Chemistryinyourcupboard:Dettol
Introduction
One hundred and fifty years ago, before the use of antiseptics, major surgery often
resultedindeathfrompost‐operativeinfections.Inthe1860sJosephListerdiscovered
phenolcouldkillgermsbutitalsocausessevereskinburns.Modernantisepticssuchas
Dettol®havebeendevelopedbasedonphenolbutwithmodificationstothechemistry
to reduce the adverse effects and produce a safer product. Many antiseptics are still
madebasedonphenolderivativesastheactiveingredient.
Linkstothecurriculum
This section contains information relevant to the following areas of your chemistry
curriculum:



Organicchemistry
Bonding
Acidsandbases
DETTOL
Antisepticsanddisinfectants
Beforethemid‐1800s,majorsurgerywasoftenadeathsentence.Amputationsof
damagedlimbswerecarriedoutasalastresortbutpatientsfrequentlydiedfrompost‐
operativeinfections.
Thischangedinthe1860swhenJosephListerdevelopedantisepticsurgeryusing
carbolicacidtosterilisewoundsandinstruments.Listerwasawareofthegermtheory
ofinfectionsdevelopedbyLouisPasteurandothers,andknewthatcarbolicacid(which
wenowcallphenol)wasabletokillgerms,Figure1.
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Figure1:Anoperationinthe1870s.Doctorsareusingacarbolicacidsprayasanantiseptic.
Noticethattheyappeartobewearingtheireverydayclothes!
(WellcomePhotoLibrary,WellcomeImages)
Itisreputedthatphenol’sgerm‐killingpowerfirstcametonoticeinabizarreway.
Sailorswhounderwentamputationsatseaappearedtohaveahighersurvivalratethan
patientsinhospital.Thisseemedtobeduetothepracticeatseaofdippingthestump
intomoltentartosealthewound.Tarcontains,amongotherthings,phenol,Figure2.
H
O
Figure2:Structureofphenol.
Nowadays,disinfectantsarejustasimportantforgerm‐killinginhospitals,homesand
elsewhere.Manyproductsarestillbasedonphenolderivativesastheactiveingredient.
ThissitelooksatDettol,madebyReckittBenckiser.
Structureactivityrelationships
Phenoliseffectiveatkillinggermsbutisotherwiseafarfromidealantisepticasit
causesnastyskinburns.Onetechniqueusedbypharmaceuticalchemistswhenfaced
withthissortofsituationistosynthesiseanumberofcompoundsrelatedtothe
substancethatisknowntobeeffective.Thisisinthehopethatoneormoreofthese
compoundswillbeasactive,orbetter,thantheoriginalbutwithfewersideeffects(such
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asskinburning).Evenbetteristhepossibilityofestablishingastructure‐activity
relationship.Thisisapatternwhichlinkssomestructuralfeatureofthemoleculewith
itspharmacologicalefficiency(iehoweffectiveitisasamedicine)inasystematicway.
Examplesofsucharelationshipcouldbethat


themoremethylgroupsattachedtoabenzenering,thebetterthegerm‐killing
power,or
themoreelectronegativeasubstituent,thelessharmfultotheskin.
Thisenablesthechemisttopredictwhichderivativesmightbemoreeffectiveand
thereforeguidethesynthesisofnewcompounds.Figure3showssomederivativesof
phenolalongwiththeirgerm‐killingpowerrelativetophenol.Allofthecompounds
(exceptphenolitself)aresubstitutedphenols.Thatisthenewgrouporgroupsattached
tothebenzeneringreplaceoneormoreofthehydrogens.
OH
Cl Cl
Cl
OH
OH
OH
OH
Cl
CH3
H3C
1
4
Cl
Cl
Cl
13
23
280
Increasing germ-killing power
Figure3:Therelativegerm‐killingpowerofsomesubstitutedphenols.
Question1
Whatsortofstructure‐activityrelationshipissuggestedbythefirstfour
compoundsinFigure3?Suggestwhatothercompoundsmightbesynthesisedto
checkthishypothesis.
Question2
ThefirstcompoundinFigure3isphenolandthelastiscalled4‐chloro‐3,5‐
dimethylphenol.Nametheotherthree.
