might be rate limiting. in the poorly understood, and the experimental conditions normally 4-6. The carbon-halogen bond might be broken

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Whitesides et al. f Reactin ctf Alkyl Halides with Magnesiunt Mechanism f Frmatin f Grignard Reagents. Kinetics f Reactin f Alkyl Halides in Diethyl Ether with Magnesiuml 217 Harld R. Rgers,2 Craig L.
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Whitesides et al. f Reactin ctf Alkyl Halides with Magnesiunt Mechanism f Frmatin f Grignard Reagents. Kinetics f Reactin f Alkyl Halides in Diethyl Ether with Magnesiuml 217 Harld R. Rgers,2 Craig L. Hill,3 Yuz Fujiwara, Randall J. Rgers, H. Lee Mitchell,a and Gerge M. Whitesides* Cnyibutin frm rhe Department J'Chemist.y, Massachusetts nstitute f Technlgy, c'ambridge, Massachusetts 02t39. Receit'ed september ll, 1978 Abstract: The rate f reactin f rganic halides with magnesium is prprtinal t rganic halide cncentratin and magnesium surface area. All rganic ididis examined and many secndary alkyl brmides appear t react with magnesium at masstransprt r diffusin-cntrlled rates in diethyl ether. The rates f less reactive rganic brmides (nepentyl, phenyl' cyclprpyl) cntain cntributins bth frm mass transprt and frm chemical reactin. Mst rganic chlrides react at rates which are nt mass transprt limited. Analysis f rate-structure prfiles indicates that free carbanins and carbnium ins are nt intermediates, and tliat the transitin-itut. structure des nt resemble an SN2 reactin. nsertin f magnesium int the carbn halgen bnd als dcs nt seem t ccur. Sluble Mg(l) can be excluded as the nly reactive species respnsible fr disappearance f alkyl halide. The tw mst plausible limiting rate-determining steps are halgen atm abstractin'frm the rganic halide by the metal and electrn transfer t the rganic halide frm the metal. ntrductin halide, either clse t r adsrbed atthe magnesium surface, The mechanisrn(s) f frmatin f Grignard reagents are might be rate limiting. in the prly understd, and the experimental cnditins nrmally 4-6. The carbn-halgen bnd might be brken used in carrying ut the reactin reflect the accumulated experience f synthetic chemists, but nt a clear understanding transitin state. 7, 8. A.rnagncsium atm at the mctal surface (r pssibly f the reactin mechanism.s 7 A central difficultl in studr in-c in slutin ) rrrrght displace helide frtn carbn in an SN2-likc rcuctin r inrcrl int thc crtrbttn halgcn bnd in a cncerted this hetergeneus rcactin mechltnism is that l btlining interpretable kinctics infrntatin. This papcr dcvclps rrpprpriate techniques and presents represcntltivc ratc \tructurc Pf r)e Css. 9. Thc rutc-linriting itcp nright invlve a species with a cvalent carbn'magnesium bnd; fr example, the rate f detachment f an rganmagnesium species frm the magnesium priil.r. Tw accmpanying papers extend thesc techniques8'e and establish that the rates f reactin f all alkyl idides and f mst secndary alkyl brmides with magnesium in diethyl surface r diffusin f the prduct Grignard reagent frm the ether, and f all alkyl brmides in plar slvents, appear t bc surface might be rate determining. 