Norbert lsenbera I - University of Wisconsin-Porkside Kenosho, Wisconsin 53140 I A Modern Look at Markovnikov'r Rule ond Marcel Grdinic University of Wisconsin Morothon CO~DUS I and the Peroxide Effect Wousou, Wisconsin 54401 I 111 1870, an early stage in the develop- ment of motlern organic chemistry, Vldimir hlarkovni- kov published his well-laiowrr and much-used Rule con- cerning the additin11 of hydrogen halides to unsymmetri- cal olefins (1). 111 its original form (1, 2) the Rule was concerned with t,he addition of hydrochloric, hydro- bromic, and hydrintlic acids to asymmetrical double- bond hydrocarbons, and, contrary to the statements in a number of modcrn organic textbooks limiting its use to unsubst,it,uted olefins, it also predicted correctly the direction of addition of hydrnhalides to halogenated olefins, acctylcucs, and allenes. Markovnikov also reported that some abnormal products are always found together with the regular addition products (3), and he modified the Rule by specifying t.he condit.ions (e.g., temperature) under xhich the additions may produce the "anti-IIarl~ovnik(~v" products (4). Such abnormal adducts were later explained by Kharash and Mayo in terms of the radical rcactiorr mechanism and peroxide effect (5). At the begiming the RIarkovniliov Rule was a purely empirical one, and no attempts were made to find a theoretical explanation. However, with the develop- ment of t,heoretical organic chemistry and availability of new, better, arrd more reliable experimental tech- niques and results, the Rule was gradually modified to accornmodatc the new developments. Today the hlarliovnikov Rule is usually expressed in the folloving nay Iu addition of the molecule X-Y to an asymmetric earboll- carbon double bond, the mare positive pert of t,he att,aeking molecule gaea to that carbon of the dorrble bond bhat is less substituted, i.e., carries the greater number of hydmgens. For example, in the case of propene and hydrogen bromide the addition could proceed in two ways CH,-CH-CH, Markovnikov product f I I 'cH,-CH-CH, Anti-Markovnikov product I I with H+ as the attacking electrophilic agent. Actually, proton attacks propene to form t,he more stable secon- dary carbonium ion, which then accepts a bromide ion Markovnikovproduct Generally, Rlarkovnikov additions are stereospecific, undergoing trans-addition, and in thecase of cyclic olefins, producing the trans-adducts (6,7). I t should be noted, however, that in some instances cis-additions may occur. This appears to be true for some less con- ventional olefins such as acenaphthylene, idene, and 1-phenylpropenes, which produce highly stabilized intermediate carbonium ions, as reported recently by Dewar and his coworkers (8a), and also in the case of somestrained cyclic olefins (86). Since it has been found that the kinetics of the addit.ion of hydrogen halides and other polar reagents to olefins is first order in olefin and approximately third order in hydrogen halide, the older explanation which assumes simple proton addition evidently cannot satisfactorily account for the mech- anism of all polar additions (9, 10). Thus, for the pro- pene-hydrogen bromide system the mechanism sequence is m W$ I Fast 1 68 6e CH + 3HBr CH---H---Br2HBr II II Hydrogen-bridged olrbonium ion Volume 46, Number 9, Sepfember 1969 / 601
Transcript
Norbert lsenbera I - University of Wisconsin-Porkside
Kenosho, Wisconsin 53140 I A Modern Look at Markovnikov'r Rule ond Marcel Grdinic
University of Wisconsin Morothon C O ~ D U S I and the Peroxide Effect
Wousou, Wisconsin 54401 I
111 1870, an early stage in the develop- ment of motlern organic chemistry, Vldimir hlarkovni- kov published his well-laiowrr and much-used Rule con- cerning the additin11 of hydrogen halides to unsymmetri- cal olefins (1). 111 its original form (1, 2) the Rule was concerned with t,he addition of hydrochloric, hydro- bromic, and hydrintlic acids to asymmetrical double- bond hydrocarbons, and, contrary to the statements in a number of modcrn organic textbooks limiting its use to unsubst,it,uted olefins, i t also predicted correctly the direction of addition of hydrnhalides to halogenated olefins, acctylcucs, and allenes. Markovnikov also reported that some abnormal products are always found together with the regular addition products (3), and he modified the Rule by specifying t.he condit.ions (e.g., temperature) under xhich the additions may produce the "anti-IIarl~ovnik(~v" products (4). Such abnormal adducts were later explained by Kharash and Mayo in terms of the radical rcactiorr mechanism and peroxide effect (5).