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Question3
a)ThebenzeneringisoftendrawnasinFigure3.Thereisahydrogenatom(not
drawn)ateach‘corner’ofthehexagonunlessthereisanotheratomorgroup
attached.Thecirclerepresentsadelocalisedringofelectronsinaπorbitalabove
andbelowthering–thisiscalledanaromaticsystem.
Explainwhycompoundswithanaromaticringinvariablyformsubstituted
derivativesratherthanadditioncompounds.
b)Stateandexplainwhattypeofreagentsismostlikelytoattackaromaticrings.
Infactthestructure‐activityrelationshipsthathavebeenestablishedforderivativesof
phenolare:


the−OHgroupisrequiredforactivity;
activityincreaseswithahalogeninthe4‐position(ieoppositethe−OHgroupin
thering);
 activityincreaseswithalkylsubstituentsofincreasedchainlength;
 increasedsubstitutionmakesthecompoundlesswater‐soluble;and
 increasedsubstitutiondecreasestoxicitytohumanswhentakenbymouth.
Phenolisalreadyaneffectivegermicide,soagreaterkillingpowerisnotreallyneeded.
Whatthegreaterefficiencyof4‐chloro‐3,5‐dimethylphenolmeansisthatmuchsmaller
concentrationcanbeusedandthereforefewerside‐effectswillbeexpected.
Dettol
Theactivegerm‐killingingredientinDettolisinfact4‐chloro‐3,5‐dimethylphenol,also
knownbyitsnon‐systematicnamepara‐chloro‐meta‐xylenolorPCMX,Figure4.The
prefixparaisanon‐systematicwayofindicatingthe4‐positiononthebenzenering,ie
oppositethe−OHgroup.Metaindicatesthe3‐and5‐positions.
Figure4:Structureofpara‐chloro‐meta‐xylenolorPCMX.
AnumberofdisinfectantandantisepticproductsareavailableundertheDettolbrand
includingdisinfectantsforsurfaces,antisepticsforuseonthebody,wipesandsprays.
Herewewillconcentrateonthe'core'product‐theliquidantiseptic,Figure5.
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Figure5:Dettolliquidantiseptic,containschloroxylenol.
Biocides,disinfectantsandantiseptics
Allthetermsaboverefertogerm(iemicrobe)‐killing.Biocideisageneraltermwhich
coversactiveingredientsthatkillmicrobes.Disinfectantskillmicrobes(bacteria,fungi
andviruses)andareusedonsurfaces(suchasworktops,sinksetc)butnotlivingthings
(suchasskin)astheymayharmthem.Antisepticsalsokillmicroorganismsbutmaybe
usedonthebodysurface(skin)althoughnotinthebody(egbymouth).
HowdoesPCMXkillbacteria?
ThedetailedbiochemistryoftheactionofPCMXandotherphenol‐basedantibacterial
agentsisbeyondthescopeofthissite.However,verysimply,theyareunderstoodto
workbythe−OHgroupofthemoleculebindingtoproteinspresentonthecell
membraneofbacteria,disruptingthecellmembraneandallowingthecontentsofthe
celltoleakout.
ThisallowsmorePCMXtoenterthecell,bindingfurtherwithproteinsandenzymes,and
effectivelyshuttingdownthecell’sfunctions.AthighconcentrationsofPCMX,the
proteinsandnucleicacidsinthecellarecoagulatedandceasetofunction,leadingto
rapidcelldeath.
Theacidityofphenols(1of3)
Phenol,C6H5OH,isweaklyacidic‐itwasoncecalledcarbolicacid(nottobeconfused
withcarboxylicacids,whichcontainthegroup−COOH).Theacidichydrogen(theone
thatislostasH+whenphenolbehavesasanacid)isthehydrogenofthe−O‐Hgroup.
Thisisbecauseoxygenismuchmoreelectronegativethanhydrogen(3.5onthePauling
scalecomparedwith2.1)sotheO‐HbondispolarisedOδ−‐Hδ+,makingitrelativelyeasy
forthishydrogentobelostasaproton(H+ion),Figure6.
