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RALPH K.
The Misuse of Markownikov's Rule Thomas A. Newton SUNY College of Technology, Utica, NY 13501 In 1950 Henne reported ( I ) that HC1 reacts with 3,3,3trifluoropropane in the presence of a catalytic quantity of AICI3t o produce 3-chloro-l,l,l-trifluoropropane:
In 1953 Ingold cited this result as an example of an antiMarkownikov addition (2).By 1960 many authors of organic chemistry texts had followed Ingold's example, and a had example it was, because Ingold had either overlooked or chosen to ignore that reaction 1did not occur in the absence of AIC13. The significance of this omission became apparent in 1970 when Myhre showed (3) that 3-chloro-l,l,l-trifluoropropane was not the only product of eq 1.Depending on the reaction conditions, significant amounts of 1,s-dichloro-l,ldifluoropropane were also formed. Myhre proposed the mechanism outlined in eq 2 to explain the formation of both products:
Thus, the AIC13 catalyses the reaction by assisting the ionization of a C-F bond to form the difluoroallylic cation 1. This ion traps a chloride ion to yield 3-chloro-1,l-difluoropropene, which then undergoes protonation a t C-2 to produce the resonance-stabilized cation 2 in preference to the secondary carbocation that would arise from addition of a proton to C-3. Competitive attack of chloride and fluoride ions on 2 yields the observed products. Although Myhre's results have been known for over 15 years, many organic chemistry textbooks still cite reaction 1 as a simple example of an anti-Markownikov addition. The mechanistic description of the reaction has changed, however. Ingold initially compared reaction 1 to the addition of HCI to propene. He ascribed the difference in the orientation of addition to the different modes of polarization of the double bond by the -CF3 and -CH3 groups: H-CI F3C-CH=CH2
C1-H H3C-CH=CH,
Subsequently an interpretation based on the relative stabilities of carbocations 3 and 4 became popular: CF,CH,CHSt 3
vs.
Geissman (4) describes the results of reaction 1in the following terms: On the basis of the same arguments we have used to account for the 'Markownikoff addition' of HBr to propylene, we would conclude from this result that ion (41)must be a more stable oroduct of the protonation of C H ~ H C than F ~ is the ion (45. Thus, Markownikoffs rule is 'violated', but for a readily discernible reason.1 Wingrove and Caret (5) also subscribe to this view. They state, As we saw. orientatiun of
addition and product distribution are well accounted for by carbocation theory. Using well-understuod mechanisms and carbocation stabilities will never fail you.
The authors then ask students t o provide a mechanistic rationalization of reaction 1. An interestine mechanistic alternative. sueeested in the solutions man& of one current introductoGtext (6),depicts the bridged ion 5 as the intermediate in eq 1:
Attack of chloride ion on the less hindered carbon of 5 yields the observed product. This interpretation reflects the present view that primary carbocations such as 3 are too unstable to he formed in solution. I have examined over 20 organic chemistry texts published since 1970 and have not found any introductory text that discusses eq 1 as Myhre has descrihed it. Only the revised editions of the advanced treatise by March (7)present Myhre's view of this reaction. Why does this example persist? One obvious answer is that many authors use other texts as sources of information and examples when writing their own books. But there are other factors. In a recent article in this Journal. also concerned with incorrect internretations of ~arkowhikov'srule, Tedder (8)suggested such misrepresentations are retained because they give predictions that are qualitatively correct, or because they affect a small area of kuowledae. Both reasons a ~ ~inl the v present case. Furthermore, ingold's misreprese&tion seems to have been retained for its novelty and its pedagoaical value. There are h i l i c reacfew, if any, examplesbf simple e ~ e c ~ r ~ ~ addition tions that violate Markownikow's rule. The addition of HI to neurine iodide, CHZ==CHN(CH~)~+I-, a reaction that is frequently cited along with eq 1,may be one. However, I would
CF,CH+CH, 4
ions (41) and (42)correspond to ions 3 and 4. respectively. Volume 64 Number 6 June 1987
531
not he surprised if a reinvestigation of that reaction revealed it to be mechanistically more complex than i t appears a t first glance. In terms of pedagogy, statements such as those by Geissman and by Wingrove and Caret, mentioned earlier, are intended to convince students that they can apply the principles they have learned to have not b r e d before. But, as Myhre concluded (3),
...the formation of apparent anti-Markownikovaddition prod-
uets in the reaction of CF3CH=CH2 with strong acids does not represent a valid basis for
of the relative of cationic intermediates resembling CF3CtHCH3or CF3CH2CH2+.
532
Journal of Chemical Education
I t is time to stop presenting i t as though it does. Literature Clted I, H,.,, A. L.: K,, S.J . A ~c h. p m . SOC.1950,92,3369. 2. Ingold, C. K.StructureandMechanism in Organic Chamistry; Cornell univ.: ~thaes. NY. 1953; p 649. 3. Myhre. P. C.: Andrews, G. D.J. Am. Chem. Soc. 197%92, 'l59676S'l. 4. Geisrmsn,T. A.Principiea o i o ~ g o n i c c h e m i s t q4th.d.; , W. H . ~reemsn:san~rsncis-
.",.".,,"".",-."". ""
,a,,...->*,
>o*
s. wingrove A. s.:care6 R.L.organic c h e m i s t r y ; ~ a r p e&r ROW ~ e ~wo r k1981: . ~302. 6. Kemp. D.8.;Vellaccio, F. Organic Chemistry: Worth: New York. 1980: p 592 7. Mmch,J. Adoan~edOrgonir Chemistry, 2nded.; McGraw-Hill: New York,1977: p 688:
3rd ed.; 1985: p 673. 8. ~ e d d e rJ. , M J. c h e m ~ d u c1884.61,%37., .
