John F. Sebasfian Miami University
Oxford, Ohio 45056
The Electronic Effects of Alkyl Groups
The past few years have witnessed the accumulation of experimental and theoretical evidence which suggests that the traditional descriptions of the electronic nature of suhstituent alkyl groups might have to be revised. This new evidence is largely a consequence of the introduction of new instrumentation (ion cyclotron resonance and IaC magnetic resonance spectroscopy, for example) and the development of sophisticated molecular orbital calculations. Since many of these recent developments in alkyl group suhstituent effects have not been reviewed or incorporated into textbooks, we here present a summary of some of the current views in this area. The discussion which follows is not meant to be an exhaustive review or survey of the literature but simply a summary of the more salient developments. The organic chemist's traditional picture of the electronic effect of alkyl groups is based on the inductive model whereby the methyl group is thought to be an electron releasing (compared to hydrogen) substituent. The inductive effect is generally defined as a successive polarizition of bonds in a chain. The effect is considered to fall-off with distance from the substituent. Thus methanol is predicted to be a weaker acid, in solution, than water because, according to the model,
substitution of a methyl group for hydrogen increases the electron density on oxygen. Qualitatively, this argument appears to be consistent with solution data. However, Brauman and Blair (1) have recently measured the gas phase acidities of alcohols by ioncyclotron resonance spectroscopy. The following order of relative acidities was found: neopentyl alcohol > t-hutyl > isopropyl > ethyl > methyl > water; and t-butyl = n-pentyl = n-butyl > n-propyl > ethyl. This order is reversed from the order observed in solution. Since the inductive effect model outlined above did not consider solvent effects a t all it should apply to the gas phase as well. It clearly does not. Brauman and Blair suggest that t-hutoxide in the gas phase is less basic than methoxide because the methyl groups, being polarisable, can stabilize the negative charge by an induced dipole. In support of this suggestion, Schuhert, et al, (g) have found that p-alkyl substituents function as apparent electron acceptors (relative to hydrogen) in lowering the energy of the nucleophilic principal electronic transitions of phenol, anisole, aniline, and N,Ndimethylaniline. These electronic spectra were determined in the gas phase (at elevated temperatures) and in heptane. The results were rationalized in terms of "substituent-polarizability" (which includes either or
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both direct polarization of the bonding electrons to the substituent and possible internal dispersion force polarization) and electronegativity. Recent microwave dipole moment determinations (3) have indicated a small electrmwithdrawing effect for methyl groups in ordinary (i.e., normal bond angles) saturated systems lacking polar substitnents. These same studies found that methyl groups are electron releasing when bonded to a carbon atom involved in a r system. The linear dependence of 13Cchemical shifts on electron density ( 4 4 ) has led Olah and White (7) to conclude, from lacnmr studies of carbonium ions, that the central carbon atom in the t-butyl cation is slightly more positive than that in the isopropyl cation. The cations in question were observed in SOe C1F-SbF, a t -20°C. Finally, Pearson and Songstad (8) have concluded, from heats of formation data, that (CH& C+ is a harder carbonium ion than CH3+. If 'alkyl groups were electron releasing one would expect the reverse. Therefore, alkyl groups, in this instance, appear to be elect,ronacceptors. Theoretical explanations for the results and conclusions cited above are not lacking. Pople and Gordon (9) carried out CNDO calculations on a variety of hydrocarbons, fluorine compounds, and oxygen compounds. The calculations yielded charge distributions and electronic dipole moments. I n summary, replacing a hydrogen atom in ethylene or acetylene by a methyl group resulted in a little charge migration between the methyl and the vinyl (or ethynyl) group. However, the methyl group did appear to repel the electrons away from the atom to which it was attached onto the p position. The studies on the fluorine compounds yielded results in sharp disagreement with the traditional description of inductive effects. The calculations suggested that induced charges (resulting from substitution of a fluorine atom for a hydrogen atom is a hydrocarbon)". . . alternate in a decaying manner. . ." Hence, the carbon atom P to the fluorine is normally negative. Aninvestigation (10) of the interaction of the methyl group and the triple bond in methyl acetylene by an accurate molecular orbital method found that the calculated dipole moment of 0.70D (0.75D by microwave) is largely due to a r-system polarization (0.88D as calculated) in the direction HC-C+C-H3+, which is reduced (to 0.70D) by an opposite c polarization of the molecule. The methyl group was calculated to donate 0.056 and 0.026 electrons, respectively, to the o and r system of the ethynyl moiety. The fact that the a's (diagonal matrix elements of the one-electron Hamil-
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tonian) of the acetylene carbon atom adjacent to the methyl group were more negative than the a's of the terminal acetylenic carbon atom led to the conclusion that ". . . the inductive model of charge transfer is not compatible with the present calculations." The CND0/2 method has been applied to calculations on the acidities of alkanes and saturated alcohols and the basicities of methylamines (11). The calculations indicated, among other results, the following: (1) stabilization of negative charge by alkyl groups; (2) the larger or more complex the alkyl group, the greater the negative charge stabilization; (3) the experimentally determined orders of gas phase acidities of alcohols and basicities of methvlamine were renroduced bv the calculations; and (4) the more substituted a saturated hydrocarbon position, the larger - the uositive charge - density on that position: Semiempirical SCF-MO calcnlat,ions for methylsubstituted borazines and hvdrocarbons Yielded the following results (12): for" the hydrocarbons the methyl group was calculated to be more electron withdrawing in the r framework than hydrogen; for the methyl borazines, the methyl group was calculated to be electron releasing through the r system, and electron withdrawing through the o framework when attached to boron, but electron releasha- when attached to nitrogen. I n conclusion, there is now substantial evidence, both experimental and theoretical, which suggests that the traditional inductive model for the electronic effects of alkyl groups needs to be modified. The results cited here apply predominantly to the gas phase and perhaps reflect more precisely the intrinsic electronic nature of alkyl groups than solution data. These studies, then, emphasize the importance of solvent effects on chemical reactivity and molecular stmctnre. Literature Cited (1) Bnaaxm, J. I.. AND B L A ~ L., K.. J . Amcr. Chem.Soc.. 90,6561 (1968). J., Tetrahedron. 17, (2) scans^^^. W. M., MURPHY,R. B., AND ROBXNB, 199 (1962). n, J . A m ? . Chem. So
