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Directive Effects in Elimination Reactions Donald Hope Froemsdorf Iowa State University

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Directive Effects in Elimination Reactions Donald Hope Froemsdorf Iowa State University Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1959 Directive effects in elimination reactions Donald Hope Froemsdorf Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Organic Chemistry Commons Recommended Citation Froemsdorf, Donald Hope, "Directive effects in elimination reactions " (1959). Retrospective Theses and Dissertations. 2155. https://lib.dr.iastate.edu/rtd/2155 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. DIRECTIVE EFFECTS IN ELIMINATION REACTIONS Donald Hope Froemsdorf A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment of The Requirements for the Degree of DOCTOR OF PHILOSOPHY Major Subject: Organic Chemistry Approved! Signature was redacted for privacy. In Charge of Ms Signature was redacted for privacy. Head of Major Departmeitff Signature was redacted for privacy. D< allege Iowa State College Ames, Iowa 1959 il TABLE OF CONTENTS Page INTRODUCTION 1 HISTORICAL 4 DISCUSSION 21 EXPERIMENTAL 51 Preparation and Purification of Materials 51 Apparatus 65 Procedures 72 Results 77 SUMMARY 105 ACKNOWLEDGMENTS 107 1 INTRODUCTION One of the more important reactions in organic chemis­ try is the elimination reaction. Elimination reactions are generally considered to be reactions in which two atoms or groups are removed from a molecule without being replaced by other atoms or groups. This occurs most commonly on ad­ jacent carbon atoms so that a more unsaturated product is produced. This type of elimination reaction is called beta- elimination or 1,2-elimination. Most of the beta-eliminations fall into two general categories. One is the eliminations that are intramolecular, initiated by heat and proceed by a unimolecular mechanism. The other is generally referred to as a heterolytic eli­ mination reaction. This heterolytic elimination reaction is broken down into two mechanistic classes - unimolecular eliminations and blmolecular eliminations. The unimolecular elimination is now commonly referred to as the El reaction and the blmolecular elimination as the B2 reaction. In both the intramolecular and heterolytic elimi­ nations a leaving group X and a fi-hydrogen are removed from the molecule resulting in the formation of an olefin and EX. When the leaving group X is in a secondary or tertiary position, there is a choice of ^-hydrogens. For example, in the following case either hydrogen atom Ha or hydrogen 2 Ha Hb Hb Ha I I / 1 , 1 -HX CHg—CH—CJhtg GRg—• CH-GRg' + OR^-OH— CR2 X atom Hb can be removed and if R is not identical with Rz, the product formed will be different, depending on which hydrogen atom is removed. This problem has been realized for more than a century, and observations have been made of the products formed with various groups X, but the underlying steric and electronic effects controlling this choice are not clear. This thesis reports some studies on the direction of elimination in the pyrolysis of some secondary and tertiary aliphatic acetates (X = -O-CO-CH^), the effect of substit­ uent s on the base-promoted elimination of 2-phenylethyl compounds by kinetic studies (X = Br, I, OSOgC^H^CH^, SCCH^g and Y ^ JD-OCH^, JD-H, J>-C1, m-Br), and some prelim- CHgCHgX B" -CH=CH, HX inary work on the decomposition of various aliphatic JD- toluenesuifonylhydrazones (R = -C^H^CHg) in the presence 3 Ha Hb Hb Ha I I , I , I CR0-C-CR52 n 2 CR2Z=CH-CR2 GRg—OHzzzCRg N-NHSOGR 4- NR HSOGR of alkali, a reaction whose mechanism is not yet known. 4 HISTORICAL The first recorded observations of a directional pre­ ference for an elimination reaction are those of Hofmann.^ He observed that the decomposition of a quaternary ammonium hydroxide containing an ethyl group and other primary alkyl groups yielded ethylene as the chief olefinic product. + (RCH2CH2)3N-CH2CH3 + OH > (RCH2CH2)3N 4- CH2=CH2 In 1875 Saytzeff2 pointed out that the elimination of secondary and tertiary halides produced predominately the olefin in which the double bond was substituted with the largest number of alkyl groups. He concluded that the olefin formed in greatest proportion was determined by the fact that the hydrogen removed comes preferentially from the carbon atom bearing the least number of hydrogens. RCH2CHCH(CH3)2 > RCH2CH=C( CH3) 2 major - X -t-A. W. Hofmann, Ann.. 78. 253 (1851). 2A. Saytzeff, ibid.. 179. 296 (1875). 