NOTES
4484
Examination of a Meerwein Reaction for an Intramolecular Isotope Effect with Carbon-14*
-CHzCH=CHCHzCl
BY Gus A. ROPP A N D VERNON F. RAAEN APRIL 1, 1954 RECEIVED
0 KOH
/'
Because of an interest in the occurrence of carbon14 isotope effects in addition reactions, it Meerw e h 2 reaction of carbon-14 labeled butadiene was examined for an isotope effect
OpN-\
Vol. 76
-CH=CHCH=CHz
+ KCl + H,!O
-
than for the liquid l-chloro-4-(~-nitrophenyl)-butene-2. This dehydrohalogenation also is essentially quantitative, and it could, therefore, be asCUCL? + H2C=CHCH = C14tI2 NaOOCCII~ sumed that the specific activity of the diene would 0 2? i truly represent the specific activity of the chloride and give &,tal. This method should be applicable generally to the evaluation of isotope effects in addition reaction of unsymmetrically labeled symmetcompounds. sz rical After the above reactions were carried out, the kl4 / -Cl4HzCH=CHCH2Cl specific activity of the purified p-nitrobenzoic acid was found to equal one-half the specific activity of O&\ the purified l-(p-nitrophenyl)-l,3-butadienewithin Use of the unsymmetrically-labeled symmetri- experimental error, thus indicating no preference on cal molecule, butadiene-l-CI4, provides a simple the part of the diazonium compound for reaction method of measurement of the ratio, k14/k12, of the with a labeled or unlabeled end-carbon atom in burate constant of the reaction of the diazonium com- tadiene. This result is understandable if one aspound with a labeled end-carbon atom of butadiene sumes that the reaction proceeds through an initial to the rate constant of the reaction with an un- rate-controlling3 break-up of the diazonium comlabeled end-carbon atom. Thus in the event k14 pound to yield an extremely active fragment such # k12, the ratio of these rate constants could be as the P-nitrophenyl radical. If every such fragevaluated as the ratio of the specific activities of the ment immediately reacts with a butadiene molecule carbon-14 in the a-carbon atom of the side-chain as rapidly as i t is formed, the distribution between of the product, l-chloro-4-(p-nitrophenyl)-butene-2, the labeled and unlabeled end-carbon atoms of a to the specific activity of the d-carbon atom of the labeled butadiene molecule should be statistical. This result is also in accord with recent results reside chain ported by Urry4 who demonstrated that the most klclklz = S a / S A The specific activity of the a-carbon atom, Sa, active of a series of free radicals appeared to give would be given by the specific activity of the p-ni- least isotope selectivity in abstracting hydrogen trobenzoic acid obtained from vigorous oxidation of from deutero-organic compounds. The synthesis of 1,3-butadiene-l-C14 from the Meerwein reaction product m e t h ~ 1 - C ' ~iodide via methyl-C14-vinplcarbinol which was dehydrated a t 300' over phosphorus / -CHzCH=CHCH2CI oxid. pentoxide involved only conventional procedures. 02s- \ The Meerwein reaction and dehydrohalogenation / -COOH HC1+ 2 HtO 3COz of the l-chloro-4-(p-nitrophenyl)-butene-2have been described elsewhere.2 The specific activity 02s- \ of the labeled diene was shown by Van Slyke combustions and radioassays of the resulting carbon Since this reaction is nearly quantitative it could dioxide to be 2.66 pc. per mmole. A 2-g. sample of be assumed that there is no isotope fractionation the labeled l-chloro-4-(p-nitrophenyl) -butene-2 was in the oxidation and that the specific activity of the boiled three hours with 15 g. of sodium dichromate, p-nitrobenzoic acid truly represents the specific 50 ml. of water and 15 ml. of C.P. sulfuric acid. activity of the a-carbon atom. In practice, isola- The resulting purified p-nitrobenzoic acid ohtion of the d-carbon atom would be more difficult, tained had a specific activity of 1.35 pc. per mmole. and i t would be much simpler to obtain the total Although the specific activity of the acid appeared specific activity of the l-chloro-4-(p-nitrophenyl)- to be about 1.5% greater than one-half the specific butene-2 and to calculate the isotope effect as activity of the diene, this difference was too small to kir/(kiz klr) = &/(Sa Sa) = S m / S t o t s l indicate definitely isotope selectivity in view of the From k14/(k12 k14), k14/k12 could be easily calcu- limited accuracy of the assay method used. lated. The value, Statal, would be measured more CHEMISTRY DIVISION readily for the easily purified solid 1-(p-nitro- OAKRIDGENATIONAL LABORATORY phenyl)-l,3-butadiene, obtained by the dehydro- OAKRIDGE,TENNESSEE halogenation?
+
-0
0
___f
0
+
+
+
+
+
( I ) This document is based upon work performed under Contract S t i m b e r W-7405-eng-28 for t h e Atomic Energy Commission a t t h e Oak Ridge National Laboratory. ( 2 ) C . A. Ropp and E. C. Coyner, "Organic Syntheses," John Wiley and Sons, Inc., New York, 1951, Volume 31, p . 80.
(3) Cf.D . H . H e y and W. A. Waters, Chem. Rcas., 21, 189 (1937); C. C. Price, "Reaction a t Carbon-Carbon Double Bonds," Interscience Publishers, Inc., New York, N. Y., p. 52. (4) W. H . Urry. Reports of the Organic Division of t h e American Chemical Society, Symposium, Denver, Colo., June, 1051, p. 30. (5) 0. I(. Neville, THIS J O U R N A L , 70, 3501 (1048).
