The Pyrolysis of Organic Esters Dissertation
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THE PYROLYSIS OF ORGANIC ESTERS DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University RICHARD JUI-FU LEE, B.S y l f .■) ®rV'«\ a.i @@ :&f a■ ' Adviser * Department of Chemistry -i- ACKMOWLEDGMEMT My sincere and profound thanks to Dr. Christo pher L. Wilson for the suggestion of the problem and his inspiring guidance throughout the research. My gratitude to the B. F. Goodrich Company, whose Research Fellowship I held for the entire span of this work. Finally, but not least, my deepest appreciation to Mr. Daniel Loughran for his ever cooperative assistance in making the infra-red analyses. -ii- TABLE OF CONTENTS Page I. INTRODUCTION AND STATEMENT OF PROBLEM 1 II. HISTORICAL INTRODUCTION........ ........... 5 1. Pyrolysis of Organic Compounds........ 5 A. Berthelot's Theory......... 5 B . Haber' s Rule ........................ 6 C. Rule of Least Molecular Deformation........................ 7 D. Bredt's Rule........................ 8 E. Blanc’s Rule................ 8 F. Nef's Theory......................... 9 2. Theory of Pyrolysis of Esters....... 10 A. Bilger and Hibbert's Mechanism.... 11 B. Hurd's "Hydrogen Bridge" Cyclic Mechanism....................... 13 C. Ritchie's Dual Reactions........... 16 D. Htickel's Concept (Tsugaev Reaction) 19 E. Alexander's Cis-Elimination Studies 20 F. Kinetics on the Pyrolysis of Esters 22 G. Some Contributions from the Kinetic Study of Dehydrochlorination of Chloroparaffins ...... 24 3. The Cyclic Transition State in Other Processes........... 32 A. General Considerations.............. 32 B. The Concept of O'Connor and Nace... 34 C. The Pyrolysis of Sulfites.......... 37 iii £a^e III. Discussion of Earlier Work,.................... 42 1. Kinetics as a Method for Mechanism Study.. 42 2. Substitution Effects on the Cyclic Transition Complex............ 48 3. The Excited Triplet State as a Transi tion State in a Special Case ....... 63 IV. EXPERIMENTAL........ 88 1. Pyrolysis of Diacetyl Cyanide (D.A.C.).... 88 A. Reagents....... 88 B. Apparatus * 89 C. Decomposition of D.A.C.................. 92 D. Isolation and Estimation of Products.. 94 E. Results................................... 96 F. By-Products................. 101 G. Preparation and Pyrolysis of Pyruvic Nitrile ........ 102 2. Pyrolysis of Di-esters....................... 104 A. Preparation of Materials........ ....... 104 B. Procedure for Pyrolysis ...... 105 C. Collection and Identification of Products ......................... 106 3. Pyrolysis of Cellosolve Acetates......... 108 A. Preparation of Materials........ ....... 108 B. Pyrolysis Procedure..................... 108 C. Collection and Identification of Products .................. 109 V,. INFRA-RED SPECTROGRAMS....................... 110 -iv- Page VI. S UMMARY......... 128 VII. BIBLIOGRAPHY................................... 129 VIII. AUTOBIOGRAPHY.......... 135 INTRODUCTION AND STATEMENT OF PROBLEM Although much is known concerning the pyrolysis of esters, most of the evidence has not been correlated. The known facts about thermal decomposition of simple esters were recorded almost as far back as a century ago by Oppenheim and Precht (l). They found that when esters containing beta-hydrogen in the alcohol portion of the molecule were decomposed thermally, acids and olefins were generally the products of pyrolysis. Their work was principally confined to the acetates. Modifications in the acid and alcohol portions of the ester molecule were investigated in succeeding researches such as those of Engler and Low (2), Peytral (3), Bilger and Hibbert (4 ) and Hurd and Blunck (5). These workers changed the structure of the ester by varying the number of beta hydrogens available and the nature of the acidsj such as acetates to benzoates, phenyl-acetates, propionates, formates, etc. From these studies they interpreted the behavior of esters on pyrolysis. In the above cases the temperature entered as a factor to be considered in that it determined the nature of products to some considerable extent. These investigators did not significantly examine the -2- temperature dependency of the primary or succeeding processes of the reaction in the cases studied, having in mind the mechanism. The collection and detection of all products formed were not too explicitly and completely reported since the secondary decompositions accompanying a rise in temperature complicated, in most cases, the results obtained. Perhaps it was due to this factor that most investigators confined themselves to simple esters, or perhaps it was the simplicity of operations that led the workers to concentrate on the beta-hydrogen effect. Nevertheless the succeeding studies by Ettckel, Tappe, and