Experiment.ppt

Experiment 12: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE Objectives To synthesize an isomeric mixture of alkenes from “E2” base promoted elimination of HBr from 2-bromoheptane under reflux. To purify the products through simple distillation. To analyze products using GC analysis of product mixture, and study the effect of base size on product distribution based on adjusted area percent. To identify characteristic absorptions in the IR spectra of reactants and products. CHEMICAL EQUATION STERIC HINDRANCE The reason that the bulkier (larger) base gives more of the less substituted alkene is that steric hindrance prevents it from approaching a hydrogen on a more highly substituted carbon. MECHANISM (MOST SUBSTITUTED ALKENE ISOMER) MECHANISM (LEAST SUBSTITUTED ALKENE ISOMER) EXPERIMENTAL PROCEDURE(SYNTHESIS) EXPERIMENTAL PROCEDURE(PURIFICATION) EXPERIMENTAL PROCEDURE(IR Analysis) EXPERIMENTAL PROCEDURE(IR Analysis) SAFETY CONCERNS Sodium methoxide and potassium t-butoxide are strong bases and corrosive! Use gloves when handling! Methanol and t-butanol are flammable! Wear safety goggles at all times and use extreme caution when heating! WASTE MANAGEMENT Cautiously add about 25 mL of water to the reaction flask and swirl to dissolve the solid salts. This forms a strongly basic solution. Place this waste in the container labeled, “BASIC AQUEOUS WASTE (E2)”. Place your product in the container labeled “ALKENE WASTE (E2)”. * * Dehydrohalogenation of 2-bromoheptane. C B r H H H H H B a s e + H B a s e B r + Removal of more hindered proton, leading to more stable, more highly substituted alkene. MAJOR product for small base, MINOR product for large base! C B r H H H H H B a s e + H B a s e B r + Removal of least hindered proton, leading to a less substituted alkene product. MAJOR product for large base, MINOR product for small base. *