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Experiment

SynthesisofIsopentylAcetate

8

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Objectives

 ToprepareisopentylacetatefromisopentylalcoholandaceticacidbytheFischer

esterificationreaction.

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Introduction

 Esters are derivatives of carboxylic acids in which the acyl carbon bears an alkoxy

substituent(‐OR)ratherthanthehydroxylsubstituent(‐OH)oftheacid.Simpleesterstendto

havepleasant,fruityodorsandarewidelyusedasflavorsandfragrances.Table1belowshows

theflavorsorfragrancesassociatedwithsomeesters.

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Ester Scent

Isopentylacetate Banana

Isobutylformate Raspberry

Isobutylpropionate Rum

n‐propylacetate Pear

Methylbutyrate Apple

Methylanthranilate Grape

Methylsalicylate Wintergreen

Ethylbutyrate Pineapple

Ethylphenylacetate Honey

Benzylacetate Peach

Benzylbutyrate Cherry

Octylacetate Orange

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Table1Estersandtheircorrespondingscents

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 The volatile compounds in natural fruits and flowers are usually complex mixtures of

compounds,whereestersfrequently predominate.Manyartificialflavoringscontainestersor

mixtures of esters. For example, the volatile oil of ripe pineapple contains several esters, as

showninTable2.

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Table2Compositionofthevolatileoilofripepineapple

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 Isopentyl acetate is known as banana oil because of its characteristic odor. This ester

has also been shown to be one of the active substances in the alarm pheromone of the

honeybee.Whenahoneybeeworkerstingsanintruder,an

alarmpheromoneissecretedalong

with the venom. The pheromone causes other bees to become aggressive and attack the

intruder.

 Esterification generally refers to the formation of esters from alcohol and carboxylic

acids,asshowninEquation1.





 The reaction proceeds by way of a nucleophilic substitution at the acyl carbon of the

carboxylicacid.Whencatalyzedbyastrongacid,usuallysulfuricacidorp‐toluenesulfonicacid,

thereactioniscalledtheFischeresterification.

ThereactionmechanismisshowninEquations2‐6.Equation2shows

theprotonationof

the acyl oxygen of the carboxylic acid. The protonation activates the acyl carbon toward

nucleophilicattack.Equation3showsthenucleophilicattackattheacylcarbonbytheoxygen

atomofthealcoholtoformatetrahedralintermediate.Equation4showsaprotontransferto

thehydroxyloxygenofthecarboxylgroup.Thisprotonationconvertsthehydroxylgroupinto

the good leaving group, water. Equation 5 shows the loss of water forming the protonated

carbox

lic aci

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ester.Finally,Equation6showstheprotontransfertoabase,resultingintheformationofthe

ester.

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 Eachstepofthereactionmechanismisreversibleand,therefore,thereactionreaches

equilibrium. Le Chatelier’s principle suggests that the amount of ester produced in an 

equilibriumreactionmightbeincreasedeitherbyusinganexcessofoneofthe reactantsorby

removingoneoftheproducts.Inpractice,anexcessofcarboxylicacidoralcohol,whicheveris

morereadily available,is added, and/or waterisremovedasthe  reaction proceeds.Awater‐

absorbingsubstancesuchasmolecularsievesmightbeincludedinthereactionmixture,orthe

watermightberemovedaspartofanazeotropewithbenzeneortoluene.

 ThemechanismsuggeststhatstericeffectsmightbeimportantinEquation3,thestep

involvingtheattackbythealcoholattheacylcarbonofthecarboxylicacid.Indeed,alkyl‐group

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branching at theα‐ orβ‐carbonofthe acidslows the rateof esterification. For example, the

relativeratesofesterificationwithmethanolfollowtheorder:



 Sterically hindered alcohols also react more slowly in the esterification reaction. The

relativeratesforesterificationofalcoholswithaceticacidfollowtheorder:



 Fischeresterificationisanexampleofanacyltransferreaction.Theacylgroupfromthe

acidistransferredtothealcohol.Acidchloridesandanhydridesalsoserveasacylatingagents.

Becauseacidchloridesandanhydridescontaingoodleavinggroups,thesecompoundsarevery

reactivetowardnucleophilicsubstitutionbyanalcohol,asshowninEquations7‐8.

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 The Fischer esterification is conducted at reflux. The purpose of reflux is to heat a

reaction mixture at its boiling temperature to form products, without losing any of the

compoundsinthereactionflask.

 Inpractice,acondenserissetverticallyintothetopofthereactionflask.Anycompound

thatvaporizeswillcondensewhenitentersthecoolenvironmentoftherefluxcondenserand

will then drain back into the reaction flask. Reflux apparatus using glassware for a semi‐

microscaletechniqueisusedintheprocedure.

 In this experiment, you will  prepare isopentyl acetate by reacting an excess of acetic

acidwithisopentylalcohol.Youwillusesulfuricacidtocatalyzethereaction.

 Afterthereactioniscomplete,youwill removetheexcessaceticacidandsulfuricacid

fromthereactionmixturebyextractionwithsodiumhydrogencarbonate.

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ExperimentalProcedure

1) Mix isopentyl alcohol (5.4 mL, via burette) and glacial acetic acid (8.5 mL, via graduated

cylinder)inaround‐bottomflask.Carefullyadd20dropsofconc.H

tothemixture.

2) Putinafewboilingchipsandassembletherefluxapparatus(Figure1).

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Figure1

Refluxapparatus

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3) Afterhavingyourset‐upcheckedbytheinstructor,heatthereactionmixtureunderreflux

conditionsfor1hour.

4) Cool the reaction mixture to room temperature. Transfer the mixture into a 125 mL

separatoryfunnelandaddcoldwater(25mL).Extractandseparatetheaqueouslayer.

5) Washtheorganiclayertwicewith5%NaHCO

(25mLeachtime).

6) WashtheorganiclayerwithsaturatedNaClsolution(25mL).

7) Dry the organic layer with anhydrous Na

. Decant the crude ester into a clean 25 mL

round‐bottomflask.Addaboilingchip.

8) Assemble a simple distillation apparatus. Distill the product and collect the 134‐143°C

fractionintoapre‐weighedtesttube.

9) Weightheproductandcalculatetheyield.Submittheproducttoyour

instructor.

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LaboratorySafetyPrecaution

1) Wearsafetygogglesandlabcoatatalltimeswhileworkinginthelaboratory.

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2) Isopentyl alcohol (3‐methyl‐1‐butanol) is irritating. Glacial acetic acid is corrosive.

Concentratedsulfuricacidiscorrosiveandoxidizing.Preventeye,skin,andclothingcontact.

Avoidinhalingvaporsandingestingthesecompounds.

3) NeutralizingacidswithsodiumhydrogencarbonategeneratesCO

gas.Venttheseparatory

funnelfrequentlytoavoidpressurebuild‐up.

4) Washyourhandsthoroughlywithsoapordetergentbeforeleavingthelaboratory.

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