is in this area that new scientific knowledge could accelerate the substitution of coal for petroleum Methane formation CO+3Hz-Cl&+HzO
Glycol synthesis 2 CO
+ -
+ 3 Hz
Hydration of coal C
Hz
HOCHzCHzOH CCHzS
Fischer-Tropsch synthesis Formation of homologous alcohols ROH
+ CO + 2 Hz
Methanol synthesis
-
RCHzOH + Hz0
Osmium and iridium carbonyls should be catalytically active in the methane formation. A homogeneous titanium complex reacts stoichiometrically under mild conditions. In the glycol synthesis (important quantity-wise) the discovery of a rhodium carhonyl cluster catalyst that is active with synthesis gas could open the possibility of replacing the ethylene with synthesis gas as the raw material. 2 CO + Hz
synthesis gas
( C O ) 2101
0.53Yz
HOCHzCHIOH
glycol
For the Fischer-Tropsch synthesis as well as for the hydrogenation of coal, a salt-melt variation has been developed.
For the hydrogenation of coal a ZuC12KCI melt is used; in the Fischer-Tropxch synthesis the catalyst Irl(C0),2 is in a NaCI/AICIq melt at 180°C and 2 atm or nreasure. A decisive breakthrouph, however, will be possible oily when newer, very noncorrosive materials are develooed. In the svnthesis of homologous alcohols the synthesis g& can serve a s k e starting base for methanol, ethanol, and higher alcohols. Outlook Homogeneous catalysis bas, within a few years, captured an important part of the technically applied processes in chemistry. Additional processes t h a t could be industrially utilized include: the introduction of new catalysts with new ligands, the discovery of new reactions (for example the activation of C-C and C H bonds), the improvement of existing processes, and the variation of the application phase. In the future energy and raw material considerations will increasinelv determine the direction of world-wide develonment nf traditional industrial organic chemistry. Since homogeneous catalysts are generally active a t low temperatures, new developments can be expected in this area. Great advances will occur where processes based on petroleum can be replaced by synthesis gas technology based on coal or where carbon dioxide as the end product in the hydrocarbon cycle can be recycled as a starting agent. Of special interest in the long term are the homogeneously catalyzed processed based on water and air in which the requried energy could be s u ~ d i e hv d the sun, an ever-available energy source. ~ x a m ~ l e s - such-applications df exist: the reduction of atmospheric nitrogen to hydrazine, the activation of oxygen for fuel cells, and the photochemical splitting of water into hydrogen and oxygen. In these processes the carbonyl catalysts as well as the cluster compounds could make a decisive contribution.
A Useful Model for the "Lock and Key" One of the most generally useful analogies for the specificityof enzymes toward substrates is the well-known "lock and key" hypothesis' originally introduced by Emil Fischer in 1894. A useful model that nicely illustrates this principle is the "SOMA" puzzle cuhe2(see figure). By using differently colored sets it is possihle to arrange them as in the figure. Manipulationsquickly demonstrate that the "enzyme" cavity will only be filled with one of the two similarly shaped "substrates." This demonstration can be extended to include an a n a l w concerning the differencesin physiological properties possessed by many enantiomeric pairs (e.g., flavor-enhancingproperties of MSG, (monosodium glutamate) versus its enantiomer). As can be quickly seen, the two "substrates" are indeed enantiomeric and only one will completely fill the "enzyme" cavity. The demonstration is made clear to a small class by using the models in the front of the class. By using either single 35-mm slides or lap-dissolve projection,3 the concepts can easily he conveyed to any size class. Another use of this relationship can he made concerning the odor differThis clearly emphasizes the conences that exist between R and S car~one.~ troversy concerning the stereochemical versus the other theories of odor recognition.
'Jencks, W. P.. "Catalysis In Chemistryand Enzymology." McGrsw-HillBwk Company. New Y a k . 1989, p. 288.
"SOMA by Parker Brothers, Inc., P.O. Box 900, Salem, MA., 01970. Harpp, D. N. and Snyder. J. P.. J. CmM. E m . , 54,68 (1977):Fine. L. W.. m p . D. N.. Krakower. E., and Snydar. J. P.. J. CmM. E m . .
54, 72 (1977).
'Russell, G. F. and Hills, J. I., Science, 172, 1043(1971);Friedman, L. and Miller, J. G., SclenC.3, 172, 1045 (1971). Arlel E. Fenster John Abbon College St. Anne de Bellwe. Que., Can&
David N. Harpp McGill University Montreal. Oue.. H3A 2K6 Canada
Joseph A. Schwarcz Vanier College Montreal. Que.. H3X 2N9 Canada
Volume 61 Number 11
November 1984
967
