Nov. 20, 1959
RADICAL ADDITIONS T O 2-BROMO-2-BUTENES
[COSTRIBUTIOS FROM
THE
DEPARTMENT OF CHEMISTRY OF
THE
5937
UNIVERSITY OF W~scossrx]
The Stereochemistry of Radical Additions. IV. The Radical Addition of Hydrogen Bromide and Deuterium Bromide to cis- and trans-2-Bromo-2-butene1J BY HARLAN L. GOERINGAND DONALD W. LARSEN RECEIVED APRIL 13. 1959 T h e radical-chain additions of H B r a n d DBr to the isomeric Z-brom0-2-butenes a t -80" in excess liquid H B r or I)9r are completely stereospecific. Under these conditions the trans addition products a r e obtained. -1s t h e reaction temperature is increased, ;.e., a s the amount of addendum in the liquid phase decreases, the stereospecificity diminishes to t h e point where both olefins give essentially the same product a t room temperature. Under all conditions the degree of stereospecificity appears to be about the same for H B r and DBr. T h e intermediate 2,3-dibromobut-2-yl radical reacts about 2.4 times faster with H B r than with DBr. Radical addition gives only 2,3-dibromobutane a n d ionic addition results in the exclusive formation of 2,2-dibroxiiobutane.
In work described in preceding papers in this halides it was clear that the radical addition of this series i t was found that the radical addition of HBr addendum to the isomeric 2-bromo-2-butenes would to l - b r ~ m o - and ~ . ~ 1-chlorocyclohexene4 gives the give 2,3-dibromobutane and ionic addition would trans addition product, cis-1-bromo-2-halocyclohex- give 2,2-dibromobutane. As shown below, in this ane, almost exclusively. More recently5 it has system stereospecific (trans) radical addition of been found that the addition of HBr to l-methyl- HBr to the isomeric 2-bromo-2-butenes results in and 1-chlorocyclopentene also gives the trans addi- the formation of different products. tion products, cis-1-bromo-2-methylcyclopentane H Br H Br and cis-1-bromo-2-chlorocyclopentane.The pre'c-c' ponderant or exclusive formation of the least stable / \ of the two possible diastereoisomers in each case CH3 CH3 shows that the addition is stereospecific in these Ia meso-dibroinide cyclic systems. H H ,4 more rigorous test of stereospecificity of addif HBr \ tion reactions than is possible with small cyclic oleBrb:C-C---H fins is determining if geometrically isomeric olefins /c=c\ Br ' \ give different products (diastereoisomers or mixCH3 CH~ Br tures of diastereoisomers of different composition). Ib di-dihromitic I n the present work we have extended our studies The paper describes a stereochemical study of the to an acyclic system, the isomeric 2-bromo-2-bu- radical additions of HBr and DBr to the isomeric tenes (I), in which this criterion of stereospecificity 2-bromo-2-butenes. can be applied. Results From the earlier ~ o r k ~ it -appeared ~ that the most likely case for a stereospecific radical addition The isomeric 2-bromo-2-butenes were prepared reaction in an acyclic system would be the addition by dehydrobromination of pure samples of the isoof HBr in excess hydrogen bromide as solvent. This meric 2,3-dibromobutane~.~~ The tram-olefin I b was inferred from (a) the observation that HBr (derived from the dl-dibromide) was purified by adds more stereospecifically than other addenda in careful fractionation with an efficient column. This cyclic systemsJ-Gand (b) evidence that the degree material was shown to be homogeneous by gas parof stereospecificity depends on the life time of the tition chromatography (g.p.c.).I1 The cis-olefin intermediate radical formed in the first (addition) I a (derived from the meso-dibromide) was more difstep.G That the transfer (second) step is more ef- ficult to purify because of its tendency to isomerize ficient with HBr than with other addenda (and to Ib. Samples containing as little as O.lyoof the thus the life time of the intermediate radical is trans isomer (g.p.c.) were obtained by rapid fracshorter) was apparent from the long chains' and tionation a t reduced pressure. However, the the relatively very low tendency for telomer forma- samples of I a used in some of the earlier experiments tion. contained as much as 2y0 of the trans isomer. The radical addition reactions were promoted by From earlier work concerning the orientation of r a d i ~ a l ~ - j ,and ' . ~ ionic7m9additions of HBr to vinyl irradiation with a quartz-jacketed Hanovia type SC-2537 lamp which fit into the test-tube-shaped (1) This work was supported by t h e United States Air Force through reaction vessel. The reaction temperature was t h e Air Force Ofice of Scientific Research of t h e Air Research and Devaried by immersing the reaction vessel in a cooling velopment Command under contract hro, AFIS(600) 1037. bath a t the desired temperature. (2) First reported as a Communication, H. L. Goering a n d D. W. Larsen, THIS J O U R N A L , 79, 2653 (1957). In the first series of experiments to be described (3) H. L. Goering, P. I. Abell and B. F. Aycock, ibid., 74, 3588 the reactants were mixed and then irradiated for (1952).
