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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.
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.
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.
OVERVIEW
Heat alkyl bromide and assigned base in solvent under reflux to synthesize products.
Purify products by simple distillation to remove unreacted starting materials.
Prepare GC sample.
Analyze and determine product ratio using GC results.
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.
CaSO4 drying tube in
thermometer adapter
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.
Keck clips here!
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
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 a site on WWW Links 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.