J O U R N A L OF T H E AMERICAN CHEMICAL SOCIETY (Registered in U. S. Patent Office)
VOLUME 81
(0 Copyright, 1959, by the American Chemical Society) NUMBER 9
MAY 14, 1959
PHYSICAL AND INORGANIC CHEMISTRY [CONTRIBUTION FROM
THE
CHANDLER LABORATORY OF COLUMBIA UNIVERSITY]
Detailed Mechanisms of Exchange Reactions between Iodine and Allyl Iodide BY WILLIAM P. CAIN'AND RICHARD M. Noms2 RECEIVEDOCTOBER 28, 1958 Allyl iodide-l-C1' has been prepared, and thermal and photochemical rates have been measured for the iodine induced isomerization t o allyl ic~lide-3-C~'. The rate of thermal isomerization is about 20% of the rate a t which iodine molecules exchange with allyl iodide, a n d the atom induced photochemical isomerization is about 50$&of the rate at which iodine atoms exchange with allyl iodide. T h e d a t a seem t o require t h a t both molecules and atoms of iodine can add t o allyl iodide t o form r-complex intermediates in which iodines become equivalent more easily t h a n the two end carbons can become chemically identical.
Introduction Previous kinetic studies demonstrated that two distinct mechanisms are available for the isotopic exchange of allyl iodide with elementary iodine. I n the dark at room temperature, the kinetics indicate t h a t the facile exchange reaction involves iodine mole~ules.~If solutions are illuminated, the rate of exchange is greatly increasedI4and this additional reaction certainly involves iodine atoms. A complex kinetic analysis of the quantum yields indicated that the atomic mechanism involves a direct substitution in which one iodine atom enters the molecule and the original one leaves either simultaneously or by dissociation of a short-lived metastable intermediate. Two alternative structures were suggested for the transition state associated with each mechanism. For the molecular mechanism, these structures were I
,. .,I* ,\
CH-CH-C
'Hn
C H - C H ~ H ~:I *
I*. . . I*. . I I I1 I n these structures and in all subsequent ones, a n asterisk (*) is used to designate an iodine atom originally present in the element, and a prime (') is used to designate the carbon atom to which the \
,
(1) Based on the Ph.D. Dissertation df William P. Cain. The original dissertation and microfilms thereof are available from the Library of Columbia University. (2) T o whom inquiries should be sent. Department of Chemistry, University of Oregon, Eugene, Ore. (3) D. J. Sibbett and R M. Noyes. Tms JOURNAL, 76,761 (1953). (4) D. J. Sibbett and R. M. Nora, ibid.. 76, 763 (1953).
organic iodine was attached originally. These designations correspond to the initial positions of the isotopic tracers in the experiments described below. For the atomic mechanism] alternative structures I11 and IV were proposed. I CH~CHC'H~
I*. . CH-CH-C'H~.
I
I*
I11
IV
Structures I and I11 involve mechanisms in which the entering and leaving iodine interact simultaneously with the saturated carbon atom; structures I1 and IV involve mechanisms in which the entering iodine interacts with the double bond. Also, exchanges through structures I and I11 produce allyl iodide in which the new iodine is attached to the same carbon atom that was occupied originally; while exchanges through structures I1 and IV produce allyl iodide in which the other end carbon is nowoccupied by iodine. Sibbett and Noyesa.' expressed preferences for structures I1 and IV on the basis of the exchange kinetics of benzyl iodide. I n general, allyl and benzyl compounds have comparable bond strengths and react a t about the same rates. However, benzyl iodide does not appear to undergo any exchange with iodine molecules, and the rate of exchange with atoms is only about 1% of the rate for allyl iodide.' These differences in reactivity indicate that the double bond in allyl iodide facili-
2031
(6) M. Gazith and R. M. Noyes, ibid., 77, 6091 (1955).