10. n sluble, reactive, magnesium(l) species might cleave mass transprt limited. Rates f reactin f mst alkl'l chlrides are slwer, and are nt mass transprt r dil'f'usicln linlited. A third paper uscs studics i thc c()rr\ip rtl'ttt.l-tjnc\llllll thc curbn halgen bnd. The kinetics data that fllw exclude runr trunsitin-statc structure shtlrving a large buildup f charge ()r surfaces t gcne ratc a qualitatir c dc:criptin trl lhc prtrcc::c' lrrrgc:tcric cf'f.ccts lirr thc rganic grup. and cstablish that \lgtl) that remvc ntltgncsiunt atnt\ frnl thc nlctal surlilcc.; \\ tlrk i: rtl thc :lc :pccic: rcspnsiblc fr brcaking carbn hrrltrgcn bnds. J lnd 6 rcnlain thc mst plausiblc tran- t be described in futurc papers.rr and that rcprtcd br thers.ll characterizes the rganic prducts l- rcactin. and sitin-statc structurcs. Thrughut this wrk, we will be cncerned with prcesses reinlbrccs the hypthesis made by Walbrsky, and dcvclped by' Bickelhaupt and thers, that radicals are inte rmediates in having velcities limited by the rate f encunter f reactants. Thesc encuntcr rates are influenced bth by mass transprt frmatin f Grignard reagents. (in We have cnsidered ten limiting transitin states 'r the turn determined primarily by the agitatin f the slutin) rganic miety, summarized by structures l-10. Here' Mg, and by dilfusin. Thc available evidence suggests that mst f represents a magnesium atm f a magnesium metal surface, the mechanistically intercsting reactins invlving alkyl halides ccur at thc magnesium-slutin interfacc. Fr cnveniencc and Mg(n) represents a magncsium in. in what fllws. we will rcfer t prcesses having ratcs limited R by smc crnbinatin f mass transprt and diffusin as RX 6 Mg,---R---X (Srl) transprt limitcd , withut specifying whether the prcess 2 trxrmg - 7 is hmgeneus r hetergeneus.l 3 3 [RX]._ 8 R;-----,X 'Mgl Resulis and Discussin 4 R- 9 RMgY Kinetics Methds. Thc rate f reactin f rganic halides s R+ l0 R---x---Mg'r with mctallic magnesium depends, in principle. n thc (mcar. rhe unperturbcd rganic haride wurd rcprcscnt,rhc il::il:l;'*'. ji' xl?:t :h::::lii3;;ilri:,l:l':il ?ll : transitin statc if the vcrall rate f reactin wcrl' limitcd by iitics f t'hc mtrgncsium surface. Thc simple.st cxpressin fr t he rate f encuntcr f this rganic halide, eithcr with thc thc r.tc f disap,-pea ra ncc f an rganic hulid , R ' X. fr'''.magnesium surface r with sme ther reactivc spccics in slutin wuld becq l. in which/(mg) takes int accunt all thc lutin. masncsrum surface charactcristics that influencc this 2. The rganic halidc mighl ildsrb at thc mctal surllcc in 'rr;.r{ the ratc-limiting step. 3. Electrn tiansicr frm rhe magnesium 1 thc rganic -d(rrx)/dt = kr(rrx)'y(mg) (l) Reprinted f'rm the,llrrnalt lhe Arnerit'an ('henrical St'ietr'. 102, Jl; (1980).1 C'pvright c l9u0 ln'the Amt'ricrrn ('hernit'al St'ietv ancl reprinterl lrr'perntissitr l'the c'ltvright rvner 2r8 Jurnal f the American Chentit'al Sciety 102:l f Januarl, 2, 1980 l-fr ) ? ft-., t:l r t rs[nrx) 7ln.xl,] l'igure l. [:ur rcprcscntltivcntpctitive Grignard rcitctins pltted accrding t cq 3 cvclhcry'l brmidc vs. 'clpcnty'l brmidc (O); cvclpcntylchlridc!'s. cvclpentrl brmidc (a); /r-pcnty-lchlride vs. c),- cltrhcrylchlride (l): ncpcntyl brnride vs. cyclpenti'l brmide (O). (Thc lirst member in each pair is pltted as R1X. the sccnd as R:X). -d(r2x)/dr = ft:(rzx) /(mg) (2) lg [(RrX),/(RrX)] = (krlkz) lg [(R:X),/(RzX)] (3) We studied initially the relative rates f reactin f tw rganic halides prescnt in thc samc slutin at a cmmn magnesiunr surface.l5 Assuming that/(mg) is the same fr tw structurally similar rganic halides cmpeting fr a magnesium surface. we