5 RCEgCHgCCCH^g > RŒ2CH= C(CH3)2 major I product X These first observations have been generalized and are now known as the Hofmann and Saytzeff rules respectively. The Hofmann rule has been shown by Ingold3 to apply to + • •+• +• 3 'onium ions (-NR3, -SRg, -PR3) in general. Ingold-J also further extended the rule and showed that it could be applied to secondary and tertiary 'onium ions. The Hofmann rule is RCH2CHCH3 •+ OH > RGH2CH= CH2 major product + S(CH3)2 RGH2C(CH3)2 + OH •> RCHpC—CHp major I product + S(CH3)2 CH3 now generally summarized in the following way. In the elim ination of 1onium salts the predominating olefinic product is the olefin that has the least number of alkyl groups attached to the double bond. The Saytzeff rule states that in the elimination of neutral-leaving groups the predomi­ nating olefinic product is the olefin that has the largest 3c. K. Ingold, "Structure and Mechanism in Organic Chemistry," Cornell University Press, Ithaca, N. Y., p. 420 (1953). 6 number of alkyl groups attached to the double bond. The scope, limitations and interpretations of these rules have been elaborated largely by Hughes and Ingold.3»4" A brief summary of their views will be recorded here. In the Hofmann elimination their view is that the direction of elimination is determined by the acidity of the -hydrogen, the most acidic being removed preferential­ ly. Thus, in the decomposition of dimethylethyl-n-propyl- ammonium hydroxide the ^-hydrogens of the ethyl group would be the most acidic. This is true because the induced positive charge at the methylene group is partially can­ celled by electron release from the attached methyl group (inductive effect). Thus, ethylene would be predicted, and is, the predominating olefin produced. By the same CH^—•CH2CH2-N-CH2CH3 4 OH —> CH^CH2CH2N( CH^)2 4" CH2—— CH2 CH^ CH3 reasoning the elimination of 2-butyldimethylsulfonium ion would be expected to give a mixture of butenes with 1- butene being the major product (the methyl hydrogens are more acidic than the methylene hydrogens). The elimination of 2-butyldimethylsulfonium ion does, in fact, give 74# 4E. D. Hughes and C. K. Ingold, Trans. Faraday Soc., 37, 657 (1941). 7 1-butane on elimination.3 CH3CH2CHCH3 OEt ^ CH3CH2CH=CH2 4- CH3CH=CHCH3 + S(CH3)2 74-% 26% The Saytzeff rule is interpreted in terms of the sta­ bility of the resulting olefinic product, the most stable olefin being formed in the greatest amount. Thus, in the elimination of 2-bromobutane the olefin formed in the greatest proportion should be 2-butene since it is more stable than 1-butene by about 2 kcal/mole.^ The base- promoted elimination of 2-bromobutane does give 2-butene as the major product.3 Likewise, the elimination of t- CH3CH2CHCH3 "OEt ? GH3CH=CHCH3 4- GH3CH2CH=Ca2 Br 81% 19# amyl bromide would be expected to give predominately 2- methyl-2-butene since it is more stable than the isomeric 2-methyl-l-butene by about 1.5 kcal/mole.5 And indeed, 2-methyl-2-butene is the predominate product.3 *C. K. Ingold, "Structure and Mechanism in Organic Chemistry," Cornell University Press, Ithaca, N. Y., 8 CH^CHgC(CH3)2 OEt ^ CH3CH-—C—CH3 CH3CH2C—CHg Br CHg CH3 71# 2# Investigations by Hughes and Ingold^i4 have shown the Hofmann rule to be limited to the E2 reaction (blmolecular elimination), while the Saytzeff rule applys to most systems in the El (unimolecular elimination) and to neutral systems undergoing the E2 reaction. This is easily rationalized when we look at the mechanism of the two reactions. The E2 reaction proceeds by a blmolecular concerted process^»4"»^»? in which the preferred stereochemistry of the /5-hydrogen and the leaving group is trans. Thus, the leaving group _n B H R R R XC^C>R > \=C^ 4- BH + X R y \ / \ R Cx R R would be expected to exert some influence on the direction &D. J. Cram, "Olefin Forming Elimination Reactions." In M. S. Newman, ed., "Steric Effects In Organic Chemistry," John Wiley and Sons, Inc., New York, N. Y., p. 305 (1956). 7j. Hime, "Physical Organic Chemistry," McGraw-Hill Book Co., Inc., New York, N. Y., p. 168 (1956). 9 of elimination. On the other hand, the El reaction proceeds by a multistage process involving a carbonium ion as an inter- mediate.6,7 Therefore, the direction of elimination /v +* H—CRG—CRG—X ^ H—CR2—CR2 4R X + 4- H—CR2—CRG > RGC—CRG "H H would not be expected to be influenced by the leaving group. The proton is lost from the same species (carbonium ion) re­ gardless of the type of leaving group involved in the ioni­ zation step. The correspondence to this prediction is un­ mistakable in the unimolecular eliminations of t-amyl bro­ mide 3 and t-amyldimethylsulfonium ion,3 the olefin pro­ portions being very nearly identical. CH3CH2C(CH3)2 El > CH3CH=C-CH3 4- C^CHgC^CHg Br CH3 CH3 82# 18# CH3CH2C( 013)2 El ^ CH3CH—Ç-CH3 4- CH3CH2C=CH2 +S(GH3)2 CH3 CH3 13# 10 Brown and Moritani® have shown that steric factors are also important in determining the direction of elimination.
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