/CH3
) €1 I. Griering : i i i d I, I, S i n i i . z b i d . , 77, Sl(i.5 (1935). >) I; 1,. Iiov;e .ind D L\' Larsen. unpublished results ( 0 ) €1. L (:oering. 11. I . R s l y e a and D W. Larsen, 'THIS J O C K X A L ,
7 8 , 318 ( l 9 ; l f j ) . (7) F R. l I a y o and C. \V:ii!ing, Cheni. Reas.. 2 7 , X.51 (1940). ( 8 ) C . \X'Valling, \I 5 iCiiirash and F. R . l f a y o , THISJ O U R N A L , 61, 1711 (1o:is) ('31 I 1 I, Goerinn a n d L. I. Silrns, i b i d . , 7 9 , ti270 (lClS7).
x,
(10) F. G. Bordweil and P. S. Landis. ibid..7 9 , 1.593 ( 1 9 5 7 j . (11) Excellent separation of t h e isomeric 2-bromo-2-butenes, "2dibromobutane a n d t h e isomeric 2,3-dibromobutanes m a s obtained with a 10-ft. column packed with t h e commercial detergent, Tide. An operating temperature of 70° and flow rate of 40 mi. of helium per minute were used t o measure t h e intercontamination of t h e isomeric 2-bromo-2-butenes. An operating temperature of 120' a a s used t o analyze mixtures of the dibromobutanes.
HARLAN L. GOERINGAND DONALD W. LARSEN
593s
five minutes. -4ir was not excluded in these experiments. At temperatures below the boiling point of HBr ( - 67 ") or DBr (prepared from DzO and pure PBr3)12 the reactions were carried out in excess liquid addendum as solvent. At higher temperatures the olefin was saturated with addendum prior to irradiation and excess addendum was passed through the liquid reaction mixture during the irradiation. After irradiation the excess HBr or DBr was removed by evaporation and the composition of the residual addition product was determined by g.p.c." In all cases components were identified by comparison of infrared spectra with those of authentic samples as well as by comparison of retention times. The results of these experiments are summarized in Table I.