write eq 2 fr a secnd alkyl halide R1X. Dividing cq b1 cq 2, making thc further assumptin that rr = l. and integrating we btain eq 3; the crrectness f the assumptin that a = is cnfirmed experimentally using a differentechnique (vide infra). Plts i cq 3 are linear t )65V cnsumptin f alkyl halide; representative data are shwn in Figure 1. Deviatins frm linearity'at high cnversins, when bserved, are due primarily t changes in the plarity f the medium at high cncentratins f Grignard reagent.s A representativc rati f the ratc cnstants (A rr'a:) btuincd using eq 3 is inscnsitivc t changes in magnesiunr sanrplc sizc. purity, and surface arca and t the additin f plausible impurities t the slutin (Figure 2).r tn. infcrence frm Figure 2 that the measured rati f rates is independent f the cxtent f agitatin is crrect nly fr certain pairs f alkyl halides. n the specific instance in which ne halide reacts at a transprt-limited rate and the secndes nt. the rati f ratcs measuredes depend n c. althugh it is entirely reprducible at a particular value i c. All i the data cllected here were btained using c,r = rpnr: dctailcd discussin f the dependence f relative rates n c,.r is deferred t the cnd f this sectin. l{igh cncentratins f magnesiurn salts d influence the rate rati by changing the plarity f the medium.8'e The scatter f pints in Figure 2 prvides a realistic illustratin f the reprducibility f the data btained using this technique. The bservatin f straight-line plts in Figure cnfirms the insensitivity f k rlkz t the cnccntratin f alkyl halides. t lw cncentratins f nragnesiurn salts and Grignard reagents in sluticln. and t the magnesium surface area. since all are changing during the curse l'a kinetic run. Equatin 3 is nt useful in trying t examinc the influence f slvent crnpsitin. slvent viscsity, r temperature n the rcactivity f a singlc rganic halidc tward rnagncsium. Fr studies f this type. we used a different prcedure. A quantit;' f magnesium very large cmpared t the quantity f rganic halide t be used (a ild excess) was immersed in slvent, and its surfac cleaned by etching using either 1,2-dibrmethane r an alkyl halide. The reactin f magnesium metal with rganic halides nrmally generates relativelv smth, plished surfaces:r that is, fr limitcd cxlents f rcactin, thc surface area f the magnesium changes very little. t is thus pssible t bring a smallquantity f ne rganic halide int cntact with the etched maqnesium surface. fllw its / /Milhey.8l A.s.T. M. K fi8, f,tf'she,,r lp.er pur t,.d k OBr Resenr 1 lll,mttincx,at 1 'll,^r,r'.^ se.ss% Avg '-RMgBr (lv) a-ms8r, (sr) /Ms -Mecr2 (sr) l.' (.. -f il'ff:, ::'^ -i-?... i;-j,-i;lij;,, ,,rjif, ];-;n -c b'rjii - rms, Ms, (.r!) rct w-j:/::' ] i ',:i',' 'i': \r{cc'z?,.1,i:.-; l-,rzr l-irtrzt l nrj,.j l'cct^ i CCl^ i l1 a ' : ':l :;\;.E F,; a = F- E i:55 E e. :E ;: a a a ; a a E b : 3# - f E B- /8? / F60 Figure 2. Thc rclativc ratcs f'rcactin f'ncpcntll brntidc und crcl- Pent\l brtlrnide uith rnugncsiurn lrc in:cn:itirc t() ntitgncsiuln t\nc. qulrntrtr.lrnd surf'acc arca. and t a nunrber 1'ptcntial cntanrin.rnts. -fhcr rrre inl'lucnccd br high cnccntratins l'rnagncsiurn halidcs;rrrd Crignurd rcrtgcnts. Thc linc indicltcd es avg rcprcscnts thc avcragc i rrluc rrbtuincd frnr scvcral dctcrniinutins f thc rclativc ratc in dictlrr crhcr rrt 0 C rrith Blkcr puriircd nragncsiunr (standard cnditins lirr rril rhc ctlrnpct iti,''c ki nctics