Stephen J. Weininger

The mechanism of phosphine-modified rhodium-catalyzed hydroformylation studied by CIR-FTIR

The mechanism of phosphine-modified rhodium-catalyzed hydrofonnylation of 1-hexene was studied by in situ infrared spectroscopy using high pressure autoclaves equipped with embedded cylindrical internal reflectance crystals (CIR-FTIR). A series of RhH(CO)2(PR3)2 complexes 1 were synthesized using p-substituted triarylphosphines where the electron density on rhodium was varied by using p-N(CH3)2, p-OCH3, p-H, p-F,p-Cl or p-CF3. The metal carbonyl and metal hydride infrared stretching frequencies were correlated by a standard Hammett treatment of the data. Reaction rates and selectivities for linear aldehydes both increased with increasingly electron-withdrawing phosphines. The IR spectra, measured under autogenous conditions of 70 †C and 200 psi (1.38 MPa), showed the presence of various intermediates in the catalytic cycle depending upon the phosphine modification, P/Rh ratio, total syngas pressure, and degree of olefin conversion. The rate and spectroscopic data permitted the assignment of a reaction mechanism involving CO dissociation from RhH(CO)2L2 1 as the primary selective hydroformylation pathway.

Mechanism of deactivation in phosphine-modified rhodium-catalyzed hydroformylation: a CIR-FTIR study

Abstract The deactivation mechanism for the hydroformylation of 1-hexene was studied by cylindrical internal reflectance infrared spectroscopy (CIR-FTIR) under autogenous conditions. These studies showed that the initial deactivation involved the conversion of the most active catalytic intermediate, RhH(CO) 2 (PR 3 ) 2 , 1 , to a less active orange dimer, [Rh(CO)(PR 3 ) 2 ] 2 , 2 , which was followed by formation of a totally inactive binuclear complex having a bridged phosphide ligand. The in situ spectra showed that when the first two complexes were observable in solution hydroformylation was facile, and when they were replaced by the phosphido complex all activity stopped. A series of hydroformylation reactions was carried out in which the triphenylphosphine ligands carried a variety of para-substituents on the phenyl rings. Deactivation was rapid when the substituents were strongly electron-donating, such as p -methoxy and p -dimethylamino.

Mastery, Insight, and the Teaching of Chemistry.

The learning of chemistry is described as a process analogous to the process of making chemical discoveries. Historical examples are given to show how chemists have used their insight to break out of a conceptual loop in order to advance the science. Having the insight to make the intuitive leap necessary to break a conceptual loop is as important as having the mastery of the pertinent facts. As in making chemical discoveries, learning elementary chemistry requires developing insight as well as acquiring mastery of the facts. However, current general chemistry teaching tends to teach facts first and insight later. Suggestions for improving this situation so that insight and facts are learned together are given. Finally, the nature of insight is probed more deeply and presented as a two-step process where the first step is an evaluation of the perceptions about science which are held. Once the student, teacher, or researcher has a clear evaluation of the validity of the perceptions that he or she holds, further significant progress toward understanding or scientific discovery is possible.

Affinity, Additivity and the Reification of the Bond

In her unique study of the semiotics of chemistry the chemist and linguist Renee Mestrallet pays particular attention to structural formulas and the characteristics they share with natural languages. As she notes, these formulas can be reduced to just two building blocks, atoms and bonds, and she explores the similarities and differences between these symbols and alphabetic characters, the fundamental building blocks of most natural languages (Mestrallet, Communication). In Mestrallet’s analysis atoms and bonds thus acquire representational parity. One might even accord a higher status to bonds because to an increasing extent the presence of atoms in molecular formulas is only implied by the intersection of lines that represent bonds (Fig. 1). This trend has been nicely illustrated in Pierre Laszlo and Roald Hoffmann’s exploration of the various representations of camphor (Hoffmann and Laszlo, “Representation”).

