chemistry in your cupboard: dettol

AlwaysreadthelabelDettolisaregisteredtrademarkoftheReckittBenckisergroupofcompanies.LastreviewedbyReckittBenckiserin2013.

Chemistryinyourcupboard:Dettol

Introduction

One hundred and fifty years ago, before the use of antiseptics, major surgery oftenresultedindeathfrompost‐operativeinfections.Inthe1860sJosephListerdiscoveredphenolcouldkillgermsbutitalsocausessevereskinburns.ModernantisepticssuchasDettol®havebeendevelopedbasedonphenolbutwithmodificationstothechemistryto reduce the adverse effects and produce a safer product. Many antiseptics are stillmadebasedonphenolderivativesastheactiveingredient.

Linkstothecurriculum

This section contains information relevant to the following areas of your chemistrycurriculum:

Organicchemistry Bonding Acidsandbases

DETTOL

AntisepticsanddisinfectantsBeforethemid‐1800s,majorsurgerywasoftenadeathsentence.Amputationsofdamagedlimbswerecarriedoutasalastresortbutpatientsfrequentlydiedfrompost‐operativeinfections.

Thischangedinthe1860swhenJosephListerdevelopedantisepticsurgeryusingcarbolicacidtosterilisewoundsandinstruments.ListerwasawareofthegermtheoryofinfectionsdevelopedbyLouisPasteurandothers,andknewthatcarbolicacid(whichwenowcallphenol)wasabletokillgerms,Figure1.

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Figure1:Anoperationinthe1870s.Doctorsareusingacarbolicacidsprayasanantiseptic.Noticethattheyappeartobewearingtheireverydayclothes!

(WellcomePhotoLibrary,WellcomeImages)

Itisreputedthatphenol’sgerm‐killingpowerfirstcametonoticeinabizarreway.Sailorswhounderwentamputationsatseaappearedtohaveahighersurvivalratethanpatientsinhospital.Thisseemedtobeduetothepracticeatseaofdippingthestumpintomoltentartosealthewound.Tarcontains,amongotherthings,phenol,Figure2.

O

H

Figure2:Structureofphenol.

Nowadays,disinfectantsarejustasimportantforgerm‐killinginhospitals,homesandelsewhere.Manyproductsarestillbasedonphenolderivativesastheactiveingredient.ThissitelooksatDettol,madebyReckittBenckiser.

StructureactivityrelationshipsPhenoliseffectiveatkillinggermsbutisotherwiseafarfromidealantisepticasitcausesnastyskinburns.Onetechniqueusedbypharmaceuticalchemistswhenfacedwiththissortofsituationistosynthesiseanumberofcompoundsrelatedtothesubstancethatisknowntobeeffective.Thisisinthehopethatoneormoreofthesecompoundswillbeasactive,orbetter,thantheoriginalbutwithfewersideeffects(such

AlwaysreadthelabelDettolisaregisteredtrademarkoftheReckittBenckisergroupofcompanies.LastreviewedbyReckittBenckiserin2013.

asskinburning).Evenbetteristhepossibilityofestablishingastructure‐activityrelationship.Thisisapatternwhichlinkssomestructuralfeatureofthemoleculewithitspharmacologicalefficiency(iehoweffectiveitisasamedicine)inasystematicway.Examplesofsucharelationshipcouldbethat

themoremethylgroupsattachedtoabenzenering,thebetterthegerm‐killingpower,or

themoreelectronegativeasubstituent,thelessharmfultotheskin.

Thisenablesthechemisttopredictwhichderivativesmightbemoreeffectiveandthereforeguidethesynthesisofnewcompounds.Figure3showssomederivativesofphenolalongwiththeirgerm‐killingpowerrelativetophenol.Allofthecompounds(exceptphenolitself)aresubstitutedphenols.Thatisthenewgrouporgroupsattachedtothebenzeneringreplaceoneormoreofthehydrogens.

OH OH

Cl

OH

Cl

Cl

OH

Cl

Cl

Cl

OH

Cl

CH3H3C

Increasing germ-killing power

1 4 13 23 280

Figure3:Therelativegerm‐killingpowerofsomesubstitutedphenols.