At the begiming the RIarkovniliov Rule was a purely empirical one, and no attempts were made to find a theoretical explanation. However, with the develop- ment of t,heoretical organic chemistry and availability of new, better, arrd more reliable experimental tech- niques and results, the Rule was gradually modified to accornmodatc the new developments. Today the hlarliovnikov Rule is usually expressed in the folloving n a y
Iu addition of the molecule X-Y to an asymmetric earboll- carbon double bond, the mare positive pert of t,he att,aeking molecule gaea to that carbon of the dorrble bond bhat is less substituted, i.e., carries the greater number of hydmgens.
For example, in the case of propene and hydrogen bromide the addition could proceed in two ways
CH,-CH-CH, Markovnikov product f I I
'cH,-CH-CH, Anti-Markovnikov product I I
with H+ as the attacking electrophilic agent. Actually, proton attacks propene to form t,he more stable secon- dary carbonium ion, which then accepts a bromide ion
Markovnikov product
Generally, Rlarkovnikov additions are stereospecific, undergoing trans-addition, and in thecase of cyclic olefins, producing the trans-adducts (6 ,7 ) . I t should be noted, however, that in some instances cis-additions may occur. This appears to be true for some less con- ventional olefins such as acenaphthylene, idene, and 1-phenylpropenes, which produce highly stabilized intermediate carbonium ions, as reported recently by Dewar and his coworkers (8a), and also in the case of somestrained cyclic olefins (86). Since it has been found that the kinetics of the addit.ion of hydrogen halides and other polar reagents to olefins is first order in olefin and approximately third order in hydrogen halide, the older explanation which assumes simple proton addition evidently cannot satisfactorily account for the mech- anism of all polar additions (9, 10). Thus, for the pro- pene-hydrogen bromide system the mechanism sequence is
m W$ I Fast 1 6 8 6e CH + 3HBr CH---H---Br2HBr II II
Hydrogen-bridged olrbonium ion
Volume 46, Number 9, Sepfember 1969 / 601
Toble 1 . Common Polar Electrophilic Reagents
Several common electrophilic reagents are given in the Table 1.
Since the stability of intermediate carhonium ions
filarkovnikov Rule may alsdbe formulatkd as follows
When an electrophilic reagent X-Y adds to an a5ymmetric carbon-carbon double boud, addition of X+ will occur in such a manner as to give the more stable carbaniurn ion.
This "carbouium ion" definit,iou of IIarltovnikov's Rule is particularly helpful in predicting electrophilic additions to some substituted olefins. For example, olefins of the type R-CH=CH-Z, vhere R is an alltyl group and Z is an electronegative atom or group, such as 4 1 , B r , -I, -OR, -SR, -X&, etc., could form either an unst,ahle primary carhonium ion or a more stable secoudary carbonium ion. Stability of the pri- mary carbonium ion would be further decreased by the negative inductive effect of Z; however, this effect is more t,han cancelcd by a strong electron-pair shift from the group Z (conjugative effect). As a result, an overall stabilization of the primary carbonium ion is achieved with the resulting formation of the Rlarkovnikov ad- duct1
+ 4 1 R-CH=CH+ + X-Y * R-CH-CH - Z +
I
If group Z is electronegative (i.e., more elcctronega- tive than carbon) but all uushnred electron pair is not, available, the primary carbon would have to form a highly destahilizcd primary carbonium ion. Instead, the reaction proceeds via the more stable secondary carbonium ion, resulting in the reversed direction of additions
I X
Unstable carbonium ion +
R-CH-CH+Z + Y--R-CH-CH+Z I
X I I
Y X More stable carbonirm ion Product
Such groups are -CFs, -CCla, -NR3+, S R 2 + , -CN,
- Presnmable
Polar reagent Electropositivity electrophile
H-Br H > Br H +
HO-CI Cl > OH C1+ H-OSOIH H > OSOIH H +
H-OOC. CH, H > OOC.CHs H +
CI-Br Br > CL Br++ H S H H > SH I1
// // -NOz, -C-OH, -C-OR, -SOzR, etc., of which the latter four may also undergo initial 0-protouation and 1,4- addition. In the case of vinyl compounds (R = H) these electronegative groups would cause what could he considered anti-Marltovnikov addition
Similarly, as many textbooks indicate, certain a,& unsaturated compounds (such as nitro-, carbonyl-, and quaternary salts, etc.) give "anti-Markovnikov" addi- tion products. For this reason, also, it has been pointed out in several textbooks that Markovnikov's Rule should be applied to unsubstituted alkenes only, since, as it is incorrectly claimed, negative substitueuts geuer- ally lead to anti-Markovnikov addition. This, as the discussion above shows, is evidently not the case. A careful application of the "most-stable-carbonium-ion" formulation of the Rfarkovnikov Rule shows that practi- cally all polar additions to carbon-carbon double bond fall into the category of normal R4arkovniltov additions.
