CHAPTER EIGHT Nucleophilic Substitution g RE 8.4 Solvation of a chloride by ion-dipole attractive forces with water. The negatively ned chloride ion interacts with the positively polarized hydrogens of water AN ENZYME-CATALYZED NUCLEOPHILIC SUBSTITUTION OF AN ALKYL HALIDE ucleophilic substitution is one of a variety of The product of this nucleophilic substitution then re- mechanisms by which living systems detoxify acts with water, restoring the enzyme to its original halogenated organic compounds introduced state and giving the observed products of the reac- into the environment. Enzymes that catalyze these tion. reactions are known as haloalkane dehalogenases The hydrolysis of 1, 2-dichloroethane to 2- several roethanol, for example, is a biological nucle ophilic substitution catalyzed by a dehalogenase CH2Cl dehalogenase CICH2 CH2CI+ 2H2O 1.2-Dichloroeth EnzymeFC-0: +HOCH2 +H3O CICH2 CH2OH+ CHCI 2-Chloroethanol Hydronium ion Chloride ion This stage of the reaction proceeds by a mechanism that will be discussed in Chapter 20. Both stages are The haloalkane dehydrogenase is believed to faster than the reaction of 1.2-dichloroethane with act by using one of its side-chain carboxylates to dis- water in the absence of the enzyme place chloride by an SN2 mechanism (Recall the reac- Some of the most common biological SN2 reac- tion of carboxylate ions with alkyl halides from Table tions involve attack at methyl groups, especially a methyl group of s-adenosylmethionine. Examples of these will be given in Chapter 16 Enzyme CHaCI CHCI Back Forward Main MenuToc Study Guide ToC Student o MHHE Website314 CHAPTER EIGHT Nucleophilic Substitution AN ENZYME-CATALYZED NUCLEOPHILIC SUBSTITUTION OF AN ALKYL HALIDE Nucleophilic substitution is one of a variety of mechanisms by which living systems detoxify halogenated organic compounds introduced into the environment. Enzymes that catalyze these reactions are known as haloalkane dehalogenases. The hydrolysis of 1,2-dichloroethane to 2- chloroethanol, for example, is a biological nucle￾ophilic substitution catalyzed by a dehalogenase. The haloalkane dehydrogenase is believed to act by using one of its side-chain carboxylates to dis￾place chloride by an SN2 mechanism. (Recall the reac￾tion of carboxylate ions with alkyl halides from Table 8.1.) ±C±O O X SN2 Enzyme CH2±Cl CH2Cl W ±C±O±CH2 O X Enzyme CH2Cl W Cl ClCH2CH2Cl 1,2-Dichloroethane 2H2O Water ClCH2CH2OH 2-Chloroethanol H3O Hydronium ion Cl Chloride ion dehalogenase enzyme The product of this nucleophilic substitution then re￾acts with water, restoring the enzyme to its original state and giving the observed products of the reac￾tion. This stage of the reaction proceeds by a mechanism that will be discussed in Chapter 20. Both stages are faster than the reaction of 1,2-dichloroethane with water in the absence of the enzyme. Some of the most common biological SN2 reac￾tions involve attack at methyl groups, especially a methyl group of S-adenosylmethionine. Examples of these will be given in Chapter 16. ±C±O O X Enzyme HOCH2 CH2Cl W H3O several steps ±C±O±CH2 O X Enzyme CH2Cl W 2H2O Cl



FIGURE 8.4 Solvation of a chloride by ion–dipole attractive forces with water. The negatively charged chloride ion interacts with the positively polarized hydrogens of water. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website