Question1Whatsortofstructure‐activityrelationshipissuggestedbythefirstfourcompoundsinFigure3?Suggestwhatothercompoundsmightbesynthesisedtocheckthishypothesis.Question2ThefirstcompoundinFigure3isphenolandthelastiscalled4‐chloro‐3,5‐dimethylphenol.Nametheotherthree.

AlwaysreadthelabelDettolisaregisteredtrademarkoftheReckittBenckisergroupofcompanies.LastreviewedbyReckittBenckiserin2013.

Question3a)ThebenzeneringisoftendrawnasinFigure3.Thereisahydrogenatom(notdrawn)ateach‘corner’ofthehexagonunlessthereisanotheratomorgroupattached.Thecirclerepresentsadelocalisedringofelectronsinaπorbitalaboveandbelowthering–thisiscalledanaromaticsystem.Explainwhycompoundswithanaromaticringinvariablyformsubstitutedderivativesratherthanadditioncompounds.b)Stateandexplainwhattypeofreagentsismostlikelytoattackaromaticrings.Infactthestructure‐activityrelationshipsthathavebeenestablishedforderivativesofphenolare:

the−OHgroupisrequiredforactivity; activityincreaseswithahalogeninthe4‐position(ieoppositethe−OHgroupin

thering); activityincreaseswithalkylsubstituentsofincreasedchainlength; increasedsubstitutionmakesthecompoundlesswater‐soluble;and increasedsubstitutiondecreasestoxicitytohumanswhentakenbymouth.

Phenolisalreadyaneffectivegermicide,soagreaterkillingpowerisnotreallyneeded.Whatthegreaterefficiencyof4‐chloro‐3,5‐dimethylphenolmeansisthatmuchsmallerconcentrationcanbeusedandthereforefewerside‐effectswillbeexpected.

DettolTheactivegerm‐killingingredientinDettolisinfact4‐chloro‐3,5‐dimethylphenol,alsoknownbyitsnon‐systematicnamepara‐chloro‐meta‐xylenolorPCMX,Figure4.Theprefixparaisanon‐systematicwayofindicatingthe4‐positiononthebenzenering,ieoppositethe−OHgroup.Metaindicatesthe3‐and5‐positions.

Figure4:Structureofpara‐chloro‐meta‐xylenolorPCMX.

AnumberofdisinfectantandantisepticproductsareavailableundertheDettolbrandincludingdisinfectantsforsurfaces,antisepticsforuseonthebody,wipesandsprays.Herewewillconcentrateonthe'core'product‐theliquidantiseptic,Figure5.

AlwaysreadthelabelDettolisaregisteredtrademarkoftheReckittBenckisergroupofcompanies.LastreviewedbyReckittBenckiserin2013.

Figure5:Dettolliquidantiseptic,containschloroxylenol.

Biocides,disinfectantsandantisepticsAllthetermsaboverefertogerm(iemicrobe)‐killing.Biocideisageneraltermwhichcoversactiveingredientsthatkillmicrobes.Disinfectantskillmicrobes(bacteria,fungiandviruses)andareusedonsurfaces(suchasworktops,sinksetc)butnotlivingthings(suchasskin)astheymayharmthem.Antisepticsalsokillmicroorganismsbutmaybeusedonthebodysurface(skin)althoughnotinthebody(egbymouth).

HowdoesPCMXkillbacteria?ThedetailedbiochemistryoftheactionofPCMXandotherphenol‐basedantibacterialagentsisbeyondthescopeofthissite.However,verysimply,theyareunderstoodtoworkbythe−OHgroupofthemoleculebindingtoproteinspresentonthecellmembraneofbacteria,disruptingthecellmembraneandallowingthecontentsofthecelltoleakout.

ThisallowsmorePCMXtoenterthecell,bindingfurtherwithproteinsandenzymes,andeffectivelyshuttingdownthecell’sfunctions.AthighconcentrationsofPCMX,theproteinsandnucleicacidsinthecellarecoagulatedandceasetofunction,leadingtorapidcelldeath.