In some instances the application of the Rlarkovnikov Rule may be ambigous. For example, the asymmetric olefin 2-pentene has two unsaturated carbons bearing an equal number of hydrogcns. Also, both the Markovni- kov and anti-Markovnikov intermediate carbonium ions are secondary. In such a case hydrohalogenation normally produces only a slight excess of one of the possible two products, e.g., 2-halopentane. This can be rationalized in terms of a more extensive delocaliza- tion of the positive charge of carbon by a stronger hyperconjugation effect (11)
Reaction of cyclopentadiene with hydrogen chloride (12) produces 3-chlorocyclopenteue as the major product
evidently involving an initial protonation to form a carbonium ion stabilized by resonance
'It should be noted that such olefins may, under the proper. reaction conditions (base catalysis), undergo nucleophilie addition with a. number of polar reagents, such as HOH, HOR, HCN, etc. The attacking nncleophiles are then HO-, RO-, CN-, etc., and the direction of addition may be predicted by considering the most stable intermediate carbanion.
602 / lournol of Chemical Educofion
Protonation in the position 1 would produce carho- nium ion which would not he stabilized by resonance
Polar electrophilic additions are generally kinetically controlled reactions. Stability of the intermediate carhonium ions determines the ratio of Afarlzovnikov and anti-hfarkovnikov products. Since, however, the kinetic controlled product is also often thermodynami- cally preferred, a calculation may be necessary if one is to establish the kinetic control character of the process. For example, in the addition of hydrogen chloride to propene both Markovnikov and anti-Marlzovnikov additions are exothermic (IS)
By applying the Gibhs-Helmholtz equation -2.305.RT.logK, = AH - TAS
and assuming that the entropy changes for hoth reac- tions are similar, one can calculate the thermodynami- cally predicted ratio of the products
Consequently, if the additions were thermodynami- cally controlled, the reaction mixture would contain - 91 mole% of isopropyl chloride and 9 mole% on n- propyl chloride. Since the actual product contains less than 0.1 mole% of n-propylchloride, the reaction is very likely kinetically contr~lled.~
Although Markovnikov's Rule was generally useful in predicting the course of additions to douhle honds, many contradictory reports concerning the addition of hydrogen hromide to alkenes appeared in the literature before 1933. In that year, the classical experiments of M. S. Kharasch and F. R. Mayo (5) demonstrated that freshly distilled alkenes react with hydrogen bromide in accordance with lMarkovnikov's Rule, while undistilled alkenes which contain small amounts of peroxides yield anti-Markovnikov Rule products. The peroxide effect was first observed with ally1 bromide (5)
no peroxide BrCH9CH=CHx + HBr -d BrCHCH-<Hz
I I Br H
peroxide BrCH2CH=CH2 + HBr ----t BrCH%CH-CH.
I I H Br
Hydrogen halide additions to double honds in violation of Markovnikov's Rule had been observed mostly with hydrogen bromide and sometimes with hydrogen
chloride (16), hut not with hydrogen iodide or hydrogen fluoride.