Thenegativeionformediscalledthephenoxideionandisoftenrepresentedintextas
PhO−,Ph,beingusedtorepresentabenzenering,whichissometimescalledthephenyl
group.Becauseitisionic,thephenoxideionismoresolubleinwaterthanphenolitself.
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H
O
O
_
+
H+
Figure6:Phenol,isalsoweaklybasic–theoxygenatomhastwolonepairs,oneofwhichcanaccept
aH+iontoformtheionPHOH2+.
ThepKavalueisameasureofthestrengthofanacid(howeasilyaH+ionislost).The
largerthepKavalue,theweakertheacid.



PCMXpKa=9.7;Ka=1.99x10−10moldm−3
phenolpKa=9.9;Ka=1.28x10−10moldm−3
ethanolpKa=15.9;Ka=7.9x10−15moldm−3
Theacidityofphenols(2of3)
Imagineaweakacid,HA,whichdissociates
HA(aq)⇌H+(aq)+A−(aq)
Theequilibriumconstantisgivenby:
Kc=
[H  (aq) ] eqm [A  (aq) ] eqm
[HA (aq) ] eqm
Foraweakacid,thisisusuallygiventhesymbolKaandcalledtheaciddissociation
constant.
Ka=
[H  (aq) ] eqm [A  (aq) ] eqm
[HA (aq) ] eqm
pKa=–log10Ka
(ThisisanalogouswithpH=−log10[H+])
ThelargerthevalueofKa,thestrongertheacid(thegreater[H+]itproduceson
dissociation).However,becauseofthe−signintheexpressionabove,thelargerthe
valueofpKa,theweakertheacid.
Question4
ThesolubilityofPCMXis330mgdm−3anditsKais1.99x10−10.CalculatethepHof
asaturatedsolutionofPCMX.
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Theacidityofphenols(3of3)
Whyisphenolsomuchmoreacidicthanethanol?Inotherwords,whyistheHoftheOH
groupofethanolsomuchlesslikelytobelostthanthatoftheOHgroupofphenol?
C2H5OH⇌C2H5O−+H+
TheanswerliesintherelativestabilitiesofthenegativeionsleftafteraH+ionhasbeen
lost.Intheethoxideion,thechargeremainslocalisedontheoxygenatom.Inthe
phenoxideion,PhO−,thenegativechargeisspreadoverthebenzeneringduetooverlap
ofap‐orbitalontheoxygenatomwiththedelocalisedπ‐systemofthebenzenering,
Figure7.
Figure7:Delocalisationinthephenoxideion.
Inthetopdiagram,theoxygenatomisshownhybridisedsp3(seenextpage)withapair
ofelectronsineachofthethreeblueorbitals.(Thefourthsp3‐orbitalisinvolvedin
formingaσ‐bondwiththecarbonatomontheringtowhichitisattached.)Inthemiddle
diagram,theoxygenatomisshownhybridisedsp2withthep‐orbitalverticalreadyto
overlapwiththearomaticsystem.Inthebottomdiagram,theoxygenatomishybridised
sp2,itsunhybridisedp‐orbitalhasoverlappedwiththe‘doughnut‐shaped’delocalised
orbitalofthebenzeneringandthenegativechargeisspreadoverthering.
Hybridisation
Hybridisationisaconceptusedtohelpexplaintheshapesofmolecules.Itisbasedon
theideathatanyatom,orbitalsofsimilarenergysuchas2sand2pcan‘mixtogether’to
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producethesamenumberofneworbitalsofadifferentshape.Thesehybridorbitalscan
thenformbondsbyoverlappingwithorbitalsonotheratoms.
Forexampleincarbon,oxygenandnitrogen,wheretheorbitalsinvolvedinbondingare
the2sand2p‐orbitals,theymaymixinthreeways.
1. s+3xp→2xsp+2xp
2. s+3xp→3xsp2+1xp
3. s+3xp→4xsp3
Thenotationsp,sp2andsp3givestheproportionofsandporbitalsinthehybrid.For
example,ansp3is¼sand¾p.Noticethattheoriginalfourorbitalsalwaysgiveriseto
fournewones.TheshapesareshowninFigure8.
Figure8:Formationofhybridorbitals‐thethreepossibilities.
