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Experiment 21:
ESTERS: SYNTHESIS AND FRAGRANCE
+ R'OHRCOH
O
RCOR'
O
+ H2OH+
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Objectives:
To synthesize an ester from acetic acid with isoamyl alcohol under reflux.
To purify your product through acid-base extraction and simple distillation.
To identify and analyze the purity of your compound using GC analysis.
To identify your product by analyzing the NMR and IR spectra.
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Before coming to lab…
Review these techniques:
Reflux
Acid-Base Extraction
Simple distillation
Drying over MgSO4
GC Analysis
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FISCHER ESTERIFICATION Fischer esterification is an acid catalyzed
nucleophilic acyl substitution.
Net effect is the replacement of an –OH of a carboxylic acid with the –OR of an alcohol to produce an ester.
It is an equilibrium reaction with an unfavorable Keq, thus we can improve product yield in several ways: Use an excess of the alcohol reactant. Use an excess of the carboxylic acid reactant. Remove water as it forms.
R O H + H O C R'
O
H+R O C R'
O
+ H O H
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FISCHER ESTERIFICATION
• Carboxylic acids are not reactive enough to undergo nucleophilic addition directly.
• By using sulfuric acid as a catalyst, the carbonyl group oxygen is protonated to give the carboxylic acid a positive charge. This makes it more reactive.
+ H OSO3H
H3CC
OH
O
+
H3CC
OH
OH
H C
CH3
CH3
OH
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MECHANISM
+ H OSO3H
H3CC
OH
O
+
H3CC
OH
OH
H C
CH3
CH3
OH
CH3C O
OH
HO
C
H3C
CH3
H
H
CH3C O
O
O
C
H3C
CH3
H
H
H
H
CH3C O
O
C
H3C
CH3
H
H
O
H
H2O H2SO4
1. Protonation of the carbonyl oxygen activates the carboxylic acid…
2. …toward nucleophilic attack by the alcohol…
3. … yielding a tetrahedral intermediate.
4. Transfer of a proton from one oxygen to another yields a second tetrahedral intermediate… 5…and converts the OH group into a good leaving group.
6. Loss of a proton and expulsion of H2O regenerates the acid catalyst and gives the ester product.
1
2
3
45
6
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OVERVIEW
Synthesize product ester under reflux.
Neutralize acetic acid with 10% NaHCO3.
Extract in ether, then wash with 10% NaHCO3.
Dry organic layer.
Perform simple distillation to remove ether from product.
Obtain final product mass, calculate % yield, prepare GC sample.
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EXPERIMENTAL PROCEDURE(SYNTHESIS)
water out
water in
heating mantle
iron ring
to voltage regulator
• Combine acetic acid, isoamyl alcohol and sulfuric acid in a 25 mL round bottom flask.
• Clamp flask to ring stand and add 3 boiling chips.
• Place water cooled condenser on top of flask, with a CaSO4 drying tube in the top.
• Heat to reflux. Reflux 30 minutes.
• Cool to room temp.
• Add 10% NaHCO3 slowly to flask.
25mL
CaSO4 tube
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EXPERIMENTAL PROCEDURE(PURIFICATION)
• Transfer liquid to separatory funnel.
• Rinse reaction flask with ether and transfer to separatory funnel.
• Wash the organic layer with 10% NaHCO3.
• Transfer the organic layer to a clean flask.
• Dry over MgSO4.
• Transfer liquid to a preweighed 50 mL round bottom flask.
• Clamp flask to ring stand. 50mL
50mL
125mL
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EXPERIMENTAL PROCEDURE(PRODUCT ISOLATION)
Heating Mantle
to voltage regulator
water out
water in
iron ring
• Set up a simple distillation apparatus, using a 25 mL round bottom as the receiving flask.
•Collect all distillate that boils under 40oC.
• Record distillation range, Ti-Tf.
• Allow reaction flask to cool to room temperature.
• Reweigh 50 mL flask to determine final product mass (actual yield) and calculate % yield.
• Prepare GC sample.
25mL WASTE
SOLVENT
50mL PRODUCT!
!!
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Table 21.1
Theoretical yield (g)Calculated value based on limiting reagent!
Actual yield (g)(50mL RB flask + product) – (Empty 50mL RB flask)
% yieldACTUAL YIELD X 100THEORETICAL YIELD
Product Appearance
PHYSICAL STATE, COLOR
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Table 21.2
CompoundGC Retention Times
(min) Area Percent Adjusted Area PercentStandard
Sample
methanol SOLVENTisoamyl alcohol
REACTANT
isoamyl acetate
PRODUCT
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EXPERIMENTAL PROCEDURE
(IR ANALYSIS)
1234
5
CH3CHCH2CH2OH
CH3
1234
5
CH3CHCH2CH2OCCH3
CH3 O
6 7
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Table 21.3
FunctionalGroup
Base Values
Acetic acid Isoamylalcohol
Isoamylacetate
Frequency(cm-1)
Frequency (cm-1)
Frequency(cm-1)
Frequency(cm-1)
sp3 CHstretch 2850-3000
C-O stretch1000-1300
OH stretch2400-3600
C=O stretch 1640-1750
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EXPERIMENTAL PROCEDURE
(NMR ANALYSIS)
1234
5
CH3CHCH2CH2OH
CH3
1234
5
CH3CHCH2CH2OCCH3
CH3 O
6 7
H3CC
OH
O
1
2
3
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SAFETY CONCERNS
CAUTION: H2SO4
is a STRONG ACID!
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WASTE MANAGEMENT
• All aqueous washes from the extraction can be flushed down the drain with plenty of water!
• Ester product and distilled ether should be placed in the bottle labeled, “ORGANIC WASTE (Esters)”.
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CLEANING
Clean round bottom flasks and distillation glassware with wash acetone only!
Separatory funnel and all other glassware should be cleaned with soap, water, and a brush if necessary, followed by a wash acetone.
DO NOT return any glassware to lab drawer dirty or wet!
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LABORATORY NOTEBOOK(Pre-lab)
• OBJECTIVE (Must clearly state…)
• What compounds will be made and how• How the compounds will be purified• How the purity of the product will be determined
• CHEMICAL EQUATION • Include the chemical equation from the top of page 177.
• TABLE OF PHYSICAL DATA (Complete the following table using a site listed on WWW Links ONLY. Wikipedia is unacceptable!)
• REFERENCE TO PROCEDURE (Must include…)
• full title including edition and author names• page numbers where actual procedure can be found
Compound MW (g/mol)
bp(oC)
d (g/mL)
HAZARDS
Isoamyl alcoholIsoamyl acetate
Acetic acidSulfuric acidDiethyl ether
methanol
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LABORATORY NOTEBOOK(In-lab)
• DATA/CALCULATIONS • Initial volumes of isoamyl alcohol and acetic acid used• Distillation range • Weight of 50 mL round bottom flask • Weight of 50 mL round bottom flask + product • Final product weight• Physical state and color of product • GC vial slot # • Theoretical yield calculation (not just value!)• % yield calculation (not just value!)• Example of an adjusted area % calculation (not just value!)
• 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 any volumes or weights of chemicals used during the experiment.• Include any mistakes, accidents or observations if applicable.