A number of explanations have been offered for the anti-Markovnikov additions, all of them involving a free radical mechanism in which a hromine atom is added to an alkene (17). In the initiation step of the free radical mechanism a free alkoxy radical may he formed from a hydroperoxide (18) (ROOH + RO' + 'OH) or from peroxides (19) (ROOR - 2RO'). The RO' radical then reacts with hydrogen hromide to produce a hro- mine atom
RO' + HBr - ROH + Br' (initiation)
Alternately, i t has been pointed out that primary attack of initiator radicals on the olefin followed by transfer with hydrogen bromide cannot he ruled out ($0)
R'O' + CHFCHR - R'OCHSCHR
R10CH2CHR + HBr - R'OCHaCHnR + B r
In the chain propagation the hromine atom reacts with the alkene to form a radical
Br' + CH-CHR - CHp+HR (addition) I
Br
The radical removes a hydrogen from hydrogen hromide CH2-CHR + HBr - CH2-CHR + B r (transfer) I
Br I
$1. H
In contrast to the ionic reactions which result in Markovnikov Rule products, free radical reactions reverse the order of the addition because the hromine radical adds first. Addition of the bromine atom may yield two different radicals
Consideration of the general order of stability of free radicals (tertiary>secondary>primary) will permit us to predict that I should he formed because it is a secondary radical, whereas I1 is a primary radical. The more stable free radical is formed more readily. In the transfer reaction the radical, I, picks up a hydro- gen atom from hydrogen bromide to afford anti-Mar- kovnikov addition to the douhle hond.
Explanations for the fact that hydrogen hromide adds to a double bond by a free radical mechanism while hydrogen chloride, hydrogen iodide, and hydrogen fluoride generally do not add in this fashion have been offered in terms of thermodynamics (19, $1, $$). While Roberts and Caserio (19) point out that the overall energetics of the free radical addition are unfavorable in case of hydrogen fluoride, they also state that addition of hydrogen chloride is relatively favorable. Pryor ($I), however, stresses the fact that only in the case of hy- drogen hromide are hoth steps in the chain exothermic. While the addition of a chlorine atom to a douhle hond is exothermic, the transfer step is endothermic (see
It has been shown that in the presence of hydrogen chloride n-propylchloride does not rearrange to isopropylchloride even on prolonged standing at mom temperature (14), nor does rearrange- ment seem to occur under the similar conditions of hydrogen chloride (16a) or hydrogen bromide (16b) addition to propene.
Volume 46, Number 9, September 1969 / 603
Table 2. Energetics of the Addition of the Hydrogen Halides to A1kenesa.b
- R O . + H X - R O H $ : F I n i t i a t i ~ n ~ , ~ X. + R'CH=CH2 - CHR'-CHz-X Addition CHR'-CHr-X + HX - CH.R'-CHI-X + X' Transfw
, AH (kcal/mole HX 1nitiationc.d Addition Trnsfer Total
a Average values of bond dissociation enemies were used See reference (10); the energy difference between a double
bond, G=C, and a single bond C-C, was calculated from values listed in T. I. COWBELL. he Streneths nf Chemical Rondq"
~ ~~
(2nd Ed.), Butterworths.'hndon. I Q S ~ ~ A H = 63 kcal/mdel. The initiation step i i ~ which the perixide decomposes to iive
free radicals, ROOR - 2RO' (AH= 35 kcal/mole (lo)), is the same for all hydrogen halides; the value of t h ~ s endothermic step is not included in the calculations.
An average value of 102 kcal/mole was used for the -OH bond in ROHj this value is lower than the bond energy, listed by others for OH (AH = 110.6), since the latter value averages in -OH bonds in several other compounds (such as water, acids, etc.).
Table 2). Thus, there is no long chain free radical addition to olefins for hydrogen chloride. There seems to be general agreement that hydrogen iodide does not add readily by a free radical mechanism because of the endothermic addition (AH = 12 kcal/mole).
When enthalpy values are used, thermodynamics seems to provide a clear-cut answer in the case of hy- drogen fluoride and hydrogen hromide. The question of hydrogen chloride and iodide is not as unambiguous (17) since the overall enthalpy values are still favorable. Free energy values would presumably provide a better answer since there may be entropy effects. At any rate, the currently used thermodynamic "explanations" somehow fail to account for all the observed phenomena.