Vol. 81
chain process. In some of the experiments the products contained small amounts of 2,2-dibromobutane which was evidently formed by competing ionic addition. The amount of ionic addition product was generally greater at low temperatures than a t high temperatures as would be e x p e ~ t e d . ~ , ' This is probably partly due to the fact that the rate is of a higher order in HBr for the ionic additionI3 than for the radical addition and the concentration increases as the temperature decreases. The amount of competing ionic addition also was found to depend on the time interval between xnixing and irradiation. When this interval was < 2 minutes oiily small amounts (e.g., ca. 1-3%) of 2,2-dibromobutane were formed a t - 78". The last column in Table I shows the composition of the 2,3-dibromide fraction, ;.e., the composition of the TABLE I radical addition product. T H E RADICAI IDI IT ION O F IIYDRUGES B R O M I D E AND In the early experiments small amounts of 1,2I J E U I .%RIULI B R O M I D E TO THE ISOMERIC 2-BlZOXo-2-BUTEXES dibromobutane were found in the product.2 This Cornpoiition was apparently due to contamination of the isoof radical X o nf addition producta meric 2-bromo-2-butenes with 1-bromo-1-butene. Temp Addendum runs dl-Dibromide,b % oc The 2-bromo-%butenes used in those experiments A. tmns-2-Brorno-2-butcne were preFared from commercial 2,3-dibromobutane I1Cr 4 1lJO - TS which contained small amounts of 1,2-dibromobu9 DBr irlc) tane. In a separate experiment it was found that dehydrobromiriation of 1,2-dibromobutane gives 2HUr 4 94 t o 1O(J -63.5 bromo-1-butene (?&)yo),cis-1-bromo-1-butene DBr 2 94 t o 100 (24%) and trans-1-bromo-1-butene (37%). With HBr 2 94 t r 1 96 -51 6 t.he coluiiin used for 6.p.c. analysis the latter comDBr 2 92 to 94 pound has the same retention time as Ia, and cis-1E u12 02 t o 91 -41 bromo-1-butene has the same retention time as Ib. -:30 0 HRr 1 b2 to 86 Thus this contamination was not detected. I n the DCr 3 s2 to s5 present work i t was found that the only dibro"3 I S: -2'3 inides resulting from the addition of HBr to pure I DBr 1 SI (prepared from pure 3,3-dibromobutane) are "3HCr 1 8': dibromobutaiie (radical addition) and 2,2-dibromo1) butane (ionic addition). DBr 1 7.9 The data in Table I show that the radical addiHBr 1 i8 25 tion reactions of HBr and DBr are essentially comDBr 1 i i pletely stereospecific (trans) at low temperatures, B czs-2-Brorno-3-butene ;.e., in liquid addendum as solvent. The cis-2nzeso-Dibr0mide.b % bromo-2--butene used in these experiments conHBr 4 90 to 9FiC - 7s tained 2y6 of the trans isomer and thus a 2,3-dibro1 DBr 92 to 97 mide fraction consisting of 98y0 of the meso isomer DBr 1 91 -63.5 corresponds to a completely stereospecific trans addition. HBr 7 46 to 85 -51.6 As the temperature is increased (as the concenDBr 2 53 to 65 tration of addendum in the reaction mixture deHRr 2 74 to 81 -41 creases) stereospecificity diminishes and the addiDBr 2 27 to 42 tion is essentially non-stereospecific a t 25 ". Under IiRr 3 36 to 42 -30 6 these conditions (i.?.,a t temperatures above - 78") DBr 4 29 to 49 the recovered olefin was partly isomerized. Since IIBr 1 3tj -22.8 the noli-stereospecific addition gives about 25y0 DBr 1 3s rneso-2,3-dibromobutane and 757, dl-2,3-dibromoHUr 4 2.5 to 29 0 butane (or the deuterated analogs if DBr is the DBr 3 31 to 34 addendum) the &-olefin I a provides a more sensiComposition of 2,3-dibroiiiobuta11e fraction. These tive measure of stereospecificity than the trans isoadducts are the ones which result from trans addition. T h e mer Ib. IVith the cis-olefin the product composition cis-2-bromo-2-butene used in these esperiments contained cn. 2Yc of the trans isomer. Thus the formation of 985) can vary from 1 0 0 ~ omeso-dibromide (stereospecific trans addition) to 2.5y0 meso-dibromide vieso-dibromide corresponds to 1007, trans addition. (non-stereospecific addition) whereas with the fransI n many of these experinieiits the only dibroinide olefin I b the product can vary only from 10070 dlformed was 2,3-dibromobutane. Since ionic addi- tlibroniicle (trans addition) to 75% db-dibromide tion gives only %,2-dibromobutane(see below) i t is (non-stereospecific addition). It appears froin the clear that the 2,3-dibrornide results from a radical ( 1 3 ) F. K. M a y o and LI.G. Savoy, Tms J O U R N A L , 6 9 , 1348 (1947); I
d
r-
d
( 1 2 ) L. C . Leitch and A T . Morse, C a n . J. C h e w . , 30, 914 (1932).
