Experiment 12:

BASE PROMOTED ELIMINATION OF HBR

FROM AN ALKYL HALIDE

+ +BaseROH

Br

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.

CHEMICAL EQUATION

+ +BaseROH

2-Bromoheptane

MF: C7H15Br

MW: 179.1

bp: 180 oC

d: 1.14 g/mL

MF: C7H14

MW: 98.2

bp: 94 oC

d: 0.697 g/mL

MF: C7H14

MW: 98.2

bp: 98-99 oC

d: 0.708 g/mL

MF: C7H14

MW: 98.2

bp: 98 oC

d: 0.701 g/mL

Br

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.

MECHANISM

C

Br

+OH

+

a

b

+

a a b

ROHH

HH HH

a a bb

b

2. …C=C bond forms…

1. Base oxygen attacks and removes a proton

from carbon adjacent to bromine…

3. …bromine is eliminated from the opposite side of the molecule.

ALL STEPS OCCUR IN A CONCERTED FASHION!

MECHANISM (MOST SUBSTITUTED ALKENE ISOMER)

C

Br

H H

H

HH

Base

+ HBase Br+

• 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)

C

Br

H H

H

HH

Base

+ HBase Br+

• 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)

water out

water in

heating mantle

iron ring

to voltage regulator

• 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)

Heating Mantle

to voltage regulator

water out

water in

iron ring

• 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

StandardRt

SMALL BASE(methanol/sodium

methoxide)

LARGE BASE(t-butanol/potassium t-

butoxide)

Sample Rt

Area Percen

t

Adjusted Area Percent

Sample Rt

Area Percen

t

Adjusted Area

Percent

methanolt-butanol ---

1-heptenetrans-2-heptene

cis-2-heptene

EXPERIMENTAL PROCEDURE

(IR Analysis)

Br

IR spectra on page 103 of lab manual!

Table 12.2

FunctionalGroup

BaseValues(cm-1)

2-bromohept

ane

1-heptene

cis-2-heptene

trans-2-heptene

Frequency (cm-1)

Frequency

(cm-1)

Frequency

(cm-1)

Frequency (cm-1)

C-Br stretch500-700

sp3 CH stretch2850-

3000

sp2 CH stretch3000-

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-heptenetrans-2-heptene1-heptenemethanolt-butanolsodium methoxide XXX XXXpotassium t-butoxide XXX XXX

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!