BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE 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 study the effect of base size on product distribution using GC analysis. To identify characteristic absorptions in the IR spectra of reactants and products.

Dehydrohalogenation of 2-bromoheptane. CHEMICAL EQUATION Dehydrohalogenation of 2-bromoheptane.

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.

ALL STEPS OCCUR IN A CONCERTED FASHION! 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.

MECHANISM (MOST SUBSTITUTED ALKENE ISOMER) + 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!

MECHANISM (LEAST SUBSTITUTED ALKENE ISOMER) + 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.

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 powder funnel. 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.

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.

Table 12.1 Remember to get data from the opposite alcohol/base pair! Compound GC Retention Times (min) and Adjusted Area Percent Standard Rt SMALL BASE (methanol/sodium methoxide) LARGE BASE (t-butanol/potassium t-butoxide) Sample Area Percent Adjusted Area Percent Adjusted methanol t-butanol --- 1-heptene trans-2-heptene cis-2-heptene Remember to get data from the opposite alcohol/base pair!

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

Table 12.2 Functional Group Base Values (cm-1) 2- bromoheptane 1- heptene cis-2- trans-2- Frequency C-Br stretch 500-700 sp3 CH stretch 2850-3000 sp2 CH stretch 3000-3100 Alkene C=C stretch 1600-1680

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

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.

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.com ONLY. Wikipedia is unacceptable) REFERENCE TO PROCEDURE (Must include…) full title, including edition and authors page numbers where actual procedure can be found Compound MW (g/mol) bp (Co) d (g/mL) HAZARDS 2-bromoheptane 180 1.14 Flammable, irritant cis-2-heptene trans-2-heptene 1-heptene methanol t-butanol sodium methoxide XXX potassium t-butoxide

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.

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!