable. These estimates introduce an uncertainty of a t least 2 kilocalories in some instances. I wish to thank Professors F. A. Matsen and i:.i V . Wheland for their advice on the preparation of this paper.
VOl. 72
naphthyllithium prepared by the direct method and purifying the product by chromatography. Preliminary experiments indicate that the preparation of tetra-1-naphthylsilane proceeds with considerable difficulty, probably brgely because of steric hindrance.
Experimental All temperatures are uncorrected unless otherwise noted. Tetra-2-naphthylsilae : (a) From Silicon Tetrachloride and 2-Naphthyllithium Made by the Direct Method.A 0.39 iV solution of 2-naphthyllithium in ctlier was inadc ind 7 g from 10 g. (0.048 mole) of ~-bromotiaphthalene~ (0.1 g. atom) of lithium i)v the u w d l method, the yield (967,) being determined 1)y acid titration? Two grain-. (0.018 mole) of silicon tetrachloride in about 50 ml. of drv ether was added to the orgatlolithium reagent with stirring. REFIXIXGTECNNICAL AND RESEARCH nIVISIOXS After refluxing for ten niinutc- the mixture was poured HLJKBLEOILPL REFINING COMPANY into dilute hydrochloric acid, the inorganic salts removed RECEIVED JUNE 14, 1950 TEXAS BAYTOWN, by extraction, and the organic Ltyer steam-distilled. The residue was extracted with about 400 ml. of petroleum cther (b. p. 60-80n) and this extkact was poured through Tetra-2-naphthylsilane L' 4 X 20 em. columii of The colored material ibas not eluted after development with 1 1. of solvent. The BY HEXRYGILMAN AKD CECILG. BRANNEN c h a n t was reduced to a small volume to obtain 5.1 g. (Sl%) of the pure hilane, m. p. 216-217" (cor.). In connection with studies on steric hindrance Calcct. for CroIT&i: Si, 5.22. Found: Si, in arylsilanes, we have synthesized tetra-2- 5.29, 5.2u. naphthylsilane. This is of interest because of a (h) From Silicon Tetrachloride and 2-Naphthyliithium generalized comment la that four naphthyl or tolyl Made by Halogen-Metal Interconversion.-To a solution groups can be bonded to silicon. Four tolyl of 10 g. (0.048 mole) of 2-bromonaphthalene, rn. p. 57in 50 ml. of dry ether was added 0.048 mole of ngroups have been attached to si1icon,lb but none .Bo, butyllithium'o in 46 mi. of ether keeping the internal temof the compounds reported has an ortho substit- perature a t 5". After stirring for fifteen minutes, 1.6 g. uent, which is of most significance in steric (0.0088 mole) of silicon tetrachloride in 15 ml. of ether was studies. There appears to be no reference con- added dropwise maintaining the temperature at j3. Test 111 was positive even after stirring for two hours cerning tetra-o-tolyl- or any tetranaphthyl-silane. aColor t 5 O followed by refluxing thirty minutes. The mixture Furthermore, experiments designed to bond four was poured into dilute hydrochloric acid; the organic layer o-tolyl radicals to silicon have demonstrated2 was rxtracted with water and steam-distilled. The residue that thc fourth p o l y ? i s introduced with dif- wrti rxtrarted mith petroleum ether (b. p . 60-80"i. In h i t cmps, 2 40 g. i51fY,)of the crude sil,tiie, m 1) 2r)2 ticulty, %Oi", tvdb i - o h t ~ d . Recrvstallizatioii from ethanol IVe have found that tetra-2-naphthyisilane can ethyl nretate ( I . I) gdVe 2 10 g. (427,) of the pure zilane, bc easily prepared b y treating silicon tetrachlo- n i . p. 21W217' (cor.) i c ) From Ethyl Silicate.-An ethereal iolution of 20.7 g. ride, ethyl silicate, .or trichlorosilane, respec(0.1 mole) of 2-bromonaphthalene and 0.1 mole of ntively, with 2-naphthyllithiuni. The reactions butyllithiuni were mixed under the usual conditions6 a t 5 " . involved in the preparation from trichlorosilane After stirring for ten minutes, 1.9 g. (0.009 mole) of ethyl possibly proceed2 through the formatian of tri- silicate, b. p. 164" .It 740 mm., iii 20 ml. of dry ether was added dropwise and the mixture refluxed for fifteen hours. 2-naphthylsilane The mirture was worked iip d s in (b) using the same sol:,RLi -t- CIJSiH ---+RtSiH 3bC1 (I! vent for crystallization. 2,2'-Binaphthyl, 0.40 g. ( 6 9 4 ) , mtlting point nnd mixed melting point with an authentic tCSH 4 K l i --+ &Si Liti (2) specimrn, 188-189)" (cor.), separated first; then came an .&Naphthyllithiuin was made both by the direct impure fraction, m. p 150-160°; foll2wed by 1.90 g. method4 from 3bromonaphthalene and lithium (40%) of the crude silane, in. p . 203-205 . Recrystallizametal and by halogen-metal interconversionj tion of the crude silane gave 1.2 g. of material, m. p. 216 217' (cor.), which showed no depression when mixed with of 2-broinonaphthalene with n-butyllithium. The the material from (a). former method introduces colored impurities Chromatographic adsorption of the petroleum ether into the product which are difficult to eliminate estract failed to achieve a separation. Fractional crysby the ordinary techniques, while the latter tallization of the solid present in the eluant showed this to contain binaphthyl and the silane. method introduces the coupled product, 2,2'- material (di From Trichforosilane.-2-Naphthylhthiurn was binaphtlipl, which is extraordinarily difficult to made it1 the same manner as in (b) using the same quantiI _
_I
+
+
remove completely. Higher yields were obtained by treating silicon tetrachloride with 2-
(1) (a) Barkhard, Rochow, Booth, and IIartt, Chem. Reoicws, 41, 105 (1947), (b) Polis, Bet , 18, 1540 (1885) (3) Unpublished studirs hy Dr G N R Smart (3) Gilman and A.Ias?ie. THIS JOURHAL, 65, 1128 (1946); Meals, :bi i , 68, 1880 ( l 9 l 0 i I Cilruan and Melvin, $bid , 71, 4050 (1949). 14) Gilman, Zoellncr and Selhy ibzd , 54, 1957 (1933. i5) Gilman mri hlunrr rlwi 62, 1813 (1040
_ _ I _
( 6 ) Prepared in 55% yield m essential accordance with the directions of Ncwman and Wise. %bad 63. 2847 (1941) (7) Gilman, Wilkinson, Fishel and Meyers. i b i d . , 45, I50 (1923). ( 8 ) Fisher Scientific ComDanv. - . 80-200 mesh (9i Sei: Gilrnan, Hofferth, Dunn and Melvin, ibid., 78, in press I1050). il0i i:ilmau, B e d . H r u n n e n . B I I I I C ~Dunn Z, and Miller, ibid.? 71, ~
1499 (194.1 !!I) Gilinnn
and Schulze, i b i d . , 47, 2002 (1925).