P.R . M a y o and J. J. Katz, r b i d . , 69, 1339 (1Q47).
Nov. 20, 1959
RADICAL ADDITIONSTO ~ - B R O M O - ~ - B C T E N E S
5939
Oxygen was excluded in these experiments. The carefully purified reactants were degassed and transferred to the reaction vessel by distillation in a high vacuum system and the reactions were promoted by light. Under these conditions when the reaction mixture was not irradiated, 2,2-dibromobutane was the only addition product. The composition of the addendum was not the same for all of the experiments listed in Table TI. However, as shown in Table I the stereospecificities for HBr and DBr are similar and thus i t would appear that the stereochemical results would be the same regardless of the composition of the hydrogen bromide-deuterium bromide mixture. It is noteworthy that the radical addition is 100% stereospecific under these conditions providing that the addendum-olefin ratio is high. As this ratio decreases (ie., as the concentration of addendum in the liquid phase decreases) stereospecificity decreases. The ratio of ionic to radical addition (;.e., 2,2dibromide//2,3-dibromide) also varies in a predictable manner as the addendum-olefin ratio varies. As mentioned above the rate of the ionic addition is probably more dependent on addendum concentration than is the rate of the radical process. If the reaction mixture is not irradiated and if the addendum-olefin ratio is high (>10) the olefin is completely consumed by ionic addition in less than an hour a t - 78". A few experiments were carried out at temperatures above the boiling point of HBr. In these cases the liquid phase (olefin saturated with HBr) was stirred with a magnetic stirrer and the progress of the reaction was followed by the change in the TABLE I1 HBr pressure above the liquid phase. Under these RADICALADDITION OF HYDROGEN BROMIDE-DEUTERIUMconditions the stereospecificity was higher than the BROMIDEMIXTURESTO THE ISOMERIC 2-BROMO-2-BUTENES experiments included in Table I. For example, a t AT -78"" 25 the 2,3-dibromobutane fractions from I a and Composition of radical I b consisted of soy0and 93% dl-dibromide, respecaddition product tively. Dibromobutane composition, 70 dl-2,3-DibromoAddendum2,22,3butane, Yo olefin ratiob The relative reactivities of HBr and DBr with A. trans-2-Brorno-2-butene the isomeric 2-bromo-2-butenes a t - 7s" were de31 10 90 100 termined as follows. A large excess (at least 1029 30 70 100 fold) of a mixture of HBr and DBr of known com23 5 95 1@0 position (determined from volumes and pressures of 9.4 7 93 100 gaseous components prior to mixing) and a pure 3.8 2 98 100 sample of I a or I b were irradiated. These reac2.4 1 99 94 tions were carried out under the same conditions l.oc 0 100 95 as those described in Table 11. Under these conditions the radical additions are essentially or comB. cis-2-Bromo-2-butene pletely stereospecific (see Table 11). The radical meso-2,3-Dibromobutane. % addition product was separated from the ionic lOOd 40 29 71 addition product by g.p.c. and the ratio of DBr to 97 21 36 64 HBr adduct was determined by infrared analysis.l 4 97 10 13 87 Control experiments showed that the compositions 96 8.0 15 85 of mixtures of meso-2,3-dibromobutaneand erythro99 5.0 3 97 2,3-dibromobutane-2-dor dl-2,3-dibromobutane and 67 2.0 1 99 threo-2,3-dibromobutane-2-d could be determined to 0.7" 0 