CH08 Instructor Notes

CH08 Instructor Notes - CHAPTER 8 Nucleophilic Substitution...

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Ch 221, Lecture Notes, Chapter 8 Page 1 of 7 CHAPTER 8 — Nucleophilic Substitution I. ASPECTS OF THE NUCLEOPHILIC SUBSTITUTION REACTION A. Reaction of a Lewis base (nucleophile) with the carbon atom of a polar C-X bond (electrophile): RX + Nu: - RNu + X: - Alkyl Halide (X = Leaving group) Nucleophile (Lewis base) We've seen this reaction before in the reaction of 1° ROH with HX B. Bond Breaking and Bond Making in the Nucleophilic Substitution Reaction 1. Alkyl halides are substrates, not alkenyl- or aryl halides X X NO NO X YES 2. C-X bond is broken heterolytically R X δ + δ - Polar Covalent Bond causes carbon to be slightly positively charged 3. Relative rates of substitution reactions: RF << RCl < RBr < RI 4. Since nucleophile has to physically come into contact with alkyl halide, bulkier RX reacts slower than less-bulky RX: 3°RX << 2°RX << 1°RX C. Common Nucleophiles 1. alkoxides RO - M + 2. carboxylate RCO 2 - M + 3. sulfide HS - M + 4. cyanide CN - M + 5. azide N 3 - M + 6. Halides X - M + M + = Li + or Na + or K + as a solution in DMSO or EtOH/H 2 O
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