6-4 A Close ook at the Mechanism:Kinetics cleophilic Substitution -oaeCgesoweaiwreo Theeacnof chloromethane with sodum hydroxideis ules int cting in a sing one of the ecular nucleophilic substitution reactions are abbreviated In the case of betwe Rate=k[CH,CI][HO:]mol aeoonsieppholesgbstutionisa oeceStem6ntehalaterggoans52 52 substitution is a one step process This is an example of a concerted reaction. tion is si n the 一0 24o-340 6-5Frone r Backside Altack? The S.2 action &apg8i69ntoaoceea88ecoat2bcateC mecular mdeand C5ueesS3aotesereochemistyataisimverteg e is observed as a product.All Sv2 proceec ●我w● 33 A Closer Look at the Nucleophilic Substitution Mechanism: Kinetics 6-4 The reaction of chloromethane with sodium hydroxide is bimolecular. The rate of a reaction can be measured by observing the appearance of one of the products, or by the disappearance of one of the reactants. In the case of reaction between chloromethane and hydroxide ion: doubling the hydroxide concentration (keeping the chloromethane concentration fixed) doubles the reaction rate. doubling the chloromethane concentration (keeping the hydroxide concentration fixed) also doubles the reaction. These observations are consistent with a second-order process whose rate law is: Rate = k[CH3Cl][HO- ] mol L-1 s-1. All of the nucleophilic substitution reactions show earlier follow this rate law (with different values of k). The mechanism consistent with a second order rate law involves the interaction of both reactants in a single step (a collision). Two molecules interacting in a single step is call a bimolecular process. Bimolecular nucleophilic substitution reactions are abbreviated SN2. Bimolecular nucleophilic substitution is a concerted, on-step process. A SN2 substitution is a one step process. The bond formation between the nucleophile and the carbon atom occurs at the same time that the bond between the carbon atom and the electrophile is breaking. This is an example of a concerted reaction. There are two distinct stereochemical alternatives for an SN2 concerted reaction: frontside displacement and backside displacement: In SN2 nucleophilic substitution reactions, the transition state of the reaction is simply the geometric arrangement of reactants and products as they pass through the point of highest energy in the single-step process. Frontside or Backside Attack? Stereochemistry of the SN2 Reaction 6-5 The SN2 reaction is stereospecific. When (S)-2-bromobutane reacts with iodide ion, there are two possible theoretical products: Frontside displacement: the stereochemistry at C2 is retained. The product is (S)-2-iodobutane. Backside displacement: the stereochemistry at C2 is inverted. The product is (R)-2-iodobutane. Only (R)-2-iodobutane is observed as a product. All SN2 proceed with inversion of configuration. A process in which each stereoisomer of the starting material is transformed into a specific stereoisomer of product is called stereospecific. The same reaction shown with Spartan molecular models and with electrostatic potential maps is: