COMMUNICATIONS TO THE EDITOR
Feb. 20, 1963
485
The infrared spectrum in the CsBr region of a benzene solution containing a 1: 1 mixture of ethyllithium-7 and t-butyllithiuni-7 exhibits a strong broad band Since - A H is positive, D(H-A) is greater than zero if centered around 500 c m - l , not found in the spectra D(X-H) - D(X-HA) is either positive or negative of either of the components. Ethyllithium and tbut less than -AH. An approximation of D(X-H) butyllithiuni have strong absorption bands a t 538 and D(X-HA) may be obtained from the Morse poand 480 an.-',respectively, attributed to vibrations tential for X H and X-HA treated as diatoms and the of the carbon-lithium framework. Summation of vibrational frequencies v(X-H) and v(X-HA). For optical densities of spectra of ethyl- and t-butyllithiumOH bonds for example i t turns out that D(X-H) 7 show that the 500 cm.-l band does not result from D(X-H9) is about 5 to 25 kcal./mole in agreement with overlapping of the bands of these components. The the expectation that the X H bond in the complex is 500 cm-l band in the EtLi7-t-BuLi7 complex shifts to longer and weaker than X H in the free molecule. about 514 ern.-' in a 1 : l solution of EtLP and tExperimental values for -AH probably are in the range B u L ~ . This ~ band may be tentatively assigned to of 2 t o 8 kcal.,'niole (if the solution values can be taken carbon-lithium framework vibrations in one or more as a guide) so that the bond dissociation energy of the new organolithium species containing both ethyl and free radical HA is about 7 to 33 kcal./mole in the gas t-butyl groups. phase. The characteristic triplet and quartet pattern in the Perhaps low temperature hydrogen atom reactions hydrogen n.m.r. spectrum of ethyllithiurn? in benzene is in the gas phase or e.s.r. experiments on H atoms not changed by addition of t-butyllithium, and the peak trapped in an inert matrix containing a diluted proton positions are only slightly shifted. This finding is not acceptor could reveal the presence of such radicals. inconsistent with complex formation, but does suggest If the Morse function is a valid description for that, if ethyl groups exist in different environments in D(X-H) and D(X-.HA), equation 5 gives a relationthe solution, exchange must be rapid compared to the ship between AH and the difference in stretching frerelaxation time. On the other hand, the singlet a t quencies v(X-H) - v(X-HA). This is being investi7 = 9.01 in the proton n.m.r. spectrum of pure t gated in more detail both experimentally and theobutyllithium in benzene is split into a doublet in soluretically. tion containing ethyllithium. A 1: 1 mixture of the Helpful discussions with Drs. S. Bauer, K . Kutschke two compounds gives t-butyl peaks a t r = 9.08 and and F. P. Lossing are gratefully acknowledged. r = 9.31, with relative intensity about 7 : 2 . Thus DIVISION OF PURECHEMISTRY t-butyl groups appear to be present in at lest two NATIONAL RESEARCH COUNCIL H. J. BERNSTEIN environments. OTTAWA, C A N A D A relation
D(H-A)
D(X-H)
RECEIVED
-
D(X-HA)
-
AH
(5)
DECEMBER 1, 1962
COMPLEX FORMATION BETWEEN ETHYLLITHIUM AND l-BUTYLLITHIUM Sir :
Cosolution of ethyllithium and t-butyllithium in benzene leads to the formation of complex organolithium compositions, differing from either pure component, containing both types of alkyl groups bonded to lithium. Pure ethyllithium1~2 and t - b ~ t y l l i t h i u mexist ~ as six-fold and four-fold polymers, respectively, in benzene solution. Apparently carbon-lithium bond breaking takes place in this solvent, leading to exchange of alkyl groups between polymeric organolithium molecules when both compounds are present. The new compounds which are produced are believed to be electron-deficient polymers of the type (EtLi),(t-BuLi),-,, where rn is a small number such as 4 or 6. Ethyllithium alone is sparingly soluble in cold benzene, but its solubility is greatly enhanced in benzene solutions of t-butyllithium. Evaporation of the benzene from a solution of both compounds leaves a lowmelting white solid residue. The white solid is highly soluble in pentane, unlike ethyllithium which is virtually insoluble in this solvent. The complex is also much more volatile than ethyllithium. I t distils readily a t 70' and 0.1 mm., forming colorless crystals. Gas chromatographic analysis of the hydrocarbons formed by hydrolysis of several samples of the distilled material show that its composition can vary. The ratio of ethyl to t-butyl groups in the resublimed composition is nearly the same as that in the original solution. Benzene solution with ethyllithium : t-butyllithium ratios of 1: 1 and 1.8:1 gave distilled products with ethyl: tbutyl ratios of 1.1:1 and 1.7:1, respectively: ni.p. 68.-72' and 56-59'. (1) T. L. Brown a n d M. T. Rogers, J . A n i . Chem. .9oc., 79, 1859 (1957). (2) R . West a n d W. Glaze, ibid., 83, 3580 (1981). (3) h l . Weiner, G. Vogel nnd I