crpcrinrcnts ) 8.,,\ \ l-^l t# n il ll L-] lll ' li r li -{ f-t t-ffi1 - i-t--fl -a# ^- c Figure 3.,\pprrrrrtus lbr lblltlu rng thc rcactin l'a singlc rganic hrrlid. rrith rr largc cxccss l-nrugncsiurn. (A) High-trquc, ad.lustublc spccti stirrcr. ( B) Srrrnpling and incrt srrs inlct thrugh a \-lir stppcr. (( l.cl l rcuctin slvcnl (trpre.rllr g11..1(x) rnl.). (l)) ['ur(tlrigirrullr) l.0t) X in. rc.rgcnt ur.itlc (()()()/( ) rr.rrlrrcsirinr r'(xl\ bunti t thc ccntrlrl :tirirrc''\xj Lt\ 1\\() JrtCe c\,)l l\-girugc :tlrrnles: \tccl\\ irc. disappearance with time, remve the slvent cntaining the initially frmed Grignard reagent and replace it u'ith fresh slvent. add a secnd halide. and fllw its disappearance with timc, all w'ithut significantll'changing the surface area f the magnesiurn. Althugh the numbers btained frm these expcriments have n abslute significance. they'can be cmpared with nc anthe r. sincel( Mg) in eq can be assumed cnstant vcr thc cursc f'thc experintents. The apparatus used in these studies is that shwn in Figurc 3. Variatins in the rate f stirrirrg cstablished that thc values f thc appare nt rate cnstant f r ccrtain alky'l brmides (e.g.. c1'clpenty'l brmide) increased with the stirring frcquencr c '(figure 4A).17 Thc rates were rcprducible at a particular value f c,;. Data were rdinarilv cllected using c = 140 rpm. T ensure the reprducibility'f the nrethd, measuremcnts were made at least in duplicate: that is, using the same magnesium assembly. kinetic runs were carried ut in succcssin with RlBr. RlBr. R1Br, and R2Br (r, when study'ing thc influence f slvent n the rate f reactin f a singlealkyl halide, in successin with slvcnts A, B, A, and B). These expcriments arc mre cmplex and less accurate than the cmpetitive kinetics cxperiments. Neverthelcss. D Whitesides e l. f Reat'tin J'Alk1,l Halides with Magnesiunt 219.! O E \ g Crft Er - LTH aah A' l't'' 2 -_; T Prlsrfr-- 3 O 0.4 LJ.b L!1 \ F.8 -? r^ =- T? v,z r RN LJ.:,.8 n7 { lv E-,l- (l ; X -] N L 6 R, (CH,taC-Hr8r R2: q C5n98r [i-' j '1 slrrrrng rle (r.p.m ) Figure 4. (A) Veriatin in thc apparent first-rdcralc cnstant fr disrppcarancc f'cvclpe ntyl brmidc in dicthyl cthcr at 0 C with stirring rutc btaincd using thc cnstant-surfacc kinetics tcchniquc. Thc santc sculc l stirring nrtcs apply t (A) and ( B). Abvc ca. 50 rpnt, vrtc\ing linrirs thc accuracy'f this nrcthd whcn applicd as in (A). Fillcd svnrbls rcprcse nt duta btlrincd ils part l'a sl,stcntatic studv l'thc rnflucncc l'r n fi.rtcnstant:pcn svnrbls rcprcscnt single cxpe rirne nts btained undcr rughll crtrpanrblc cnditins. (B) Variatin in thc rclativc rate cnstants lirr rcaclin f'tw alkyl hrrlidcs uith rrragnesiunt. as u f'unclin l-stirring nrtc. The tw alkyl halidcs were prcsent in thc sanre slutin and thc duta rrcrc cllcctcd irnd analvzed using the crnpctitive kinctics tcchnique, bul thc cxpcrimcntal cnfiguratin uscd fbr slirring was that typical i thc cnstant-surfacc tcchniquc (Figure-1). The tp sct f data in this figurc ( O. rr-c sh 1 Br vs. c-(.stlqbr) re prescnts a cmpctitin bctwccn tw brrlidcs bth rcacting irt r clsc t a transprt-lintitcd ratc. The ccntral sct (0, (Ctlr)rC--Cll2Br vs. c-cst-l,rllr) givcs data lirr cxperinrenis which nlv ne halidc is clcarly lransprt linritcd. The bttnr set (O. rr-c5ll11cl vs. c-c-shecl) givcs rcsults fr cmpetitins in which ncithcr halide is lnrnsprt lirrited. (Cl) Thc rati tlf ratcs f