Theacidityofphenols(1of3)Phenol,C6H5OH,isweaklyacidic‐itwasoncecalledcarbolicacid(nottobeconfusedwithcarboxylicacids,whichcontainthegroup−COOH).Theacidichydrogen(theonethatislostasH+whenphenolbehavesasanacid)isthehydrogenofthe−O‐Hgroup.Thisisbecauseoxygenismuchmoreelectronegativethanhydrogen(3.5onthePaulingscalecomparedwith2.1)sotheO‐HbondispolarisedOδ−‐Hδ+,makingitrelativelyeasyforthishydrogentobelostasaproton(H+ion),Figure6.

ThenegativeionformediscalledthephenoxideionandisoftenrepresentedintextasPhO−,Ph,beingusedtorepresentabenzenering,whichissometimescalledthephenylgroup.Becauseitisionic,thephenoxideionismoresolubleinwaterthanphenolitself.

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O

H

O_

+ H+

Figure6:Phenol,isalsoweaklybasic–theoxygenatomhastwolonepairs,oneofwhichcanacceptaH+iontoformtheionPHOH2+.

ThepKavalueisameasureofthestrengthofanacid(howeasilyaH+ionislost).ThelargerthepKavalue,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=eqm(aq)

eqm(aq)eqm(aq)

][HA

][A][H

Foraweakacid,thisisusuallygiventhesymbolKaandcalledtheaciddissociationconstant.

Ka=eqm(aq)

eqm(aq)eqm(aq)

][HA

][A][H

pKa=–log10Ka

(ThisisanalogouswithpH=−log10[H+])

ThelargerthevalueofKa,thestrongertheacid(thegreater[H+]itproducesondissociation).However,becauseofthe−signintheexpressionabove,thelargerthevalueofpKa,theweakertheacid.

Question4ThesolubilityofPCMXis330mgdm−3anditsKais1.99x10−10.CalculatethepHofasaturatedsolutionofPCMX.

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Theacidityofphenols(3of3)Whyisphenolsomuchmoreacidicthanethanol?Inotherwords,whyistheHoftheOHgroupofethanolsomuchlesslikelytobelostthanthatoftheOHgroupofphenol?

C2H5OH⇌C2H5O−+H+

TheanswerliesintherelativestabilitiesofthenegativeionsleftafteraH+ionhasbeenlost.Intheethoxideion,thechargeremainslocalisedontheoxygenatom.Inthephenoxideion,PhO−,thenegativechargeisspreadoverthebenzeneringduetooverlapofap‐orbitalontheoxygenatomwiththedelocalisedπ‐systemofthebenzenering,Figure7.

Figure7:Delocalisationinthephenoxideion.

Inthetopdiagram,theoxygenatomisshownhybridisedsp3(seenextpage)withapairofelectronsineachofthethreeblueorbitals.(Thefourthsp3‐orbitalisinvolvedinformingaσ‐bondwiththecarbonatomontheringtowhichitisattached.)Inthemiddlediagram,theoxygenatomisshownhybridisedsp2withthep‐orbitalverticalreadytooverlapwiththearomaticsystem.Inthebottomdiagram,theoxygenatomishybridisedsp2,itsunhybridisedp‐orbitalhasoverlappedwiththe‘doughnut‐shaped’delocalisedorbitalofthebenzeneringandthenegativechargeisspreadoverthering.

HybridisationHybridisationisaconceptusedtohelpexplaintheshapesofmolecules.Itisbasedontheideathatanyatom,orbitalsofsimilarenergysuchas2sand2pcan‘mixtogether’to

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producethesamenumberofneworbitalsofadifferentshape.Thesehybridorbitalscanthenformbondsbyoverlappingwithorbitalsonotheratoms.

Forexampleincarbon,oxygenandnitrogen,wheretheorbitalsinvolvedinbondingarethe2sand2p‐orbitals,theymaymixinthreeways.

1. s+3xp→2xsp+2xp2. s+3xp→3xsp2+1xp3. s+3xp→4xsp3

Thenotationsp,sp2andsp3givestheproportionofsandporbitalsinthehybrid.Forexample,ansp3is¼sand¾p.Noticethattheoriginalfourorbitalsalwaysgiverisetofournewones.TheshapesareshowninFigure8.

