35-Chemical and Biological Catalysis

35-Chemical and Biological Catalysis - 5.111 Lecture # 35....

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Unformatted text preview: 5.111 Lecture # 35. CHEMICAL AND BIOLOGICAL CATALYSIS [Pages 565—570 from the “Chemical Principles” textbook, 4111 edition, by Peter Atkins & Loretta Jones, Freeman, New York, 2008] How to accelerate chemical reactions? Importance of catalysis in industry and in living organisms. Let’s first consider the phenomenon of catalysis in more mechanistic detail than thus far. Catalysts accelerate chemical reactions without being consumed. They do so by providing an alternative reaction pathway (mechanism) with a lower activation energy, Ea — [Slide 35.1]. Since both forward and reverse reactions are accelerated to the same extent, the equilibrium constant is unchanged by a catalyst. Typical reactant-to-catalyst ratios. Homogeneous and heterogeneous catalysts. A homogeneous catalyst is present in the same phase as the reactants. Consider the following reaction in aqueous solution catalyzed by dissolved Br2: 2 H202 (3(1) —’ 2 H20 (1) + 02 (g) (1) The first step of this reaction involves the reduction of bromine with hydrogen peroxide: Br2 (aq) + H202 (aq) —> 2 Br' (aq) + 02 (g) + 2 H+ (am (2) followed by the oxidation of the bromide formed with another molecule of hydrogen peroxide in the next step to regenerate the catalyst: Al c0323.. ".0 mmwhmoi anus—wok. fan 63233 yo mm mu: “#9601 fan twin—35 fan. 359.0 <—— Afilaua |eg1uaxod 2 Br' (aq) + H202 (aq) + 2 H+ (aq) —> Brz (aq) + 2 H20 (1) (3) In the sum of equations (2) and (3), Br'(aq) and H+(aq) cancel to yield equation (1). Thus although the catalyst bromine participates in the reaction, it is not consumed. Nevertheless, the concentration of the catalyst does appear in the rate law; e.g., if the reaction (2) above is rate—limiting, then the rate for the overall reaction, (1), is Ratem z Ratea) = k[BI‘2] [H202] A heterogeneous catalyst is present in a phase different from that of the reactants. Typically — a solid catalyst for gaseous or liquid reactants. Finely divided or porous solid catalysts; Why? Consider the previously mentioned hydrogenation of ethylene to ethane catalyzed by powdered Pt: CH2=CH2 (g) + H2 (g) —" CH3CH3 (g) Dissociation of H2 to ZH‘ on the Pt surface; formation of CH3=CH2-, etc. —-— [Slide 35.2]. Another important example: the catalytic converters of automobiles —— [Slide 35.3]. Poisoning (inactivation) of heterogeneous catalysts and its mechanisms. Heavy metals, especially lead, as poisons for catalytic converters; hence Pb-free gasoline. Enzymes Nature’s catalysts; their role in living organisms. Enzymes as specialized protein molecules — [Slide 35.4]. Substrates. NOW ~No ~Nz Inez #:0353323 .OU The active-site concept. How enzymes work: the (adjustable) lock-and-key and induced- fit mechanisms — [Slide 35.5]; enzyme-substrate complementarity in the transition state. Simplest kinetic equation of enzyme action: E+S<—>E-S—-—>E+P Acidic (e.g., Asp and Glu) and basic (e. g., His and Lys) groups brought together within the active site upon folding of the enzyme molecule; other (non-protein) molecules and ions ofien present in enzyme active sites. Remarkable features of enzymes as catalysts compared to typical chemical catalysts: 0 extremely high catalytic activity (“perfect” enzymes); 0 keen selectivity (substrate and stereo-) of action; 0 ability to work under mild conditions (in water, at ambient temperature and pressure); 0 enzymatic activity is finely regulated by appropriate biomolecules. Enzyme inhibition — [Slide 35.6]. Pharmaceuticals as inhibitors of enzymes. Gene therapy. Many enzymes work together in living organisms. Enzymatic pathways. Matabolism: catabolic and biosynthetic pathways. ( 5:30.. .23... «sham \\ .. mum w>=u< =>wx= , Bahama—3m ...
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This note was uploaded on 02/08/2010 for the course CHEM 143 taught by Professor Mike during the Spring '10 term at École Normale Supérieure.

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35-Chemical and Biological Catalysis - 5.111 Lecture # 35....

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