RESEARCH
Inductive effects stabilize carbanions Kinetic studies show fluoroalkyl anions are stabilized by inductive field effects, not hyperconjugation Inductive effects, rather than hyperconjugation, appear to have overriding influence on the stability of fluoroalkyl anions. Dr. Andrew Streitwieser, Jr., and his coworkers at the University of California, Berkeley, believe they have compelling evidence that effectively rules out previous theories of the importance of hyperconjugation in the stability of these anions [/. Am. Chem. Soc, 88, 692, 693 (1967)]. This work is part of Dr. Streitwieser's research effort in the area of acidity of hydrocarbons. For his achievements he will receive the 1967 ACS Award in Petroleum Chemistry in April at the 153rd ACS National Meeting in Miami Beach, Fla. (see page 110). Hyperconjugation, or no-bond resonance, is generally considered to be significant in. the electronic structure of carbonium ions. It involves the leakage of electrons from a neighboring bond to the electron-deficient carbon of the ion. In effect, this results in spreading the positive charge around a greater volume of the molecule. Electron repulsions are thus diminished, leading to greater stability of the ion. The hyperconjugation theory has also been invoked to explain the stability of carbanions. For example, Dr. John D. Roberts of Caltech, while working at MIT, suggested that negative hyperconjugation contributes to the resonance structure of p-trifluoromethylaniline. The negative hyperconjugation concept was used by Dr. Linus Pauling to interpret bond lengths, and at Georgia Tech by Dr. Jack Hine (now at Ohio State University) and Dr. Robert C. Rosscup to interpret solvolytic reactivities of polyhalogenated compounds. Dr. Sam Andreades of Du Pont, a former graduate student of Dr. Streitwieser's, also used the theory to explain the results of isotope exchange experiments involving a series of fluorinated hydrocarbons. He found, for example, that tris (trifluoromethyl) methane forms a carbanion a billion times faster than does fluoroform with a solution of sodium methoxide and methanol. Dr. Streitwieser, however, reasoned that inductive effects between the carbon and fluorine atoms might explain the differences of carbanion stability in 54 C&EN FEB. 6, 1967
Dr. Andreades' series of compounds. To check his theory, he and graduate student David Holtz examined the isotope exchange rate with lH-undecafluorobicyclo [2.2.1 ]heptane. In this bridged-ring compound, all but one of the carbon atoms are fully fluorinated. Moreover, the lone C—H bond is at the bridgehead of the molecule. This location is significant because the anion formed at this position during base-catalyzed exchange cannot overlap to any significant extent with neighboring C—F bonds to delocalize negative charge by hyperconjugation. Overlap is precluded because of the great strain that would be produced by forming even a partial double bond at the bridgehead of the bicyclo[2.2.1] system, Dr. Streitwieser notes. Inductive effects, however, have no such
stereochemical limitations, he says. Measurement of the isotope exchange rate of the bicyclic compound presented special problems. Experiments had to be performed from —45° to —74° C. because at higher temperatures the isotope interchange rate was too fast to be measured. The Berkeley chemists were able to obtain their results at these unusually low temperatures for kinetic studies by a simple, yet ingenious method. They inserted a fragile bulb containing tritiated methanol into a test tube that held a mixture of methanolic sodium methoxide and a sample of the fluorinated bicycloheptane, supplied by Dr. J. C. Tatlow of the University of Birmingham, England. Then they placed the test tube assembly in a thermostatically controlled bath and allowed
RAPID EXCHANGE. Dr. Andrew Streitwieser, Jr., studies the carbanion form of lH-undecafluorobicyclo[2.2.1]heptane. Tritium exchange with the compound's lone hydrogen is faster than that with tris(trifluoromethyl)methane, indicating the anion is stabilized by inductive effects
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A fluorinated carbanion might be stabilized either by negative hyperconjugation . . .
. . . or by inductive effects
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it to reach temperature equilibrium. To start the kinetic run, the fragile bulb was smashed against the bottom of the test tube. This introduced tritium into the reaction mixture and allowed isotope exchange between the fluorinated hydrocarbon and the tritiated alcohol. At a specific time during each reaction, Dr. Streitwieser and Mr. Holtz added toluenesulfonic acid to neutralize the sodium methoxide and stop the reaction. Using a liquid scintillation counter, they measured the tritium content of the isolated fluorocarbon. Results from a number of such experiments showed the exchange is first order. Surprisingly, the bridged-ring fluorocarbon forms a carbanion five times faster than tris(trifluoromethyl)methane does. Thus no-bond resonance doesn't provide significant stabilization in the tris(trifluoromethyl) methyl ion, Dr. Streitwieser and Mr. Holtz reason. It is probable, therefore, that negative hyperconjugation is not involved in the other fluorocarbon systems for which it has been postulated. The
relative stability of these fluorinated carbanions undoubtedly derives largely from an inductive field effect, they say. To further check this conclusion, Dr. Streitwieser, with Dr. A. Hadyana Pudjaatmaka (since returned to Bandung University, Indonesia) and Dr. Alan P. Marchand (now at the University of Oklahoma), carried out a series of proton-exchange experiments using 9-substituted-9-tritiated fluorenes and methanolic sodium methoxide. The substituent groups included methyl, ethyl, methoxymethyl, benzyl, and trifluoromethyl. On plotting the rate of anion formation against pK, or acidity, of the corresponding substituted acetic acids, the Berkeley chemists find that the value of each derivative, and especially that of 9-trifluoromethyl, falls on the same straight line. This substantiates the inductive effect theory of anion formation, Dr. Streitwieser claims. "If negative hyperconjugation were involved, we would expect the —CF 8 group to yield an anion more readily than it actually does," he says.
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