Figure8:Formationofhybridorbitals‐thethreepossibilities.

Theanglesbetweenthehybridisedorbitalsareimportantastheyhelptoexplaintheshapesofmolecules.Thesporbitalsareat180°tooneanother,thesp2at120°andthesp3pointtothecornersofatetrahedron(109.5°).

Hintsp2ispronounced‘esspeetwo’not‘esspeesquared’.

Makingtheactiveingredientsoluble(1of3)PCMXis,asexpected,notverysolubleinwater;only330mgdm−3.

AlwaysreadthelabelDettolisaregisteredtrademarkoftheReckittBenckisergroupofcompanies.LastreviewedbyReckittBenckiserin2013.

Question5WhywouldyouexpectPCMXtoberelativelyinsolubleinwater?Thisisbecauseonlythe–O‐Hgroupcanformhydrogenbondswithwater.

Question6Drawadiagramtoshowhowawatermoleculecanformahydrogenbondwithaphenolmoleculea)viaahydrogenatomonthewater.b)viaanoxygenatomonthewater.

Becausetheyareacidic,phenolsandsubstitutedphenolsaremoresolubleinalkalinesolutions.However,stronglyalkalinesolutionsarecausticanddamagetheskinsoaslightlyalkalinesolution,pH10,isused.Infact,PCMXshowsslightlylessactivityatpH10thaninaneutralsolution.

Dettolactuallycontains4.8%w/vPCMX.

w/vstandsforweightperunitvolume,sothisis4.8gin1000cm3or48gdm−3(48,000mgdm−3),nearly150timesasmuchasitssolubilityinpurewater.Howisthisachieved?

PCMXissolubleinrelativelynon‐polarsolvents‐onesuchispineoilwhichisusedintheformulationofDettol.Thisoilisderived,notsurprisingly,fromneedles,twigsandconesofpinetrees.Pineoilisamixturecomposedlargelyofalpha‐terpineol,whichismoderatelypolarbecauseofitssingle−OHgroup,Figure9.

Figure9:Structureofalpha‐terpineol.

Therestofthepineoilconsistslargelyofhydrocarbons.Itcontainsterpenes,agroupofhydrocarbonswhichareessentiallypolymersofisoprene,Figure10.

Figure10:Structureofisoprene.

AlwaysreadthelabelDettolisaregisteredtrademarkoftheReckittBenckisergroupofcompanies.LastreviewedbyReckittBenckiserin2013.

Question8 Whatisthemolecularformulaofisoprene? Whatistheempiricalformulaofisoprene?

Question9Whatisthesystematicnameofisoprene?

Makingtheactiveingredientsoluble(2of3)TheanswertoincreasingthesolubilityofPCMXistouseasoap.

Soapsanddetergentshave‘tadpole‐shaped’moleculesinthattheyhaveanon‐polar‘tail’andapolarorionic‘head’.The‘tail’canformvanderWaalsbondswithnon‐polarmoleculeswhilstthe‘head’canformhydrogenbondswithwater.Thisisanexampleofthe‘likedissolveslike’rule.

ThesoapusedinDettolismadefromcastoroilwhichcontainsricinoleicacid(systematicname12‐hydroxy‐(cis)‐9‐octadecenoicacid),Figure11.

OH

O

OH

Figure11:Structureofricinoleicacid.

Question10Explainthesystematicname12‐hydroxy‐(cis)‐9‐octadecenoicacid.Historically,thepositionsofsubstituentsarounddoublebondshavebeenindicatedbythetermscis(onthesameside)andtrans(ontheoppositeside).

Analternativenotationmakesitpossibletodealwithmorecomplexcases.TheE‐Znotationisbasedonatomicnumbers.Welookattheatomsattachedtoeachofthecarbonatomsinthedoublebond.Whenthetwoatoms(ofeachpair)ofhigheratomicnumberareonthesamesideoftheC=C,theisomerisdescribedasZ,fromtheGermanwordfortogether,zusammen.IfnotitisE,fromtheGermanwordforopposite,entgegen.

So,ifthetwoatomswiththegreatestatomicnumberareonthesamesideofthedoublebondasinricinoleicacid,theconfigurationisZ.