100 78 within about 1%. a In these reactions the addendum and olefin were deIf i t is assumed that the radical addition involves gassed and transferred to the reaction flask on a vacuum line. addition of a bromine atom (reaction 1) followed by After the mixture had warmed up from t h e liquid air temtransfer (reaction 2), the nature of the product deperature, a t which the components were frozen out, to Dry Ice temperature it was irradiated. Molar ratio. In this pends only on the second step. In other words, reaction approximately 10% of the unreacted olefin was the relative rates of radical addition of HBr and
data in Table I that there is little, if any, difference in the stereospecificities of the additions of hydrogen bromide and deuterium bromide. Under conditions where the radical additions are stereospecific, the addition of DBr to dimethylacetylene gives pure dl-2,3-dibromobutane-2,3-d2by two successive trans addition reactions. When the reactions were not irradiated, varying amounts of radical and ionic addition products were obtained together with unreacted olefin. Evidently under these conditions the radical addition is promoted by oxygen or adventitious peroxides. I n some cases a t -78" the ionic addition product, 2,2-dibromobutane, was formed exclusively. At temperatures of over -30" only the radical addition product, 2,3-dibromobutane, was isolated. The radical addition product was examined for a series of dark reactions a t different temperatures. The compositions of the 2,3-dibromobutane fractions (;.e., the stereospecificities) a t various temperatures corresponded well to those observed for the light-promoted additions (Table I). As in the case of the light-promoted additions, the stereospecificities for HBr and DBr were about the same. I n another series of experiments mixtures of HBr and DBr of known composition were added to the isomeric 2-bromo-2-butenes a t - 78'. These experiments were designed primarily to measure the relative reactivities of the two addenda in radical addition reactions. However, information concerning (a) the stereochemistry of the radical addition reaction and (b) competition between ionic and radical addition reactions was also obtained and is presented in Table 11.
recovered. Corrected for intracontamination of the cisolefin with trace amounts of the trans-olefin.
(14) H. E. Zimmerman, TUISJOURNAL, 79, 8554 (lQ57).
1-01. s1
HARLA?; L. GOERIKG AXD DONALD W. LARSEN
.iT)DIl ION
TABLE 111 sion. If the radical is non-planar and inverts BROMIDE-DECTERIUM BROMIDE (oscillates) rapidly, the planar structure represents
OF
DROGLN
h I I X T l - R E S TO THE I S O M E R I C 2-HRORIO-?--HUTENESn Cnrnpri or iddendurn
L%
DRr
Compn of product % DBr adductb
kH
tmns-2-Bromo-2-butene 65
oi I
'kD
2 5
2 4 2 7
53 31 22 1s 1 r)
2 2
2 5 2 5 2 4
B. Lis-2-Bronio-%-buterie 69 2 2 6i 2 4 56 76 2 5 ria 54 2.6 7.3. 55 2.3 51 29 2 6 13 2 4 26 a These reactions were run at - 78" with oxygen-free mixtures of addendum and olefin. * The radical addition product (2,3-dibromobutane) was separated from the ionic addition product by g.p.c. These compositions are for the pure 2,3-dibromobutane fraction. 8:i 8r,
DBr will be the same as the relative rates a t which they undergo the transfer step. Thus relative rates of addition provide a measure of the isotope effect ( k 'Kn) ~ for reaction 2. \\ /' \ T) ./
(1 1
\\
+ Br.