Delayed Reaction: The Tardy Embrace of Physical Organic Chemistry by the German Chemical Community*

The emergence of physical organic chemistry, which focuses on the mechanisms and structures of organic reactions and molecules using the tools of physical chemistry, was a major development in twentieth-century chemistry. It first flourished in the interwar period, in the UK and then in the US. Germany, by contrast, did not embrace the field until almost a half century later. The great success of classical organic chemistry, especially in synthesis, encouraged indifference to the new field among German chemists, as did their inductivist research philosophy, as enunciated by Walter Hückel’s ground-breaking textbook (1931). This author also resisted new concepts and representations, especially those of the American theoretician, Linus Pauling. The arrival of the Nazi regime reinforced such resistance. Postwar conditions initiated a reaction against this conservative, nationalistic attitude, especially in the American Occupation Zone. Exposure to American textbooks and visiting lecturers influenced attitudes of younger chemists. The accompanying shift towards a more explanatory, less hierarchical mode of pedagogy was consonant with larger social and political developments.

Benzene and Beyond: Pursuing the Core of Aromaticity

Summary Kekulé first suggested a hexagonal structure for benzene in 1865. For over a half-century after, chemists struggled to reconcile proposed structures for benzene and other aromatic compounds with their resistance to chemical transformation and tendency to maintain the type during reaction. The combined structural and reactivity features of these compounds were eventually covered by the term ‘aromaticity’. Kekulé, Bamberger and Thiele had each proposed a criterion for aromaticity; all were either empirically contradicted or incapable of evaluation. In the 1930s, two rival quantum mechanical methods succeeded in establishing a physical basis for aromaticity. Using valence bond theory, Pauling attributed benzenes stability to its being a resonance hybrid of several Lewis structures. Calculating resonance energies was challenging but manipulating Lewis structures was not; that procedure provided qualitative insights into aromatic structure and reactivity. Resonance theory appealed especially to organic chemists and eclipsed Hückels contemporaneous molecular orbital approach, which remained relatively inaccessible. In the 1950s, however, simple rules derived from Hückels mathematics, combined with proton NMR data, provided seemingly universal criteria for aromaticity. In the event, post-1950 discoveries of non-organic, three-dimensional compounds such as ferrocene and the fullerenes that exhibit aromatic properties led chemists to doubt the utility and universality of ‘aromaticity’ as a concept. A recent consensus maintains that aromaticity is a multi-variable phenomenon that cannot be reduced to a strict definition, a property it shares with other core chemical concepts such as ‘acidity’ and ‘reactivity’.

Paper Tools from the 1780s to the 1960s: Nomenclature, Classification, and Representations

The four articles in this issue ofAmbixwere among seven papers presented in a symposium at the fall 2016 national meeting of the American Chemical Society on the occasion of the 2016 History of Chemistry Division’s Award to Ursula Klein for Outstanding Lifetime Achievement in the History of Chemistry. Klein’s symposium keynote paper appeared in 2017 in the Division’s Bulletin for the History of Chemistry. The four articles that follow in this issue cohere around major themes in Klein’s work, most notably the role in chemical investigation and documentation of paper tools such as classifications, formulas, models, diagrams, and other images. In her book, Experiments, Models, Paper Tools, Klein has this to say about paper tools:

Private Philanthropy and Basic Research in Mid-Twentieth Century America: The Hickrill Chemical Research Foundation

The Hickrill Chemical Research Foundation, located north of New York City on the estate of its patrons, Sylvan and Ruth Alice Norman Weil, had a short (1948–59) but productive life. Ruth Alice Weil received a Ph.D. in organic chemistry in 1947, directed by William von Eggers Doering of Columbia University. She intended that Hickrill contribute to cancer chemotherapy while providing resources for Doerings more speculative research. Ultimately, Doerings commitment to theoretical organic chemistry set Hickrills research agenda. Lawrence Knox, an African American with a Harvard Ph.D., supervised the laboratorys daily activities. Hickrills two dozen postdoctoral fellows produced path-breaking results in Hückel aromatic theory and reactive intermediate chemistry, fostering the postwar emphasis on “basic science.” This essay places the Laboratorys successes in the wider context of postwar politics and scientific priorities. Private philanthropic support of basic science arose because it received little pre-World War II government support. In the immediate postwar period, modest organisations like Hickrill still met a need, but the increasing governmental defence- and non-defence-related support for science eventually rendered them unnecessary.