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Makingtheactiveingredientsoluble(3of3)Thericinoleicacidisreactedwithsodiumhydroxidetoformtheionicsaltsodiumricinoleate.Thissalthasanionicheadandalargelynon‐polartail–theclassicshapeofadetergentorsurfactantmolecule,seeFigure12.

OH

O

O- Na+

Ionic 'head'

Non-polar 'tail'

Figure12:Structureofsodiumricinoleate.

Inthebottle,DettolconsistsofanalmostclearliquidinwhichthePCMXisheldinsolutionbythesodiumricinoleate.

Ondilutioninwater,however,acloudyliquidforms.ThisconsistsofdropletsofpineoilcontainingdissolvedPCMX.Thesearehelddispersedinwaterbyalayerofsoapmoleculesarrangedwiththeirtailsinthepineoilandtheirheadsinthewater,Figure13.Thesedropletsarebigenoughtoscatterlight,hencethecloudinessofthesuspensionwhichiscalledanemulsion.

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Figure13:PCMXinaqueoussolutionexistsinanumberofforms.

ThePCMXinthedropletsofpineoilisnotavailabletokillbacteria‐itisthefreeaqueousPCMXthatdoesthis.However,anequilibriumexistsbetweentheemulsifiedPCMXinthedropletsandfreePCMXdissolvedinthewater.AsPCMXisusedupinkillingbacteria,moreisreleasedfromthedropletstokeeptheaqueousPCMXconcentrationessentiallyconstant.SothedropletsactasareservoirofPCMX.

Question11WhatchemicalprinciplepredictsthatastheaqueousPCMXisusedup,morewilldissolvefromthedropletsandthemicellestomaintainitsconcentration?ThereisafurtherreservoirofPCMXinso‐calledmicelles.Thesearegroupsofsoapmoleculesclusteredtogetherwiththeirnon‐polartailsentwined.Non‐polarPCMXmoleculescanexistinsidethesemicellesandbereleasedinasimilarwaytothoseinthedroplets.AlsoinsolutionarefreesoapmoleculeswhichinfactaidthepenetrationofPCMXintothebacterialcellwalls.Inaddition,pineoilitselfhasamildantibacterialaction.

Pineoilalsohasapleasantsmellofpineandovertheyears,thishasbecomeassociatedinthemindofthepublicwithantisepticanddisinfectantaction,somuchsothatmostdisinfectantproductsareformulatedwithapinesmell.

Althoughimprovementshavebeenmadeovertheyears,thebasicformulationofDettolandsimilarproductshasremainedthesamesincethe1930s.

OtheringredientsandformulationsThereareanumberofotheringredientsintheformulationofDettolincludingpropan‐2‐ol(isopropanol),Figure14.

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Figure14:Structureofpropan‐2‐ol(isopropanol).

ThishelpstomakethePCMXsolubleinwaterandhelpstostabilisethemicelles.Caramelgivestheproductitsyellow‐browncolour.

AswellasDettolliquidantiseptic,Dettolbrandedproductsaresoldinanumberofotherformswithdifferentformulations.MostoftheDettolproductsarenotmedicinesandthereforedonothaveregisteredactives,Figure15.

Figure15:DifferentformsandformulationsofproductssoldundertheDettolbrand.

FurtherinformationDettolissoldintheUKbyReckittBenckiser(www.dettol.co.uk).Thereareotherbrandedproductswhichworkinasimilarway.

AcknowledgementsTheRoyalSocietyofChemistrywishestothankReckittBenckiserforhelpinpreparingthismaterial.

AlwaysreadthelabelDettolisaregisteredtrademarkoftheReckittBenckisergroupofcompanies.LastreviewedbyReckittBenckiserin2013.

TheRoyalSocietyofChemistrygratefullyacknowledgesthatthisprojectwasinitiallysupportedbyReckittBenckiserin2007.ReckittBenckiserconductedafinalreviewin2013sopleasenotethatcertaininformationmaybeoutofdate.