an average configuration. In any event, the isomeric radicals are interconverted by rotation about the CzC3 bond and in no other way without bond rupture. According to this interpretation the isomeric radicals are intercepted in liquid HBr and DBr more rapidly than they undergo rotation about the CPC, bond. As conditions are made less favorable for a rapid transfer step (by decreasing the addendum concentration) interconversion of the radicals competes with the transfer and stereospecificity diminishes. It is interesting to note that the isomerization of the olefin under conditions where the reaction is not stereospecific is consistent with this picture. It has been shown previously17that the barrier for rotation (interconversion) in a radical similar to that involved here is comparable to and probably a little larger than that for reversal of the addition step. In other words, if the radical is not intercepted before it loses its stereochemical identity it would be expected to return in part to isomerized olefin. The intermediate radicals would be expected to be converted to the trans addition products providing the barrier for rotation is higher than that for transfer. The advantages for forming the trans adducts are perhaps most apparent from the end-on views of the radicals shown a t the right. Bond formation in the direction indicated by the arrows not only avoids interactions with the Ca-bromine, but also permits a smooth transition to product without the eclipsing of bonds.
(2)
\\
The value of the isotope effect ( K H ' K D ) for the reaction of the 2,3-dibroniobut-2-yl radical was determined from the relationship of the composition of the adderidurn and product using the equation. k l I Qu
=
[(I.)Br:, (HBr',] [(oirfin,IIRr I '(c~lefin~DI3r'1]
In this equation (olefin.HBr) and (olefin.DBr) represent the hydrogen bromide and deuterium bromide adducts, respectively. The data in Table 111 show that in the present case the isotope effect for reaction 2 is ?.-I 0.1.
*
Discussion If the radical chain addition is a two-step processlj i t is clear that isomeric intermediate radicals are formed from the isomeric 2-bromo-2-butenes. ;is illustrated below, if the process is written in its simplest form (i.e., if the radicals are assumed to have classical structures) the radicals are rotational isomers. In these illustrations i t is assumed that the bromine atom approaches from a direction perpendicular t o the c-bonds in the double bond4 and that the radicals are planar. It should be pointed out: Lhat the latter assumption, which is probably corrcct,ls is not important in the following discusI(hara?ch, 11. Engelmann and F. R . h l a y o , J . Ovg. C h r m . , D. H. H e y and W.A . XX'aters, Chem. Revs., 2 1 , 109
);
, l~.l:37), (lljl
ch $11
Apparently methyl radicals are planar, T. Cole, H . 0. PritR . DnT-idian a n d H. I f . IIcConnell, Moleciilar P h y s . , 1, :ind i t would seem t h a t replicement of hydrogen by larger j
Br.
t w n a addition I,:/
,
dibrumide
According to the above picture an apparently second-order transfer step competes successfully with rotation about a u-bond. Clearly the activation energy for the transfer step--from bond dissociation energies i t appears that ill the present system this reaction is about thermally neutral-must be very low.1s Even so i t would appear that the energy required for the two reactants to diffuse together would be comparable t o t h a t for interconsubstituents would make t h e planar coniiguratiori ninre fa\-orablc relative t o non-planar configurationb. T h e r e iire other i n d i c a t i o n s t h a t t h e planar configuration is t h e most stiible one; e.g , : e e J . I i i n c , "Physical Organic Chemistry." IIcGrarv-Hill Book C o . , Inc.. Piew York, N. Y . , 1956, p. 4 4 4 , and A . Rajbenbach znd hI. Szwarc, Proc. Chem. SOL., 347 (195.8). (17) H. Steinmetz and R. I f . No).es, THISJ o r - ~ i ~ n7r4, , -1141 (1952). (18) T h e activation energy for CHa. H B r (vapor phase) 1s 1.5 i 1 kcal.; H. C. Andersen and G. B.Kistiakovsky, J . Chens. Phys., 11, 6 (1943).