Lengutke (6), Stevens and Richmond (7), Houtman, van Steenis and Heertijis (8), Wibaut and van Pelt (9), Ritchie, Jones and Burns (10), and many others, till the very recent studies of Bailey (ll) have been concentrated largely on the selectivity of the beta-hydrogen elimination phenomenon. Chronologically there has been no cessation of effort in the field of ester decomposition. But there were more prominent fads through certain periods of history than others, and these peaks gave welcomed respite for the researcher to stop and take inventory. As with all other well practiced reactions, pyrolysis of esters enjoyed and is enjoying today a very practi cal and industrialized position. The patents of Filachione, Fisher, Ratchford, Rehberg, Fein and Smith (12) and many others bear out that this unique elimination has been much favored for synthesis. It is not clear, however, why theoretical con siderations of ester pyrolysis have not kept pace with this practical application. Recently there has been some quantitative work, but very little of it concerns carboxylic esters. Some studies have been made of surface catalysis (8) rate measurements (13), homogeneity of the system and activation energies required (14), however some skepti cism remains about applying these data to the decompositi of esters in general. Hence prediction of products from the molecular structure of the ester or from a knoifledge of the mechanism of decomposition is still whimsical. We have tried in the present work to broaden the scope of the now fairly acceptable postulate of intra-molecular decomposition of esters by testing such a mechanism on species that do not undergo normal elimination, but decompose rather with some degree of rearrangement. The simple but different products from these reactions, taken together with what the litera ture has revealed concerning normal eliminations, should enlighten us about the transition state. It -4- should prove interesting to observe whether any of the postulated transition states will fulfill the needs of both normal and abnormal eliminations. -5- HISTORICAL INTRODUCTION Pyrolysis of Organic Compounds A . Berthelot*s Theory If one wishes to decide what was the beginning of the theory of pyrogenic reactions, Berthelot (15) may well be considered as the pioneer. His general theory may be summarized in three parts (l6 )~: (1 ) In addition to pyrolytic reactions of decom position, there are also reactions of synthesis. In the latter, there is progressive hydrogen elimination, accompanied by the gradual formation of complex hydro carbons which eventually may result in the deposition I of carbon. (2) The building-up processes and the tearing- down processes are considered to limit or oppose each other since the lower ones reform to produce the higher one. This leads to a complicated equilibrium between / an increasing number of hydrocarbons. Berthelot*s conception of the decomposition of methane is as follows: 2CH4 - ♦ CH2 =CH2 + 2H2 2CH4 - * chhch; + 3H2 ch2=ch2 ♦ GH=CH + H2 HChCH + h2 —* ch2 =ch2 -6- GH2=CH2 + H2 ------ f CH3-CH3 2 CH3-CH3 ______ , 2 CH^ + HC=CH + H2 (3 ) These reactions occur whether the hydrocarbon is in contact with hydrogen or with other hydrocarbons. B . Haber's Rule Considerable criticism of Berthelot's theory was made by Haber (17). He pointed out that Berthelot's first proposal was an arbitrary interpretation of the observation that in the gasification of hydrocarbons at progressively high temperatures, graphite always appeared; however the carbon was never free from hydro gen. Furthermore, am: equilibrium postulated by Berthe lot indicated a permanent slate eventually reached because of a permanent set of external conditions. Since the effect of temperature on the equilibrium was obscure, it is hard to accept Berthelot's "methane equilibria". Furthermore, the irreversibility of the formation of benzene from acetylene makes the explanation awkward. To supplant Berthelot's ideas Haber gave a general rule of his own. He concluded that the C-C linkage in the aromatic series is more stable than the C-H linkage and the reverse is true in the aliphatic series. This rule, however, failed to explain many embarrassing data such as the hydrogen production in preparing propylene from propane, the pyrolysis of diphenyl ethane, -7- and the pyrogenic reaction of ethane to yield hydrogen. Many more cases that the Haber Rule cannot predict make the usefulness of the concept limited. G. Rule of Least Molecular Deformation. The trend toward the importance of molecular structure initiated by the previous considerations led to the concept of "Least Molecular Deformation". Al though this general rule could have been demonstrated by the earlier works on pyrolysis it was not formulated until the early part