Recently, the influence of steric effects in the addition of hydrogen bromide to cyclic olefins has been reported (23, 24). Since an open intermediate radical did not account for these observations, another structure was proposed, namely a three-membered cyclic intermediate radical (28,24)
As Bohm and Abell point out (241, there is some justi- fication for this type of structure despite the three electron bonds. These bonds must involvc atoms which are very similar and which have electronegativity differences of 0.5 or less (25). The difference between the electronegativities of carbon and bromine is 0.3 and it is small enough to permit three-clcctron bond forma- tion. Failure of hydrogen chloride to enter into free radical additions to double bonds (and to yield anti- i\'Iarkovnikov products) may be due to a smaller stabil- ity of the threc-membered intermediate chloroalkyl
radical compared to the bromoalkyl radical. Several factors may account for the smaller stability of the chloroalkyl radical; the larger electronegativity differ- ence between chlorine and carbon (0.5) and the smaller size of the chlorine atom. [In solvolysis reactions, for example, Winstein and Grunwald have shown that chlorine is a poorer participating group than bromine (26)l. Therefore i t is quite likely that a cyclic chloro- alkyl intermediate either is not formed or is much less stable than a bromoalkyl radical. If a three-mem- hered halolalkyl radical is involved, then this inter- pretation may account, in part, for the fact that, in general, anti-Markovnikov addition to olefins is re- ported for hydrogen bromide only.3
An intermediate ?r-complex has also been considered (19,24)
Abell and Piette (28) cite evidence from electron para- magnetic resonance spectroscopy which seems to sug- gest such a structure for symmetrical olefins (VIII, R=Rf=CHa).
I n the absence of convincing evidence to support the formation of a cyclic intermediate, the classical radical structure is acceptable provided that it is modified to take into account the stereochemical consequences. An extremely rapid chain transfer step (e.g., HBr) may avoid inversion of configuration a t the carbon hearing the free electron, while slower chain transfer (e.g., thiols) may lead to some inversions. For hydrogen chloride the radical addition to propene has been postulated as follows (29)
CL' C=c-C - C C - c - c - c 4
&I CI I IX X 1 do.., IICl ( faat, HCI
iY-CH-CJI. CH-CH-CH3 I Cl
Transfer to the less hindered radical, X, is faster than transfer to IX and isopropyl chloride is formed.
In addition to the aforementioned intermediates, a hydrogen bromide olefin complex has also been pro- posed (30)
Br' - 4 :'
C=C
' R' I Br XI
Attack of the bromine atom from the sidc away from the complexed HBr would account for t,rans-addition.
Neureiter and Bordwell (81) explain the stereo-
Skell and coworkers also proposed a scheme involving the formation of a bridged radical to account for the selective bro- minatian of l-bromo-2-methylb!1ta1,e which yields optically active 1,2dibromo-2-methyIb11ta11e, while chlorination yields all possible dichlorides including optically inactive l,2-dichlaro-2- methylbutane (87).
chemistry in terms of the polar character of the transfer reaction. They propose an intermediate dipole, XII, in which the negative end is directed away from t,he site of attack
Such a dipole is postulated to be preferentially oriented away from the largest permanent (negative) dipole on the saturated carbon. The molecule engaged in the transfer approaches the radical center trans to the most negative group on the saturated carbon.
It is clear that an unequivocal interpretation is not possible a t this time and that the question of free radical additions of hydrogen halides to olefins needs further experimental studies.
There are other free radical additions to alkenes besides those involving hydrogen halides (see Table 3). The addition of mercaptans to alkenes depends upon the presence or absence of peroxides and is thus similar to hydrogen bromide addition (52)
0, CHzCH=CHz + RSH -+ CHn-CH-CHI
I I
Other free radical additions include polyhalomethanes, such as tetrabromomethane and bromoform (35).
Free radical additions of peroxides to double bonds have been known for some time (54).
I t was shown recently that some additions could be effected in neutral solution if carried out photochemi- cally in the presence of a high energy photosensitizer. Surprisingly, as all the evidence suggests, the reaction is not a free radical addition, but rather proceeds via carbonium ion intermediates derived from the first ex- cited triplet state of the olefin, giving Markovnikov's products (55).
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