|MadSci Network: Chemistry|
Hi Christine, What a fantastic question! Catalysis is a tremendously complex subject and its importance extends into virtually every area of chemistry. There is more than one meaningful definition of a catalyst. One I learned as an undergraduate is that a catalyst is a chemical species which participates in a chemical reaction but is not consumed in the reaction. This definition doesn't allude to the real value of catalysts: their ability to lower the "activation energy" of a reaction. The activation energy is the energy "invested" in the reaction to make it go. When the reaction is done that energy is returned (and then some) if the reaction is exothermic (products are lower energy than starting materials). The figure below is a graphical representation of this principle: It is important to understand that the rate of a reaction is directly related to its activation energy. A higher activation energy requires a greater energetic investment and therefore slows the rate of the reaction. It is important , also, to know that most reactions are reversible and a catalyst will act to lower the activation barrier between products and reactants such that the reverse reaction is catalyzed as well. An example of a simple, reversible, catalyzed reaction is the acid catalyzed hydrolysis of an ester: In the forward direction, the ester associates with an acidic proton (H+) to form an "activated" intermediate that is attacked by a water molecule to form the next intermediate that can then lose a proton and decompose to the carboxylic acid and alcohol products of hydrolysis. The proton is catalytic because it is consumed and regenerated, then can be reused in the reaction cycle. The catalyst does not determine which direction the reaction goes, but that can be influenced by other factors (temperature, pressure, and reactant concentrations). The most important catalysts on earth are enzymes. Without them there would be no living things (and thus no man-made catalysts!). The products of enzyme catalysis are proteins, DNA, RNA, simple to complex organic products, and enzymes themselves. In fact there is nothing in your body except water and simple inorganic salts (like NaCl) that isn't the product of a catalytic process. Chemicals like diatomic oxygen, vitamins, and some amino acids are not made in your body but are products of catalytic processes in plants and other organisms. Enzyme catalysts possess the ability to perform selective catalytic reactions beyond anything currently possible in a laboratory with a non proteinogenic catalyst. Your body possesses enzymes called "esterases" which perform the identical ester hydrolysis reaction as shown above but have the ability to select a specific ester and only lower the activation energy of hydrolysis of that ester! In contrast, an ordinary acid catalyst will catalyze any ester hydrolysis. The most important industrial catalytic processes produce polymers (plastics), methanol, ammonia, hydrogen, and carbon monoxide. This is by no means a complete list but these few simple compounds are the starting materials for an uncountable number of consumer products (how many plastic things do you use each day?) from fertilizers to pharmaceuticals to bulk construction materials. Hoola-hoops would not have been possible without a special polymerization catalyst that produces the stereo-regular plastic required for them. A special class of catalysts that is growing in importance in the pharmaceutical industry is stereoselective catalysts. These catalysts have the ability to catalyze reactions to give optically active products. These products can be the precursors of optically active pharmaceuticals that are free of the ineffective (and sometimes dangerous) stereoisomer present in a racemic mixture. (see my Madsci post 891001341.Ch for more detailed information on the importance of optical activity in pharmaceuticals). One example of this is the Noyori asymmetric hydrogenation catalyst used in the industrial preparation of the OTC drug sodium naproxin. I hope this information was helpful! Jeremy.
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