Theanglesbetweenthehybridisedorbitalsareimportantastheyhelptoexplainthe
shapesofmolecules.Thesporbitalsareat180°tooneanother,thesp2at120°andthe
sp3pointtothecornersofatetrahedron(109.5°).
Hintsp2ispronounced‘esspeetwo’not‘esspeesquared’.
Makingtheactiveingredientsoluble(1of3)
PCMXis,asexpected,notverysolubleinwater;only330mgdm−3.
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Question5
WhywouldyouexpectPCMXtoberelativelyinsolubleinwater?
Thisisbecauseonlythe–O‐Hgroupcanformhydrogenbondswithwater.
Question6
Drawadiagramtoshowhowawatermoleculecanformahydrogenbondwitha
phenolmolecule
a)viaahydrogenatomonthewater.
b)viaanoxygenatomonthewater.
Becausetheyareacidic,phenolsandsubstitutedphenolsaremoresolubleinalkaline
solutions.However,stronglyalkalinesolutionsarecausticanddamagetheskinsoa
slightlyalkalinesolution,pH10,isused.Infact,PCMXshowsslightlylessactivityatpH
10thaninaneutralsolution.
Dettolactuallycontains4.8%w/vPCMX.
w/vstandsforweightperunitvolume,sothisis4.8gin1000cm3or48gdm−3
(48,000mgdm−3),nearly150timesasmuchasitssolubilityinpurewater.Howisthis
achieved?
PCMXissolubleinrelativelynon‐polarsolvents‐onesuchispineoilwhichisusedin
theformulationofDettol.Thisoilisderived,notsurprisingly,fromneedles,twigsand
conesofpinetrees.Pineoilisamixturecomposedlargelyofalpha‐terpineol,whichis
moderatelypolarbecauseofitssingle−OHgroup,Figure9.
Figure9:Structureofalpha‐terpineol.
Therestofthepineoilconsistslargelyofhydrocarbons.Itcontainsterpenes,agroupof
hydrocarbonswhichareessentiallypolymersofisoprene,Figure10.
Figure10:Structureofisoprene.
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Question8
 Whatisthemolecularformulaofisoprene?
 Whatistheempiricalformulaofisoprene?
Question9
Whatisthesystematicnameofisoprene?
Makingtheactiveingredientsoluble(2of3)
TheanswertoincreasingthesolubilityofPCMXistouseasoap.
Soapsanddetergentshave‘tadpole‐shaped’moleculesinthattheyhaveanon‐polar‘tail’
andapolarorionic‘head’.The‘tail’canformvanderWaalsbondswithnon‐polar
moleculeswhilstthe‘head’canformhydrogenbondswithwater.Thisisanexampleof
the‘likedissolveslike’rule.
ThesoapusedinDettolismadefromcastoroilwhichcontainsricinoleicacid
(systematicname12‐hydroxy‐(cis)‐9‐octadecenoicacid),Figure11.
OH
OH
O
Figure11:Structureofricinoleicacid.
Question10
Explainthesystematicname12‐hydroxy‐(cis)‐9‐octadecenoicacid.
Historically,thepositionsofsubstituentsarounddoublebondshavebeenindicatedby
thetermscis(onthesameside)andtrans(ontheoppositeside).
Analternativenotationmakesitpossibletodealwithmorecomplexcases.TheE‐Z
notationisbasedonatomicnumbers.Welookattheatomsattachedtoeachofthe
carbonatomsinthedoublebond.Whenthetwoatoms(ofeachpair)ofhigheratomic
numberareonthesamesideoftheC=C,theisomerisdescribedasZ,fromtheGerman
wordfortogether,zusammen.IfnotitisE,fromtheGermanwordforopposite,
entgegen.
So,ifthetwoatomswiththegreatestatomicnumberareonthesamesideofthedouble
bondasinricinoleicacid,theconfigurationisZ.
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Makingtheactiveingredientsoluble(3of3)
Thericinoleicacidisreactedwithsodiumhydroxidetoformtheionicsaltsodium
ricinoleate.Thissalthasanionicheadandalargelynon‐polartail–theclassicshapeofa
detergentorsurfactantmolecule,seeFigure12.
