Nucleophilic substitutions may be reversible. em8Caaog GLCHGLGLGN GICLCIONO! 6-9 Structure and S2 Reactivity:The Substrate the folon ituents on the reacting carbon can be seen i THHLE 5A e82hg2,t地m.yoneortwocarbona a4gh tom by a es22ee g0的电 二器 0西由8 77 Nucleophilic substitutions may be reversible. Halide ions (except F- ) are both good nucleophiles and good leaving groups. The SN2 reactions of these halides are reversible. The solubility of the sodium halides dramatically decreases in the order: NaI > NaBr > NaCl. NaCl is virtually insoluble in propanone so reactions involving the displacment of Cl- can be made go to completion by using the sodium salt of the attacking nucleophile: When the nucleophile in a SN2 reaction is a strong base (HO- , CH3O- , etc.) it becomes a very poor leaving group, and SN2 reactions involving strong bases as nucleophiles are essentially irreversible. The relative reaction rate of iodomethane with a variety of nucleophiles illustrates the previous points: 6-9 Structure and SN2 Reactivity: The Substrate Branching at the reacting carbon decreases the rate of the SN2 reaction. The effects of substituents on the reacting carbon can be seen in the following data: The transition states of the reaction of OH- with methyl, primary, secondary and tertiary carbon centers explain the decrease in activity: The steric hindrance caused by adding successive methyl groups to the electrophilic carbon decreases the transition state stability to the point that substitution at a tertiary carbon does not occur at all. (fast) Methyl > primary > secondary > tertiary (does not occur) (very slow) Lengthening the chain by one or two carbons reduces SN2 reactivity. Replacement of one hydrogen in chloromethane by a methyl group to form chloroethane reduces the rate of SN2 displacement of the chlorine atom by about a factor of 100. Replacement of the hydrogen by an ethyl group to form chloropropane reduces the rate of SN2 displacement of the chlorine atom by another factor of 2. The gauche conformer in the 1-propyl case has similar reactivity to the ethyl case