New class of reducing agents shows promise A new class of reducing agents—alkoxyaluminum hydrides—is now avail able to organic chemists. Prof. E. C. Ashby and students Burgess Cooke and John Lott at Georgia Institute of Technology, Atlanta, have found that selectivity of reduction with com pounds of the new group differs sig nificantly from that with other reduc ing agents such as aluminum hydride, hydridoaluminum halides (the "mixed hydride" reagents), and the classic lithium aluminum hydride. The result of this development and of previous work on related reducing agents, in Dr. Ashby's view, is that compounds of the general formula, HA1X2, promise to be an important class of selective reducing agents in organic chemistry. The "X" in the formula can be a chloride, bromide, iodide, methoxy, isopropoxy, tertiary butoxy, phenoxy, or dimethylamino group. The three Georgia Tech chemists uncovered the selectivity and mecha nisms of alkoxyaluminum hydride re ductions in work on conversion of epoxides to alcohols [/. Org. Chem. 33, 1132 (1968)]. Reactivity of the alkoxyaluminum hydrides toward epox ides—less than that of mixed hydrides but more than that of LiAlH4—de pends on the compounds' Lewis acid ity. Acidity also determines the pre dominant reaction path and product composition. In turn, the acidity can be explained by the compounds' de gree of polymerization. The alkoxyaluminum hydrides' dis tinctive reducing role was demon strated after several hypotheses con cerning their existence within the past two years. A group under Dr. P. T. Lansbury at the State University of New York at Buffalo earlier postulated that the alkoxyaluminum hydrides are intermediates in epoxide reductions involving mixed hydrides. Dr. Ashby states that a group under the direction of Prof. E. L. Eliel at the University of Notre Dame is responsi ble for chemists' interest in mixed hy dride reductions. Their work led to Dr. Ashby's interest in expanding the versatility of the reagent by varying the nature of the group attached to Al-H compounds. Prof. H. C. Brown and his associates at Purdue University have also used alkoxy substituents to modify the reducing power of LiAlH 4 . The mechanisms of mixed hydride reductions are described in Dr. Ashby's and Mr. Cooke's latest work on the mixed hydrides [/. Am. Chem. Soc, 90, 1625 (1968)]. The new study expands on Dr. Ashby's earlier discov ery that reduction follows two princi 40 C&EN APRIL 1, 1968
pal paths—one involving direct reduc tion, the other involving migrationafter initial complexation of the reduc ing agent and epoxide. Dr. Ashby points out that mixed hydride reagents (H n AlX 3u ) and alk oxyaluminum hydrides (H n A10R 3 n ) can be prepared readily, standardized, and stored for future use. To prepare the alkoxyaluminum hydrides, Dr. Ashby uses the reaction of aluminum hydride and the corresponding alcohol in tetrahydrofuran. A report of this method was recently published by Prof. H. Noth (now at the University of Marburg, West Germany) and K. Suchy, who did their work at the University of Munich. Epoxide reductions by hydrides are very sensitive to the choice of reducing agents, according to Dr. Ashby. For example, reductions of β-diisobutylene oxide by aluminum hydride, dichloroaluminum hydride, iodoaluminum dihydride, and dimethoxyaluminum hy dride produce considerably different product ratios for the four products formed. Product differences follow a pattern based on Lewis acidity of the reducing agent. Mixed hydride reagents such as dichloroaluminum hydride, which
show strong Lewis acidity, give a substantial percentage of migration compounds. These compounds are formed by opening the three-member epoxide ring and transferring one car bon-bonded substituent group to the adjacent carbon in the ring. For in stance, the reaction of /?-diisobutylene oxide and dichloroaluminum hydride in tetrahydrofuran at room tempera ture gives a 6 1 % fraction of 2,2,3,3tetramethylbutanol in a 75 to 78% product yield. On the other hand, alkoxyaluminum hydride reduction of epoxides yields much less of the migration product. This indicates weaker Lewis acidity. For example, the reaction of /?-diisobutylene oxide and ieri-butoxyaluminum dihydride reacts to give a 12 to 24% yield, only 3 % of which is 2,2,3,3-tetramethylbutanol. This weaker Lewis acidity is unex pected, the Georgia Tech chemists point out. It is hard to see how re placement of hydrogen in aluminum hydride with an alkoxy group, which supposedly functions only as an induc tive electron withdrawer when bound to aluminum, can yield a more weakly acidic reagent than aluminum hydride. The aluminum hydride is itself a
PRODUCT ANALYSIS. Graduate student Burgess Cooke (left), undergraduate John Lott, and Prof. E. C. Ashby of Georgia Tech examine vapor phase chromatograph chart. Vapor phase chromatography is used to analyze reduction products of alkoxyaluminum hydrides and other reducing agents
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weaker acid than any of the mixed hy drides. The answer, Dr. Ashby says, lies in the greater role of molecular associa tion in the alkoxyaluminum hydrides. Noth and Suchy in Munich found that the newer hydrides, which are solvated with tetrahydrofuran, exist as dimers, trimers, or insoluble polymers in ben zene. Bridge bonding in these ag gregates involving alkoxy groups re duces the expected Lewis acidity of these reagents, the Georgia chemists state. There is also the question of the alkoxyaluminum hydrides* lower total yields. Here again, Dr. Ashby says, the highar degree of association in the oxygen-containing reducing agents contributes to a lesser reactivity be cause of steric hindrance. Because of the low product yields with β-diisobutylene oxide, Dr. Ashby and his associates also studied alkoxy aluminum hydride reduction of sty rène oxide, a less hindered epoxide. In this case, as might be expected, higher yields and considerably more direct-reduction products resulted compared to the more hindered epoxide. For example, teri-butoxyaluminum dihydride and styrene oxide react to form 79% 1-phenylethanol (direct reduction product) in a 97% product yield. By contrast, the reduction of β-diisobutylene oxide produced only 28% of the direct reduction product, 2,4,4-trimethyl-2-butanol. Further contrast, Dr. Ashby adds, is shown in that reduction of styrene oxide by HA12C1 produces only 1% of the di rect reduction product.