Non-polar 'tail'
Ionic 'head'
OH
O-
Na+
O
Figure12:Structureofsodiumricinoleate.
Inthebottle,DettolconsistsofanalmostclearliquidinwhichthePCMXisheldin
solutionbythesodiumricinoleate.
Ondilutioninwater,however,acloudyliquidforms.Thisconsistsofdropletsofpineoil
containingdissolvedPCMX.Thesearehelddispersedinwaterbyalayerofsoap
moleculesarrangedwiththeirtailsinthepineoilandtheirheadsinthewater,Figure
13.Thesedropletsarebigenoughtoscatterlight,hencethecloudinessofthe
suspensionwhichiscalledanemulsion.
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Figure13:PCMXinaqueoussolutionexistsinanumberofforms.
ThePCMXinthedropletsofpineoilisnotavailabletokillbacteria‐itisthefree
aqueousPCMXthatdoesthis.However,anequilibriumexistsbetweentheemulsified
PCMXinthedropletsandfreePCMXdissolvedinthewater.AsPCMXisusedupinkilling
bacteria,moreisreleasedfromthedropletstokeeptheaqueousPCMXconcentration
essentiallyconstant.SothedropletsactasareservoirofPCMX.
Question11
WhatchemicalprinciplepredictsthatastheaqueousPCMXisusedup,morewill
dissolvefromthedropletsandthemicellestomaintainitsconcentration?
ThereisafurtherreservoirofPCMXinso‐calledmicelles.Thesearegroupsofsoap
moleculesclusteredtogetherwiththeirnon‐polartailsentwined.Non‐polarPCMX
moleculescanexistinsidethesemicellesandbereleasedinasimilarwaytothoseinthe
droplets.Alsoinsolutionarefreesoapmoleculeswhichinfactaidthepenetrationof
PCMXintothebacterialcellwalls.Inaddition,pineoilitselfhasamildantibacterial
action.
Pineoilalsohasapleasantsmellofpineandovertheyears,thishasbecomeassociated
inthemindofthepublicwithantisepticanddisinfectantaction,somuchsothatmost
disinfectantproductsareformulatedwithapinesmell.
Althoughimprovementshavebeenmadeovertheyears,thebasicformulationofDettol
andsimilarproductshasremainedthesamesincethe1930s.
Otheringredientsandformulations
ThereareanumberofotheringredientsintheformulationofDettolincluding
propan‐2‐ol(isopropanol),Figure14.
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Figure14:Structureofpropan‐2‐ol(isopropanol).
ThishelpstomakethePCMXsolubleinwaterandhelpstostabilisethemicelles.
Caramelgivestheproductitsyellow‐browncolour.
AswellasDettolliquidantiseptic,Dettolbrandedproductsaresoldinanumberofother
formswithdifferentformulations.MostoftheDettolproductsarenotmedicinesand
thereforedonothaveregisteredactives,Figure15.
Figure15:DifferentformsandformulationsofproductssoldundertheDettolbrand.
Furtherinformation
DettolissoldintheUKbyReckittBenckiser(www.dettol.co.uk).Thereareother
brandedproductswhichworkinasimilarway.
Acknowledgements
TheRoyalSocietyofChemistrywishestothankReckittBenckiserforhelpinpreparing
thismaterial.
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TheRoyalSocietyofChemistrygratefullyacknowledgesthatthisprojectwasinitially
supportedbyReckittBenckiserin2007.ReckittBenckiserconductedafinalreviewin
2013sopleasenotethatcertaininformationmaybeoutofdate.
QUESTIONSANDANSWERS
Question1
Whatsortofstructure‐activityrelationshipissuggestedbythefirstfour
compoundsinFigure3?Suggestwhatothercompoundsmightbesynthesisedto
checkthishypothesis.
Itsuggeststhatthemorechlorinesubstituents,thegreaterthegerm‐killingactivity.
Morehighly‐substitutedderivativescouldbemade,suchas2,3,4,5‐tetrachlorophenol.
Question2
ThefirstcompoundinFigure3isphenolandthelastiscalled4‐chloro‐3,5‐
dimethylphenol.Nametheotherthree.