QUESTIONSANDANSWERSQuestion1Whatsortofstructure‐activityrelationshipissuggestedbythefirstfourcompoundsinFigure3?Suggestwhatothercompoundsmightbesynthesisedtocheckthishypothesis.Itsuggeststhatthemorechlorinesubstituents,thegreaterthegerm‐killingactivity.Morehighly‐substitutedderivativescouldbemade,suchas2,3,4,5‐tetrachlorophenol.

Question2ThefirstcompoundinFigure3isphenolandthelastiscalled4‐chloro‐3,5‐dimethylphenol.Nametheotherthree.2‐chlorophenol;2,4‐dichlorophenol;2,4,6‐trichlorophenol.

Question3a)ThebenzeneringisoftendrawnasinFigure3.Thereisahydrogenatom(notdrawn)ateach‘corner’ofthehexagonunlessthereisanotheratomorgroupattached.Thecirclerepresentsadelocalisedringofelectronsinaπorbitalaboveandbelowthering–thisiscalledanaromaticsystem.Explainwhycompoundswithanaromaticringinvariablyformsubstitutedderivativesratherthanadditioncompounds.Aromaticcompoundaremorestablethanexpectedduetothedelocalisedringofsixelectrons.Formingadditionproductswouldrequireoneormoreoftheseelectronstobeusedinformingbond(s)withaddedgroup(s)andthereforedestroythearomaticstability.

b)Stateandexplainwhattypeofreagentsismostlikelytoattackaromaticrings.Electrophiles.Theseattackthehighelectron‐densityofthedelocalisedsystemofelectrons.

Question4ThesolubilityofPCMXis330mgdm−‐3anditsKais1.99x10−10.CalculatethepHofasaturatedsolutionofPCMX.

Ka=eqm(aq)

eqm(aq)eqm(aq)

][HA

][A][H

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=eqm(aq)

eqm(aq)eqm(aq)

][PCMX

][PCMX][H

SincesomePCMXdissociates,[PCMX(aq)]eqm=[PCMX(aq)]–[H+(aq)]eqm

So1.99x10−10=eqm(aq)eqm(aq)

eqm(aq)eqm(aq)

]H][PCMX

][PCMX][H

[

Since[H+(aq)]eqm=[PCMX‐(aq)]eqmand[H+(aq)]eqmissmall

1.99x10−10=eqm(aq)

eqm2

(aq)

][PCMX

][H

1.99x10−10= 3-

eqm2

(aq)

10 x 2.11

][H

[H+(aq)]2eqm=4.2x10−13

[H+(aq)]eqm=6.48x10−7moldm−3

pH=−log10[H+]=6.19

Question5WhywouldyouexpectPCMXtoberelativelyinsolubleinwater?TheonlypolarpartofthemoleculewhichcanhydrogenbondtowatermoleculesistheOHgroup,therestisnon‐polarandunabletointeractstronglywithpolarwatermolecules.

Question6Drawadiagramtoshowhowawatermoleculecanformahydrogenbondwithaphenolmolecule

viaahydrogenatomonthewater

viaanoxygenatomonthewater

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Question7Writeoutthestructuralformulaofisopreneusingadisplayedformula

C

C

C

C

H H

C

H

H

H

H

H

H

Question8

Whatisthemolecularformulaofisoprene? Whatistheempiricalformulaofisoprene? C5H8 C5H8

Question9Whatisthesystematicnameofisoprene?2‐methylbuta‐1,3‐diene

Question10Explainthesystematicname12‐hydroxy‐(cis)‐9‐octadecenoicacid.

octadecindicatesthatthereare18carbonatoms

oicacidindicatesacarboxylicacid(COOH)functionalgroup

eneindicatesanalkene,acarbon‐carbondoublebond(C=C)

the9indicatesthepositionoftheC=Cbetweencarbons9and10(countingfromtheCOOHendofthemolecule)

hydroxyindicatesanalcohol(OH)functionalgroup

the12indicatesthattheOHgroupisoncarbon12(countingfromtheCOOHendofthemolecule)

cisindicatesthatbothchainsattachedtotheC=Careonthesameside

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Question11WhatchemicalprinciplepredictsthatastheaqueousPCMXisusedup,morewilldissolvefromthedropletsandthemicellestomaintainitsconcentration?LeChatelier’sprinciple.