tatcs exchange in a way t h a t is not available to I)c.iizyl iotlitle. .i ~ n o r espwific test o f ~iieclianisiiiis to tleterriiine whether eschangc~is :iccoiiiiiunietl by :L shiit i n t h e carbon to which the iodine is :tttachetl. One mctlicid that was considered was to distinguish the c:irlxins by attaching a inethyl group to oiie as was done by England and Hughes' in their clever t1enioristr:itii)ri of the SK?'reaction hetweeri lironiide ion Lint1 the methyl allyl brcimidcs. Howel-er, the 1-11iet1iyl a n t 1 Y-tneth>-l allyl iodides aI)p;irently have never been prepared separately, :ind intcrconversion is T v y rapid.' T h e procedure finally selected wzs suggested by the synthesis of allyl broniide-l-Ci4 by Systrorri aritl 1,e:ik.' Tlie corresponding allyl iodide was synthesized by a similar procedure, ant1 the distribution of carbon-14 was determined by ozonizing the allyl iotlide and isolating the number-:; carbon 21s formddchyde. Then simultaneous trxcer es~ ~ e r i n i c n with ts carbon and iodine were used to iiie:istire I-late w n s alxiut 80' i . 7 1 t ~ et l J S y l 3 t e then was added to a mixture of potassium iLitlidc ivith iiietallic mercury and silver powder in ethylene gi! < - t i l , .ind the \\-illJle mixture w:is stirred in total darknes, , ~ ice t teriipcrdture under an atmosphere of nitrogen. S i i m e iL),liiie \ \ : I \ f~iriiied after t h e initial addition (if t h e alI>-l t,,.! late, b u t it \v:is removed bl- t h e metals t h a t wcre preient. 'The mixture friiin t h e reaction was washed n-ith \v,iter : i n t i ;iqttcoiis ,iiltite and mixed with silver poivder, antl the vol:ttile Lillyl i(idide w:ts vacuum distilled in the dark :it t ~ i i t t i temperature through phosphorus pentoside i n t o a liqtiitl nitrogen t r a p . Tile product was Jreighed b y the liglit of s i ruby tl
b e d in addition t o t h e ozonization reaction. Despite t h e low yields, we are reassured b y the consistent stoichiometric equivalence of the two products. Other Reagents.-The hexane solvent was pr' pared from petroleum ether b y treatment with fuming sult'u-ic acid as described previouily.'* I t contaitied about 2 X 1 U -c, i i i i ~ l e / I . of material capxblc of forming org:tnic:illy botinc' iodine under tlie cmditions of our experiments. Reagent grade resublirned iodine was used without urther purification. Solutions were activated with carrier free iodine-131 obtained froin t h e Oak Iiitlge National I,, bor'ttory on authorization of t h e United Stittcs .ltiiniie Energy Commission. For some experiments not requiring carbon-14, CIJI irnercia1 allyl iodide was purified b y the method of Sibliei t antl so ye^.^
General Kinetic Procedures.-In the therinal experii ients, the concentration of allyl iodide \vas SI) much grcate tlian t h a t of iodine and t h e fractional rate of isurneriz'itioi was so slow relative to t h a t of exchmge t h a t virtuall). all I i f a n y iodine-l:3 1 initially present in t h e element would h ' e entered the allyl iodide before there was any iiieasur:ililc isuinerization of the carbon-14 distribution. Therefore, eschange and isomerization experiments were carried o u t separately. T h e appropriate solutions \\-ere pilxttctl into separate coinpartmetits in a n opaque glasb-stoppered flask :ind therino.t:rted a t :30°. T h e r u n KLS -tartetl by inverting a n d shaking the fla5.k and returning i t to the thermostat. At the end of the r u n , t h e iodine was reduced a n d eXtr,icted nitti aqueous sulfite, and customary procedurez \\ere used t o measure either the fraction of iodine-131 in tlic ; I C ~ U C O U S layer or the specific activity of carbon-14 iii tlie numhcr-:3 carbon of t h e allyl iodide. I n photochemical experiments, t h e times ( I f illuniiriation rnriged from :3,9 to 11)seconds and were too short f i J r xccur'ttc measurement, IIon'ever, the qu:llitity of intere5t to u s i.; tlic ratio ( i f rates of escliange :mil isomerizati(1n r,itiier than tiie absolute value ( J f either rate. Since for both r e x t i o n s the lognritiiin I J f tlic diitance from equilibrium is :L linear function of tlie time, the ratio of the rcitesW;LS cdculalile from the fractirmal distmce e:~cIi had giiiie t o equilibriuiii eveii \vitliout a n y irifi~riiiation 3 s t o tlie xctuul tiiric of re,ictioii. Solutions of iodine-131 and of allyl iodide-I-C1' ~ v c r emixed antl illuniiiiated for short periods with the 4 a mercury itrc. .in aliquot sample o f the sulution was C Y tracted with aqueims sulfite, arid t h e rndioactivity of t h e LlqLlelJus layer was coinpared with t h a t of a control solution of iodine. T h e number-3 carhirn of the allyl iotlitle tlien was converted tc) the forinaldehyde diinctliime, .tiid its activity \\-,is also iiie:isured. Calculation of Rates.-The rate of exchange (equation 2 ) can be cnlciilnted b y standard equations. If x is the fraction of iodine-131 present in t h e element a t time t , and if tlie subscripts 0 and m refer t o initial and equilibrium \-,tliies
where
I n these expressions, .I1 refers t o allyl iodide, and K,, is t h e totxl rate of reaction 2 by all mechanisms. T h e specific complications IJf application t o thermal and photrichenlical d a t a are discussed in the sections on results. T h e rate of isomerization (equation 1) cnn lie calculateti froin tile expression fli,
where r: is defined by equation 3 . T h e factor of two entcrs the denominator because the equilibrium state for rcactirm 1 represents t h e change of only half t h e molecules from their original configuration just a s in t h e case of optical activity where t h e rate a t which molecules invert configuration is half the rate of racemization a s customarily defined.
Thermal Results Measurements of Exchange.~--~.iswas indicated above, measurements of theriiial isomerization antl (12) J . Ziinmermnn unci R. lr. S o y e ? , J . ( ' h ? i u P l i j y . , 18, ''?.->8 (1950).
exchange could not be made on the same solutions. The thermal data of Sibbett and Noyes3 indicate t h a t in our concentration range I