rcactin l'ncpenrl,l brnride rlnd cvclpentyl brrnidc in diethyl cther. as a lunctin f stirring ratc, tlbtuincd using the cntpctitive tcchniquc (nragnctic stirring bur and nlrgncsiunr t urnings). agrecmcnt between runs was usually *.l}v, with n evidence fr an imprtant systematic decrease in ratc due t a decrease in magncsium surface area. Typical data are shwn in Figure Since data btained using each f the tw kinetics techniques will be utilized in the discussins that fllw, it is useful t have a dcscriptive label fr each. We will call the first prcedure lime (min) l'igure 5. Pscud-first-rdcr plts fr thc disappearance l- cyclpentyl brnridc in dicthy'l cther btained in fur cnsccutivcxperiments using thc cnstant-surhrcc tcchnique (r. iirst run; E, third run) and tetrahydrl'urun (t. sccnd run. a. furth run). dcscribcd the cmpetitin f tw rganic halides present in a singlc slutin fr the same magnesium surface-the cmpetitive technique . The secnd prcedure-the cmparisn f data btained frm successive reactin carried ut s that a magnesium surface f apprximately cnstant area is maintained will be referred t as thc cnstant-surface tcchnique . Thc cnstant-surface technique serveseveral useful functins. First. it permits the cmparisn f rates f reactin f a single rganic halide with magnesium in different slvents and at different temperatures. Secnd, it prvides a direct experimental measure f the expnent a in eq and 2: since a plt f lg (RBr),/(RBr) vs. time is linear (Figure 5), a cquals l. Third, it prvides a methd f checking results gathcrcd using the simpler cmpetitive techniques based n cq 3. The ratis f rate cnstants btained by measurements f tw alkyl halides in sequence using this mre cmplex technique agree (perhaps frtuitusly) with thse inferred frm thc simple cmpetitivc prcedure. Fr example, measurement f the ratcs f reactin f cyclpentyl chlride and cyclpentyl brmide in sequence using the cnstant surface prcedure gave the rati k '+tr tlkcslqbr = 0.053; measurement based n the cmpetitive kinetics technique and eq 3 gave This last bservatin is directly relevant t a general questin cncerning thc cmpetitive kinetics methds (eq 3) which is particularly imprtant fr this wrk: viz., what is the significance f the relative rates measured using this technique when ne f the cmpeting substrates reacts at a transprt-limited rate and the secnd reacts at a rate slwer than diffusin r mass transprt?17 Cnsider the influence f agitatin in the three pssible limiting rate regimes fr reactin f tw alkyl halides (A and B) with magnesium. i A and B bth react at transprt-cntrlled rates, and bth have similar diffusin cnstants (as is the case in virtually all f this wrk), knlks - will be independent f the stirring rate. f A and B bth rcact at rates that are much less than transprt cntrlled, kn and ks will be independent f mass transprt and k,rlks will be independent f stirring rate. lf, hwever, A reacts at a rate less than mass transprt but B reacts at a mass-transprtcntrlled rate, the analysis f relative rates becmes cmplicated: kn will be independent f stirring rate, ks will depend n mass transprt and will increase with increasing stirring rate, and k nl ks will decrease with increasing stirring rate. Figurc 4B shws rate data btained using pairs f alkyl halides that fit these three limitins cnditins. These data were 220 Jurnl f'the American Chemit'al Sciet.t' f 102:l f Januarv'2, 1980 q) = -^ x (Jl -- to 3.O 2.O r. X = Brra \x= cr i;ffg r Fas,-'\ / i\ t^'t \,i-l :0 E Rt+Mg RBr. MS RC. Mg J. OA2A34 surfce re (rbitrry units) t (t X & Figure 6. The relativc rittes f rcactin f cyclpcntyl brnridc (O) and crclpcntyl chlridc ( O ) with nlagncsiunr is prprtinal t thc cxpscd surl'rrcc lrrclt f thc tttltgncstum. --z:rbr ' BuaSnH btained using the apparatus shwn in Figure 3 (t facilitate Figure 7. Structurc rcactivitv prfilcs fr thc reactin f rganichlridcs, cnlparisn with Figure 44) but using the cmpetitive kinetic brnridcs. and ididcs u'ith magnesium (tp) and rganic brmidcs with tcchnique; that is, bth alkyl halides f a cmpeting pair were tri-rr-bulvltin htdridc (bttm). Thc ring sizc f the cyclalkvl halidcs is dcnted by rr . All reactins wcrc run in dicthylether a10 (. Thc pint present in slutin at the same time, and reacting at the same rcprcscntcd b1 lrrt-butvlirr rcrrctin f'alkyl chlridc r,'ith rnlrgncsilrnr magnesium surface. Evidence presented later in this paper and rr rrs btirincd usrng 2-rrrcthy l-j-buty chlridc. cf. lintntc b. Tablc l. in a fllwing paper8 indicates that cyclpentyl brmide reacts at a transprt-cntrlled rate; n-pentyl brmide is n the verge The mst imprtant cnclusin frm this rate law is that the f transprt cntrl (its rate is ca. 90V Lhat f cyclpentyl rate-determining step in the reactin requires rcactin f rganic halide at the magnesium surface. n particular, it cx- brmide); nepentyl brmide reacts significantly mre slwly than a transprt-cntrlled rate (ca. 507that f cyclpentyl cludes several cnceivable reactin mechanisms invlving brmide); n-pentyl chlride and cyclpentyl chlride react generatin f sluble Mg(l) by reactin i Mg(0) and nruch mre slwly than cyclpentyl brmide. Figure 4A Mg(ll),r': either in the rate-determining step r in a prir dcmnstrates that the rate f reactin f nepentyl brmide equilibrium (eq 5-7). n the first instance, the reactin ratc with magncsium shws a cnsiderably smaller dependence n wuld be indcpe ndent f rganic halide cncentratin. n the c,; than des that f cyclpentyl brmide, and cnfirms that sccnd, the rate rvuld depend upn the cncentratin f transprt limitatins are lcss imprtant with the l'rnler than magnesium-cntaining species in slutin, but wuld bc independent f the quantity f magnesium metal present, since with the lattcr. As expected, the measured values f ft,-( sr 1 Br/ k,j-1'.1 iqtlr?hd kr-c.t-t r r c t / ft.-1't 11 qcl are independent f stirring rate. while that fr k1c rr.,),cc'n:r3r/ft.-1.11n13,. the thermdynamic activity f magnesium metal in the system is indcpendcnt f its mass as lng as sme amunt f the mctal dccreases as the stirring rate increascs. The valuc mcasured is prescnt and as lttng as thc cncentratin f Mg(l) is truly at fr ft1c rrrtr('('1:br/ftc-c'sljqbr using the standard cmpctitil c cquilibriurn. Thus, structures such as l0 can be discarded as kinetics technique was fund t be independent f the magnetic exclusiue representatins fr the transitin state fr this reactin. Nte, hwever. that thc kinetic analysis utlined in a stirring frcquency in the range used (400*800 rpm, Figure 4C). A similar value was measured fr this rati f rates using the fllwing paper8 suggests that snle hmgeneus reactin hybrid experimental technique that frnts the basis fr Figurc may cntributc t thc disappearance f alkyl halide in cmpetitin rvith thc hetcrgeneus re
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