APPLIED KINETICS and chemical reaction engineering
This is the third of a series of state-of-the-art books growing out of summer symposia sponsored jointly by l&EC and the Division of Industrial and Engineering Chemistry. The 15 papers from the 1966 symposium which were published from September 1966 to June 1967 in INDUSTRIAL AND ENGINEERING CHEMISTRY are combined in this book. Reaction engineering, with its focus on the chemical transformation itself, lays some claim to being the discipline that uniquely differentiates chemical engineering from other branches of engineering. The recently developing interest in reaction analysis has served to reorient chemical engineering research toward the reaction itself and increasingly to consider unit operations from the viewpoint of their interaction with the chemical transformation. This book offers much to the engineering researcher and reactor designer as well as the practicing chemical engineer. Robert L Gorring and Ver η W. Weekman of the Systems Research Division of Mobil Oil Corporation and co-chairmen of the symposium, contributed the introduction. Chapter titles and the authors appear below: M i x i n g a n d C o n t a c t i n g in C h e m i c a l R e a c t o r s K e n n e t h B. Bischoff K i n e t i c C o n s i d e r a t i o n s in S u r f a c e C a t a l y s i s J o h n H. S i n f e l t
RESEARCH IN BRIEF
Suitable modification of the coeffi cients of the Benedict-Webb-Rubin equation of state permits extension of the equation to the low temperatures involved in liquefying light hydrocar bons. Theoretical studies by Dr. Her bert E. Barner and Stanley B. Adler of M. W. Kellogg, Piscataway, N.J., have verified that their modified BWR equa tion is accurate for natural gas mixtures at low temperatures. (The original BWR equation was published almost 30 years ago). Dr. Barner told the Natural Gas Processors Association, meeting in New Orleans, La., that lowtemperature phase equilibria and en thalpy predictions are more reliable for close-boiling mixtures such as nitro gen-methane-ethane than for widerboiling mixtures containing heavier (and higher boiling) hydrocarbon components such as propane and bu tane. M. W. Kellogg is an engineering and construction firm which designs and builds natural gas plants.
P h o t o c h e m i c a l Reaction Engineering A . E. C a s s a n o . P. L. S i l v e s t o n , and J . M. S m i t h R e a c t i o n M e c h a n i s m s for E n g i n e e r i n g D e s i g n H u g h M. H u l b u r t and Y. G. Kim Is S o p h i s t i c a t i o n R e a l l y N e c e s s a r y ? Rutherford Aris Disguised Kinetics James Wei S u r f a c e M o d e l s in H e t e r o g e n e o u s C a t a l y s i s G i u s e p p e Parravano Y i e l d in C h e m i c a l R e a c t o r E n g i n e e r i n g J a m e s J . Carberry A c e t y l e n e a n d H y d r o g e n f r o m t h e P y r o l y s i s of M e t h a n e J o h n Happel and Leonard Kramer R e a c t i o n R a t e M o d e l i n g in H e t e r o g e n e o u s C a t a l y s i s J . R. Kittrell and R. Mezaki S e g r e g a t i o n Effects in P s e u d o l a m i n a r F l o w R e a c t o r s W . M. Edwards and D. I. Saletan T h e T h e o r y of O s c i l l a t i n g R e a c t i o n s J o s e p h Higgins T h e C o n c e p t of D i f f u s i o n in C h e m i c a l K i n e t i c s T h o r A . Bak and Edward R. Fisher S t o c h a s t i c M i x i n g M o d e l s for C h e m i c a l R e a c t o r s F. J . Krambeck, R. Shinnar, and S. Katz T u r b u l e n t H e a t T r a n s f e r to a N o n e q u i l i b r i u m , C h e m i c a l l y R e a c t i n g Gas P. L. T . Brian and S. W . B o d m a n To order, fill out the coupon below AMERICAN CHEMICAL SOCIETY Special Issues Sales / 1155 Sixteenth Street, N. W. / Washington, D. C. 20036 Please send copies of Applied Kinetics and Chemical Reaction Engineering at $7.50 each. 224 pages (9x12) with index. Clothbound. An I4EC Reprint. (3rd I4EC Division Summer Symposium) Π Check enclosed (to American Chemical Society). Γ Ί Send bill.
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