17.Hydrolysis.11

17.Hydrolysis.11 - Abiotic Reactions: Hydrolysis...

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Unformatted text preview: Abiotic Reactions: Hydrolysis Introduction to Hydrolytic Reactions Nucleophilic Substitution Reactions Reactions and Substrates Hydrolytic Reaction Rates Summary and Conclusions I. Introduction to Hydrolytic Reactions Definition – nucleophilic substitution reaction via water. Water is capable of serving as a weak nucleophile. Some of the most common environmental substrates include alkyl halides, esters, and amides. Catalyzed in organisms by hydrolases such as carboxylesterases and amidases. Hydrolytic rates can be influenced by pH. II. Nucleophilic Substitution Reactions Displacement of one group on a molecule by another via the attack of a nucleophile (electron-rich constituent) on an electron-poor group (electrophile, often carbon). The attacking nucleophile must be a Lewis base (possesses an atom with a pair of electrons available for bonding; B:). 2 Bases include HO-, HS-, CN-, H O, SCN-, etc. General types include Sn1 and Sn2 mechanisms. ome General Types of Substitution Reaction Water can serve as a weak nucleophile due to the unshared electron pairs on oxygen. Sn1 Nucleophilic Substitution Reactions Generally involve the brief formation of a carbocation. There is no structure inversion. Reaction is dependent on the stability of the carbocation, which is highly reactive. o o o Stability (= reactivity): 3 > 2 > 1 > methyl cation. Sn2 Nucleophilic Substitution Reactions Do not involve the formation of a carbocation. There is structure inversion. Branching of the α carbon reduces the reaction rate via steric hindrance. o o o The rate of reaction is methyl > 1 > 2 > 3 . Reactivity Summary fluence of Leaving Group on Hydrolytic Rate Reaction rates also dependent on leaving groups, which are dependent on C-X bond strengths: I = Br > Cl > F. Good leaving groups are weak bases, but strongly electrophilic. III. Reactions and Substrates Alkyl halides: RX + H2O HX + ROH Esters: RCOOR’ + H2O RCOOH + R’OH Amides: RCONR2 + H2O RCOOH + HNR2 Chemical Persistence and Hydrolysis Many newer-generation pesticides contain functional groups susceptible to hydrolysis to reduce persistence. Esters – esfenvalerate, permethrin (insecticides) Carbamates – carbaryl, carbofuran (insecticides) Ureas – monuron, diuron (herbicides) Thiophosphoric acid esters – methyl & ethyl parathion (insecticides) Thioesters – malathion Lactams – include antibiotics such as penicillins, which are susceptible to lactamases in resistant bacteria. Neutral Hydrolysis of Carboxylic A involves the Neutral hydrolysiscid Esters slow attack of water. The intermediate is unstable and quickly rearranges. The leaving group ultimately becomes an alcohol. II. Rates of Hydrolysis Effect of pH on Hydrolytic Rate Base-Catalyzed Hydrolysis of an Ester This form of hydrolysis involves nucleophilic attack by HO-, which is a stronger nucleophile than water. The resultant intermediate quickly rearranges. The leaving group ultimately becomes an alcohol. Base-Catalyzed Hydrolysis of an Amide In general, base catalysis is important in the environment. For instance – seawater has a pH of 8.0. ase-Catalyzed Hydrolysis of Methyl Bromide Methyl bromide is an effective soil fumigant for strawberries. Susceptible to base-catalyzed hydrolysis, but a threat to the ozone layer. Potentially to be replaced by methyl iodide. Acid-Catalyzed Hydrolysis of an Amide This form of hydrolysis involves protonation by hydronium ion. The carbocation is then attacked by water (weak nucleophile). The resultant intermediate quickly rearranges. The leaving group ultimately becomes an amine. IV. Hydrolytic Reaction Rates Neutral hydrolysis can be described by: h 2 dC/dt = -K C [H O]; Kh is the rate constant (h1). Since [H2O] is constant (excess) in solution, n rate only varies with changes in C: dC/dt = -K’ C (K’n = Kh [H2O], or the neutral rate constant; h1). C=concentration of pesticide Acid-Base Catalyzed Reaction Rates Acid-catalyzed hydrolysis can be described by: a + dC/dt = -K C [H ]; Ka is acid-catalyzed rate constant (h-1). [H+] can be derived by pH = - log [H+] Base-catalyzed hydrolysis can be described by: b dC/dt = -K C [-OH]; Kb is base-catalyzed rate constant (h-1). [-OH] can be derived by pH = - log [H+] Overall First-Order Rate Constant Defined as Kt, and is a function of all the rate constants: t n b a K=K +K +K 1/2 Hydrolytic half life may be calculated by t 0.693 / K K can be either Kt, Kn, Kb, or Ka = V. Summary and Conclusions Hydrolysis involves nucleophilic reaction with water. Reactions can be either Sn1 or Sn2. Hydrolytic rate can be influenced by environmental pH. Products again have unique physical-chemical and toxicological properties. Many newer-generation pesticides contain functional groups susceptible to hydrolysis to reduce their persistence. ...
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This note was uploaded on 03/10/2011 for the course ETX 102A taught by Professor Ronaldtjeerdema during the Winter '10 term at UC Davis.

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17.Hydrolysis.11 - Abiotic Reactions: Hydrolysis...

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