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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 study the effect of base size on product distribution using GC analysis. • 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 1. Base oxygen attacks and removes a proton from carbon adjacent to bromine… ALL STEPS OCCUR IN A CONCERTED FASHION! 2. …C=C bond forms… 3. …bromine is eliminated from the opposite side of the molecule.

  6. 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!

  7. 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.

  8. EXPERIMENTAL PROCEDURE(SYNTHESIS) • Add 2-bromoheptane and alcohol solvent to 25 mL flask w/3 boiling chips. • Clamp flask to ring stand. • Add solid base to flask using powderfunnel. • Place the condenser on the flask. • Using a thermometer adapter, place a CaSO4 drying tube in the top of the condenser. • Begin water flow, apply heat. • Reflux the solution for 60 minutes. • Cool flask using a beaker of tap water.

  9. EXPERIMENTAL PROCEDURE(PURIFICATION) • Once cooled, arrange a simple distillation apparatus. • Remember to clamp both flasks to ring stand! • Apply water flow and heat. • When the first drop reaches the receiving flask, mark the temperature (Ti). • Collect ~5 mL in the receiving flask. • Read the temperature again (Tf). Remove the heat. • Prepare a GC sample.

  10. Table 12.1 • Remember to get data from the opposite alcohol/base pair!

  11. EXPERIMENTAL PROCEDURE(IR Analysis) IR spectra on page 103 of lab manual!

  12. Table 12.2

  13. 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!

  14. WASTE MANAGEMENT • After adding water to the reaction flask, pour this and product solution into container labeled “LIQUID ORGANIC WASTE”.

  15. CLEANING… • Clean round bottom flasks with soap/water/wash acetone. • Clean all remaining reflux/distillation glassware with wash acetone ONLY while in your lab hood! DO NOT REMOVE THIS GLASSWARE FROM YOUR HOOD! • Funnel and beaker can be cleaned with soap/water/wash acetone.

  16. LABORATORY NOTEBOOK(Pre-lab) • OBJECTIVE (Must clearly state…) • What compounds you will make and how • How you will purify the compound • How you will determine the purity of your compound • CHEMICAL EQUATION • Include the general chemical equation from page 99. • TABLE OF PHYSICAL DATA(Complete the following table using MSDS sheets from http://hazard.com/msds/ or www.chemexper.comONLY. Wikipedia is unacceptable) • REFERENCE TO PROCEDURE (Must include…) • full title, including edition and authors • page numbers where actual procedure can be found

  17. LABORATORY NOTEBOOK(In-lab) • DATA/CALCULATIONS • Initial weight of 2-bromoheptane used • Initial weight and identity of base used • Theoretical yield calculation (not just the value!) • Physical state and color of product • GC vial slot # • GC sample solvent identity • Give an example of an adjusted area % calculation • EXPERIMENTAL PROCEDURE • In paragraph form, briefly describe the procedure that you actually followed during the lab. • Paragraph must be written in PAST TENSE, PASSIVE VOICE. • Include ACTUAL volumes or weights of chemicals used during the experiment. • Include any mistakes, accidents, or observations if necessary.

  18. For Next Lab… • Lab Quiz 3 will be given at the end of the lab period next lab! • We will perform Experiment 8 at the beginning of lab next week. • The pre-lab notebook entry for Experiment 8 will be due at the beginning of lab!

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