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Experiment 12:

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.

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Experiment 12:

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  1. Experiment 12: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE

  2. 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.

  3. CHEMICAL EQUATION Dehydrohalogenation of 2-bromoheptane.

  4. 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.

  5. MECHANISM (MOST SUBSTITUTED ALKENE ISOMER) B r B a s e H C H H H H 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!

  6. MECHANISM (LEAST SUBSTITUTED ALKENE ISOMER) B r B a s e H C H H H H 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.

  7. EXPERIMENTAL PROCEDURE(SYNTHESIS)

  8. EXPERIMENTAL PROCEDURE(PURIFICATION)

  9. EXPERIMENTAL PROCEDURE(IR Analysis)

  10. EXPERIMENTAL PROCEDURE(IR Analysis)

  11. 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!

  12. WASTE MANAGEMENT • Cautiouslyadd 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)”.

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