.8 Branching next to the reacting carbon also retards haee2a2dnetypropanea 6 Important Concepts Hgakan0-Ana内haae,naygopnd 3. on 品品 d is 品29时c ary h o Important Concepts Important Concepts 5. S2 Rea 9.Polar Aprotic Solvents-AccelerateS2 s 2 Transition State bron philicity-Increases: own in periodic table In aprotic solvents. 8 8 Replacement of a hydrogen in a halomethane by a carbon chain of 3 or more atoms shows no additional effect over a carbon chain of 2 atoms. Multiple substitution at the position next to the electrophilic carbon causes a dramatic decrease in reactivity in SN2 substitution reactions. 1-Bromo-2,2-dimethylpropane is virtually inert. Branching next to the reacting carbon also retards substitution. The explanation for the decrease in reactivity is in the stabilities of the transition states involved: In 1-bromo-2-methylpropane two gauche methyl-halide interactions occur in the only conformation permitting nucleophilic attach by the OH- . In 1-bromo-2,2-dimethylpropane there is no conformation allowing easy approach of the OH- and the reaction is blocked almost completely. 6 Important Concepts 1. Haloalkane – An alkyl halide, an alkyl group and a halogen 2. Haloalkane Properties – Strongly affected by the C-X bond polarization and the polarizability of X. 3. Nucleophilic – When a lone pair of electrons on a reagent attacks a positively polarized (or electrophilic) center. If a substituent is replaced, the reaction is termed a nucleophilic substitution. The substituent replaced is called the leaving group. 4. Nucleophilic Substitution Kinetics – For primary and most secondary haloalkanes the reaction is 2nd order. These reactions are termed SN2. They are concerted reactions where bonds are simultaneously made and broken. 6 Important Concepts 5. SN2 Reactions are Stereospecific - These reactions proceed by backside displacement. The configuration at the reacting center is inverted. 6. SN2 Transition State – • sp2 carbon center • Partial bond making at nucleophile and electrophilic carbon. • Partial bond breaking at leaving group and electrophilic carbon • Both nucleophile and leaving group bear partial charges. 7. Leaving Group Ability - Roughly proportional to the strength of the conjugate acid (especially good leaving groups: chloride, bromide, iodide, sulfonates). 8. Nucleophilicity – Increases: • With negative charge • Farther to the left and down in periodic table • In aprotic solvents. 6 Important Concepts 9. Polar Aprotic Solvents - Accelerate SN2 reactions. 10. Branching At The Reactive Center - (or at the carbon next to it) sterically hinders the transition state and decreases the rate of SN2 substitution