HYPERCONJUGATION: AN ELEMENTARY APPROACH RICARDO CARVALHO FERREIRA Recife, Pernambuco, Brazil
When an atom or group of atoms with an available THE CONCEPT of hyperconjugation, first suggested by Baker and Nathan seventeen years ago (1) is seldom electron-pair is bonded to a conjugated system of mentioned in the introductory organic chemistry double bonds there occurs resonance between structures classes, although others equally "advanced" and not I and 11, resulting from the electronic displacements: less difficultsubjects have won their places in these same courses. This is unfortunate since a knowledge of the principal features of the hyperconjugation effect is essential to the understanding of a number of facts in the chemistry of the unsaturated hydrocarbons. At the advanced level there are detailed reviews of the suhIn the same way, when an alkyl group is bonded to a ject (2, S), which has been treated by the molecular conjugate system there may occur resonance between orbital method by Mulliien and his co-workers (4, 5). structures I11 and IV: However, apparently there is not in the literature an N N N elementary approach to the subject and we believe that H-C-C=C-C=Cthe following treatment may be useful to many organic chemistry teachers who wish to introduce the concept of H C=C-C=C+ H-C-C=C-C=C IV hyperconjugation together with that of "normal" reso111 nance to their sophomore or junior students. The number of resonance structures of the types I11 Possibly the best manner to teach the concept of and IV will increase as the number of the C-H bonds of hyperconjugation to the beginner student is to give an account of its experimental basis. For didactic reasons the carbon atom in the alkyl group bonded to the conjugated system increases. Since there are three such the empirical evidences of hyperconjugation may be C H bonds in methyl, two C-H bonds in ethyl, one divided into three categories, namely, kinetic, thermo- C H bond in isopropyl, and no C H bond in ter-butyl chemical, and spectroscopic evidences. (the carbon atom refers to the one directly bonded to the conjugated system), the effect will be greater for methyl KINETIC EVIDENCE and will progressively decrease as far as ter-butyl, as In 1935 Baker and Nathan (I), studying the following found by Baker and Nathan. Mulliken, Rieke, and bimolecular reaction between p-substituted benzyl bro- Brown (4, 5) have shown that this hypothesis is commides and pyridine with dry acetone as solvent, patible with the molecular orbital theory and called it hyperconjugation2 to distinguish it from normal conX o H y B r +N jugation. Many other kinetic evidences for hyperconjugation H2 may be found in the original literature but we will not describe them here (6, 7). found that the reaction rate increases in the order X = THERMOCHEMICAL EVIDENCE NOz < H < alkyl radicals, that is, with the increasing Even those students who have had only the fundaelectron release tendency of the substitute group. It is a firmly established fact that the electron-release ten- mental principles of the resonance theory know that a dency of the alkyl radicals due to the inductive effect resonating structure is always associated with lowered alone follows the order Me < E t < iso-Pr < ter-Bu. free-energy content, that is, increased stability of the Rather surprisingly, however, Baker and Nathan found molecules. The hyperconjugation effect should likethat the rate of this reaction increases in the order X = wise produce greater stability of the alkyl-substituted olefins compared with the nonsubstituted ones. Table H < ter-Bu < iso-Pr < E t < Me1. We may explain this apparently anomalous behavior 1 shows the heat of hydrogenation of some mono-olefins as determined by Kistiakowsky and collaborators (8). as follows: It can be seen from the above data that the stability
+
-
+
This behavior is described in the literature as the "BakerNathan effect."
2
Second order hypereonjugation need not be considered here.
NOVEMBER, 1952 TABLE 1 No.nf C-H hyperconjugated bonds
-AH Kg.-c$./rnol
Olefin
B F H-6 =&-H
0
32.8
H H
I
I
I
I
3
H-C=C-CHa H CH,
30.1
6
H-C=C--CH1 H CHs
28.4
9
HaCC=C-CHJ CHa CHJ
26.9
HaC-C=C-CHs
2fi. 6
I
I
I
I
12
of the olefins increases (since their heats of hydrogenation decrease) as the number of hyperconjugated &H bonds increases. From thermochemical data it can also be shown that hyperconjugation, such as is present in propylene, produces a resonance energy of the same order of magnitude at the conjugation between two double bonds, as in l,3-butadiene. In fact, the difference in the heats of hydrogenation of ethylene and propylene is 2.7 kg.-cal./ mol and that between 1,4-pentadiene (isolated double bonds) and 1,3-butadiene is 60.8 57.1 = 3.7 kg.-cal./ mol (9).