2‐chlorophenol;2,4‐dichlorophenol;2,4,6‐trichlorophenol.
Question3
a)ThebenzeneringisoftendrawnasinFigure3.Thereisahydrogenatom(not
drawn)ateach‘corner’ofthehexagonunlessthereisanotheratomorgroup
attached.Thecirclerepresentsadelocalisedringofelectronsinaπorbitalabove
andbelowthering–thisiscalledanaromaticsystem.
Explainwhycompoundswithanaromaticringinvariablyformsubstituted
derivativesratherthanadditioncompounds.
Aromaticcompoundaremorestablethanexpectedduetothedelocalisedringofsix
electrons.Formingadditionproductswouldrequireoneormoreoftheseelectronstobe
usedinformingbond(s)withaddedgroup(s)andthereforedestroythearomatic
stability.
b)Stateandexplainwhattypeofreagentsismostlikelytoattackaromaticrings.
Electrophiles.Theseattackthehighelectron‐densityofthedelocalisedsystemof
electrons.
Question4
ThesolubilityofPCMXis330mgdm−‐3anditsKais1.99x10−10.CalculatethepH
ofasaturatedsolutionofPCMX.
Ka=
[H  (aq) ] eqm [A  (aq) ] eqm
[HA (aq) ] eqm
330mg=0.33g
0.33/156.6mol=2.11x10−3molin1dm3
ietheinitialconcentrationofPCMX,[PCMX(aq)]=2.11x10−3moldm−3.
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PCMX→H++PCMX−
1.99x10−10=
[H  (aq) ]eqm [PCMX  (aq) ]eqm
[PCMX (aq) ]eqm
SincesomePCMXdissociates,[PCMX(aq)]eqm=[PCMX(aq)]–[H+(aq)]eqm
So1.99x10−10=
[H  (aq) ] eqm [PCMX  (aq) ] eqm
[PCMX(aq) ] eqm [H  (aq) ] eqm
Since[H+(aq)]eqm=[PCMX‐(aq)]eqmand[H+(aq)]eqmissmall
1.99x10−10=
[H(aq) ]2eqm
[PCMX(aq) ]eqm
1.99x10−10=
[H (aq) ]2 eqm
2.11 x 10-3
[H+(aq)]2eqm=4.2x10−13
[H+(aq)]eqm=6.48x10−7moldm−3
pH=−log10[H+]=6.19
Question5
WhywouldyouexpectPCMXtoberelativelyinsolubleinwater?
Theonlypolarpartofthemoleculewhichcanhydrogenbondtowatermoleculesisthe
OHgroup,therestisnon‐polarandunabletointeractstronglywithpolarwater
molecules.
Question6
Drawadiagramtoshowhowawatermoleculecanformahydrogenbondwitha
phenolmolecule
 viaahydrogenatomonthewater

viaanoxygenatomonthewater
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Question7
Writeoutthestructuralformulaofisopreneusingadisplayedformula
H
H
C
H
H
C
C
H
C
C
H
H
H
Question8
 Whatisthemolecularformulaofisoprene?
 Whatistheempiricalformulaofisoprene?
 C5H8
 C5H8
Question9
Whatisthesystematicnameofisoprene?
2‐methylbuta‐1,3‐diene
Question10
Explainthesystematicname12‐hydroxy‐(cis)‐9‐octadecenoicacid.
 octadecindicatesthatthereare18carbonatoms
 oicacidindicatesacarboxylicacid(COOH)functionalgroup
 eneindicatesanalkene,acarbon‐carbondoublebond(C=C)
 the9indicatesthepositionoftheC=Cbetweencarbons9and10(countingfrom
theCOOHendofthemolecule)
 hydroxyindicatesanalcohol(OH)functionalgroup
 the12indicatesthattheOHgroupisoncarbon12(countingfromtheCOOHend
ofthemolecule)
 cisindicatesthatbothchainsattachedtotheC=Careonthesameside
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Question11
WhatchemicalprinciplepredictsthatastheaqueousPCMXisusedup,morewill
dissolvefromthedropletsandthemicellestomaintainitsconcentration?
LeChatelier’sprinciple.
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