+
Nov. 20, 1959
RADICAL ADDITIONS TO 2-BROhIO-2-BUTENES
version of the radicals. The scheme can be modified in two ways to accommodate the observation that under favorable conditions the transfer reaction can be isolated from the interconversion of the isomeric radicals. I n one of these the barrier for rotation about the C2C3 bond in the radical is increased. I n the other the transfer step is facilitated and in fact becomes first order. It seems possible that the barrier for rotation about the CzC3 bond may be higher than expected because of interaction between the Ca-bromine and the trivalent carbon, e.g., the radical may have a bridged structure. This was proposed in an earlier paper3 to account for the preferred trans addition in cyclic systems and could well account for the fact that with a reactive transfer agent present in high concentration the radicals are intercepted before being interconverted. The other modification of the chain process involves complexing of the addendum and olefin prior to addition. If the bromine atom attacks the substrate from the side away from the complexed HBr molecule trans addition would be expected because the newly formed radical can transfer immediately as illustrated below. According to this scheme, which was also suggested earlier, the transfer step is a first-order process and does not require two species to diffuse together. This picture accommodates the observation that HBr and DBr show about the same stereospecificity. This is difficult to rationalize in terms of a competition between transfer and rotation about the C2C3 bond because HBr apparently transfers 2.4 times faster than DBr. If both have about the same tendency to complex with olefins, similar stereospecificities would be expected. It should be pointed out that if this picture is correct the relative rates of addition of HBr and DBr determined by the competition method will not correspond to the deuterium isotope effect for the transfer step. Indeed, the relative rates in this case will depend in part on the relative tendencies to complex with the limited amount of olefin. Br;
Br
From the present results i t seems doubtful that stereospecific radical additions in acyclic systems will be observed with other addenda. Judging from chain length and tendencies to give telomers, HBr and DBr are much more efficient transfer agents than other addenda. With these compounds stereospecificity is observed only under conditions which are the most favorable for a rapid transfer step. It would appear that sulfhydryl compounds would be the next most likely type of addenda to add stereospecifically. In this connection i t is noteworthy that preliminary results indicate that the addition of hydrogen sulfide to the isomeric 2-chloro-2-butenes in excess liquid hydrogen sulfide is not stereospecific. The formation of identical copolymers from the copolymerization of vinyl
5941
acetate with the isomeric dichlor~ethylenes~~ indicates that the propagation step in polymerizations probably does not intercept the radical fast enough to preserve stereochemistry. From this i t would appear that addenda which transfer so sluggishly that telomers are formed with reactive monomers will not add stereospecifically. In this connection i t is interesting to note that the addition of bromotrichloromethane to the isomeric 2-butenes is not stereospecific.2o From the data presented in Table I1 i t is apparent that the highest degree of stereospecificity is observed under conditions least favorable for isolating the radical addition from the ionic addition. Even with the present system, which is relatively unreactive toward electrophilic addition, ionic addition competes under conditions where the radical addition is completely stereospecific. Since a halogen atom greatly reduces the reactivity of a double bond for ionic additionz1 and slightly increases it for radical addition,22i t is clear that competing ionic addition would be more serious with dialkylethylenes than with the present system. The present techniques probably could be modified to reduce ionic addition somewhat, e.g., by shortening the time between contact of the reactants and irradiation. However, it appears that dialkylethylenes may be consumed a t least in part and perhaps largely by ionic addition under conditions required for stereospecific radical addition. To be sure, the radical addition can be readily isolated from the ionic addition with dialkylethylenes' (but evidently not with trialkylethylene~~) . However, under these conditions the addition is not stereospecific in acyclic systems. Experimental Materials-Anhydrous hydrogen bromide was obtained from the Matheson Co., and was used without purification except by distillation. Deuterium bromide was prepared from DzO (Stuart Oxygen Co., >99.5% deuterium) and PBr:+.lz T h a t quite pure DBr was obtained was shown by the n.m.r. spectrum of dZ-2,3-dibromobutane-2,3-d2, the adduct obtained with 2-butyne. This spectrum showed no indication of the presence of the undeuterated adduct. nzeso-2,3-Dibromobutane was prepared in about 95"; yield by the addition of bromine to trans-2-butene (Matheson Co., >99% trans isomer).10 This product which contained
