Lecture.Packet.8.transformation

Lecture.Packet.8.transformation - 4.4 ABIOTIC AND BIOTIC...

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CEE 440 © 2011 Charles J. Werth, University of Illinois at Urbana-Champaign. All rights reserved. 1 4.4 ABIOTIC AND BIOTIC TRANSFORMATION PROPERTIES My teaching goals for this section of the notes are for you 1) to understand the major factors effecting abiotic and biotic reactions 2) to evaluate the thermodynamic feasibility of abiotic and biotic redox reactions (via general rules and thermodynamics) 3) to apply the rate laws that describe kinetic reaction rates 4) to calculate the extent that reactions affect contaminants that are undergoing advection, dispersion, and sorption - In the natural environment chemicals can be transformed to other chemicals (hopefully of lesser environmental concern). This section of notes focuses on the types of reactions that occur, their thermodynamic feasibility, and their rates.
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CEE 440 © 2011 Charles J. Werth, University of Illinois at Urbana-Champaign. All rights reserved. 2 Chemical reactions can occur alone in solution, on a solid surface (such as a mineral surface), or in the presence of a catalyist. Furthermore, these reactions may be biotic or abiotic. Abiotic reactions - occur in the absence of microorganisms Biotic reactions - microorganisms responsible for reaction Abiotic transformations in the subsurface are often either nucleophilic substitution reactions or oxidation-reduction reactions. Biotic transformation in the subsurface (and elsewhere) are usually oxidation- reduction reactions. Hence, nucleophilic substitution reactions and oxidation-reduction reactions are of high environmental significance and we will examine them in more detail.
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CEE 440 © 2011 Charles J. Werth, University of Illinois at Urbana-Champaign. All rights reserved. 3 4.4.1 Nucleophilic substitution reactions These reactions involve the displacement of a good leaving group, X, with a nucleophile, :Nu - , from its bond with carbon (McMurry, Organic Chemistry, 1992). Nucleophile: nucleus loving, electron rich sites which donate pair of electrons to electron poor site, often negatively charged R-X + :Nu - R-Nu + :X - (4.1) or CH 3 CH 2 CH 2 X + :Nu - CH 3 CH 2 CH 2 Nu + :X - (4.2) where X = Cl - , Br - , I - Nu = HO - , HS - , NH 2 - , CN - , HPO 4 2-
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CEE 440 © 2011 Charles J. Werth, University of Illinois at Urbana-Champaign. All rights reserved. 4 Good leaving group: typically weak bases (i.e. those that stabilize the negative charge best). Reaction rate often increases with decreasing basicity of leaving group (i.e. reactivity increases from Cl - <Br - <I - , basicity increases from I - <Br - <Cl - ) The nature of the nucleophile may or may not affect the reaction rate. When it does, general rules for determining a good nucleophile are beyond the scope of CEE440.
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CEE 440 © 2011 Charles J. Werth, University of Illinois at Urbana-Champaign. All rights reserved. 5 4.4.1.1 Hydrolysis The most ubiquitous nucleophile in natural systems is OH - , which comes from water (H 2 O). Reactions with water are termed hydrolysis reactions.
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Lecture.Packet.8.transformation - 4.4 ABIOTIC AND BIOTIC...

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