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RALPHK. BIRD WHIST EL^ University of West Florida Pensacola. FL32504
textbook Forum A Textbook Error or Failure of the Saytzeff Rule? Manfred G. Reinecke and William 6. Smith Texas Christian University, Fort Worth, TX 76129
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The auuarent source of the textbook uroblem is a 45A oroblem i n a uouular organic chemistrv text ( 1 ) re.. qut!it.i the mechanism for the isomrri7ati0n of 2.7-diywr-old paper that used infrared : ~ n dchemical methodsof uhoiuhnric acid to 1.1-distructurc dctc~minatmn 6 , .Of thr t h r w ~ r u d u c t sIA. R. rnethvlocta-2.6-dime 1 with . methil-2-isop~openylcyclopentane 4. The expected answer & C1 formed i n a ratio of 3l:4:65, C was asLumed and A & B were demonstrated to be cvcloalkenes bv their uutake of involves urotonation to the more stable 3" carbocation 2 which adbs to the remaining double bond to give the more one equivalent of hydrogen. A was tentatively assigned structure 4 based on the identity of the infrared of its restable 3" carbocation 3 that loses a proton to form 4, not t h e m o r e s u b s t i t u t e d 2-isopropylidene-1,l-dimethylcyclop entane 5. This apparent failure of a logical extension of Saytzeff's rule led to the following discussion, calculations, and lit2 erature search. 1 Because of the alkenelike tran/+l:8.1\ sition state of the product-determining step of a n E l reaction, the most stable alkene is formed preferentially. According to Saytzeff's rule t h i s should be the most substituted alkene 5. How8 g ever, if 5 were the less stable isomer, i t would not be t h e pre- - 2 5 .O t147.9 t144.7 - 2 5 . 9 /-:5.1 ferred product. Such exceptions to SaytzefF's rule are known ( 2 ) . Possible reasons for a n increased Readion scheme and calculated heats of formation. energy content of 5 m i g h t be s t e r i c hindrance between t h e gem dimethyl groups and one of duction product to that of a synthetic sample of 1,l-dithe allylic methyl groups andlor the bond angle distortion methyl-2-isopropylcyclopentane.However, the authors necessary to place the double bond exo to the five-membered ring. This hypothesis was tested and discarded on state that "The position of the double bond in A is still unthe basis of the calculated heats of formation [Hfl of the certain. From the strongly deformed isopropyl doublet of energy minimized conformations of 4 and 5 that show the the infrared spectrum, i t is probably in the exocyclic posilatter to be more stable by 3.5 kcaWmol (see the figure.)' tion a s shown", i.e. 4. Clearly the structure of A i s equivoMight the reaction of 1 to 4 involve an E2 mechanism cal and none were offered for B and C. that sometimes gives the least substituted alkene accordThis fact was recognized in 1979 as part of another study ing to Hofmann's rule? An example ofjust such a phosphoin which the reaction of 1 with phosphoric acid under the ric acid catalyzed dehydration of a secondary alcohol by conditions of reference 6 was repeated and the actual prodthe E2 mechanism was reported recently in this Journal ucts shown to be 5 , 7 , and 9 by proton and carbon NMR (7). (31. Normally, however, Hofmann orientation is observed The yields were not given, but the related reaction of the only diol precursor of 1 with pyridinium hydrobromide gave under strongly basic conditions where the relative acidities products 4, 5, 7 , and 9 in yields of 2, 18, 35, and 35%, of the fl-hydrogens, an ElCB mechanism, or the effective steric hulk of the charged leaving groups are important ( 4 ) 'Modelling was carried out with a modified MM2 (MMX) force field with molecules in which eonformational restrictions do not using the Program PC. MODEL 4.50 available from Serena Softpermit the preferred trans-coplanar geometry of the leaving ware, P. 0. Box 3076, Bloomington, IN 47402-3076. This force field groups to he achieved in the E2 transition state as in the has been parameterized to provide energies for carbocations of the dehydrahalogentaion of menthyl chloride (5). type reported here and does so with an accuracy approximately equal to that of semi-empiricalMO programs using the AM1 or PM3 The reaction conditions are not strongly basic, however, hamiltonians. Each of the appropriate alkenes was generated first and even if a n oxonium salt related to 3 were an intermeand minimized. The carbocations were constructed from these minimized structures and themselves minimized. All calculations corrediate in the formation of 4, its conformational mobility spond to conditions in a vacuum at 25". would eliminate the second explanation. A
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Volume 72 Number 6 June 1995
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respectively. The remaining 10%consisted of the acyclic diene 1and two of its double bond isomers. It, therefore, appears that no 4 and little 5 is produced from the reaction of 1 with phosphoric acid. The major products, 7 and 9, arise by R-hydride rearrangement of the shift to initial carbOcation to followed by a carbocation 8. Elimination of the R-protons gives 7 and 5 from 6 and 9 from 8, compatible with the relative stabilities of the alkenes [7>5=9>>>4]and the cations [6>>>8=3] determined from the calculated HEs in the figure.' in hi^ study reveals the necessity of including any text revisions that incorporate new or corrected re-
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Journal of Chemical Education
sults. In this instance a revised prohlem asking for the mechanism of formation of 7 and 9 from 1 probably would be more challenging and certainly more correct than the Literature Cited I. Morrison, R. T:Boyd, R. N. Drgonie Chemislo, 6th ed.:PreniieeHall: Englewood cliff&, NJ, 1992; P 423. 2. Ho, I.: Smith, J.G. lktrahedrnn 1970,26,4277. ,
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4. March. J. ~ d v o n c e dorponic chemzstry,~ e o d i o n r~, e c h o n i s m and s structure, 3rd ed.:Wiley-lnkrstienee: New York. 1985. pp R9CL892. 5. ~eferenceI, p 472. 6. St"="=.F G-; S~auldiw.S. C..Jr J. Amer Cham. Soc. 1949,71.1687. 7. Oauzonne, D.; PlaUer, N.; Demerseman, P.: Lang C.; Royer R. Bull. Soc. Chim. Fr
ls79,506.