-
SPECTROSCOPIC EVIDENCE
A similar effect has been observed in alkyl-substituted -ethylenes. For example, X,, for ethylene is 1630 A, for propylene it is 1730 $ and in the case of tetramethylethylene X,, = 1840A (10,11). Mulliken, Rieke, and Brown (4) have shown that this is precisely what one may expect to observe as the number of hyperconjugated C-H bonds increases. The evidence for hyperconjugation from dipole moment measurements (13) is not conclusive and recently has been disproved. This refers to the fact that although the dipole moments of the saturated hydrocarbons and of ethylene are zero, this is not the case with some alkylsubstituted mono- and di-olefins (see Table 2). These dipole moments could be attributed to two factors, namely, hyperconjugation involving the C-H bonds of the methyl groups or charge transfer due to the difference in the electronegativity of a saturated carbon (sp3 hybridization) and an unsaturated carbon atom (sp2hybridization). In the last few years it has been shown by theoretical eaJculatious that the inductive effect alone is responsible for the dipole moments of these substances (16). Since hyperconjugation represents a stabilizing influence it plays an important role in the thermodynamics of certain reactionsof themono-and poly-olefins. An interesting and simple account of the applications of the concept of hyperconjugation to these subjects has recently been published by Baker (17).
I t is found experimentally that a compound with conjugated double bonds absorbs light of a longer wave- LITERATURE CITED length than a similar compound with isolated double (1) BAKER,J . W., AND W. 8. NATHAN, J . Chem. Soe., 1935, bonds. Furthermore, as the number of conjugated 1844; see also WHELAND, G. W., J. Chem. Phys., 2 , 474 (1934). double bonds increases, the absorption shifts to pro(2) DEASY, C. L., Chem. Reu., 36, 145 (1945). gressively longer wave lengths. For instance, the ex(3) CRAWFORD, V. A,, Quart. Reu., 3 , 226 (1949). treme ethylene absorption band is 1900-2000 A, that of R. S., C. A. RIEEE,A N D IV. G. BROWN, J. Am. (4) MULLIKEN, Chem. Soc., 63, 41 (1941). 1,3-butadiene is 2100 and that of 1,3,5-hexatriene is R. S., AND C. A. RIERE,ibid., 63, 1770 (1941). 2500 k (10, 11). I n the case of the substance 2,4,6,8,- (5) MULLIKEN, (6) HUGHES, E. D., C. K. INGOLD, AND M. TAHER,J. Chem. 10,12-tetradecahexaene the absorption occurs in the Soe., 1940, 949. visible band and the compound is therefore colored. (7) BAKER,J . W., AND M. L. HEMMING, ibid., 191 (1942). (8) WHELAND, G. W., "The Theory of Resonance," John Wiley This fact is attributed to resonance between conjugated & Sans., Ino.., New York. 1944... n. 54. double bonds the number of the resonating structure (9) WHELAND, G. W., ibid., p. 87. increasing with the number of conjugated bonds. (10) PRICE,W. C., AND W. T. TUTE, PTOC. Roy. Soe., A174, 207 (144ni --,. \-"
TABLE 2
Hvdroearbon
Formula
H Propylene HIC=C-C& H H H 2,4Pentadiene Hd-C=C-C=CB rr 1L
din Debves)
Refe7-
0.35
13
0.68
14
enees
%Methyl-1,shutadiene
H2C=C-C=CH2
0.38
14
Toluene p-Xylene' o-Xylene
CH, CaHrCHa p-HsC-CQH,-CHa O-HIC--C~H+~CH~
0.37 0.00 0.62
13 15 15
I
nSyrnmetricalmolecule
(11) PRICE,W. C., A N D A. D. WALSH, ibid., 174, 220 (1940). (12) WHELAND, G. W., 8p. eil., pp. 133-4. (13) MCALPINE, K. B., AND C. P. SMYTH, J . A m . Chem. Soe., 55, 453 (1933). (14) HANNAY, N. B., AND C. P. SMYTH, ibid., 65, 1931 (1943); 68, 244 (1946). (15) HURDIS,E. L., AND C. P. SMYTH, ibtd., 64, 2212 (1942). (16) These calculations are mainly due to C. A. COULSON, H. C.
LONCUET-HIGGINE, A. PULLMAN, B. P U L L ~ R. N ,DAUDEL, and other investigators in England and France. See, for instance, PULLMAN, B., AND A. PULLMAN, '*Les theories Blectroniques de la ehimie organique," Masson & Cie., Paris, 1952, p. 411. J. W., in "Ternas Actuales de Quimica,)' Conseja (17) BAKER, Superior de Investigaciones Cientificas, Madrid, 1950, pp. 2346.
