Reactions and Properties of Aromatic Compounds
Recognised ias ithe isimplest iorganic, iaromatic ihydrocarbon, iBenzene iis ia icolourless iliquid
i with ia imolecular imass iof i78 iand ia imolecular iformula iof iC6H6. iRepresented iby ia ihexagon
i consisting iof i3-double ibonds, iits istructure ihas ia isix-carbon iring. iThe icarbon iatoms iare
i represented iby ia icorner ithat iis ibonded ito iother iatoms.
i Figure i1 i– iIllustration iDepicting ithe iStructure iof iBenzene
All ithe iatoms iin ithe ibenzene istructure iare ihydrogens. iWithin iits istructure, ithe idouble
i bonds iare iseparated iby ia isingle ibond, ihence iits iconfiguration ito ihave iconjugated idouble
i bonds. iA icircle iis iused ias ian ialternative isymbol iinside ithe ihexagon ithat iis iused ito
i represent isix ipi ielectrons. i
Benzene iis icategorized ias ia ihydrocarbon ias ia iresult iof iits ichemical iformula. iComprised iof
i only ihydrogen iand icarbon iatoms. iBased ion iits istructure iand iformula, ibenzene iis
i identified ipure ian iaromatic ihydrocarbon. iIt iis isaid ito ibe ia icompound icomposing
i hydrogen i& icarbon ihaving ialternate idouble ibonds iin ithe iform iof ithe iring.
August iKekulé ivon iStradonitz, ia ipronounced iGerman iorganic ichemist iwas ithe ifirst ito
i propose ia ireasonable iconstruction ifor ibenzene. iAligned iin ia ihexagon, iKekulé iproposed
i single ibonds iand iinterchanging idouble ibonds ibetween ithe icarbon iatoms i(C-C---C=C).
i Conjoined ito ievery icarbon iatom iwas ia ihydrogen. iWhile iKekulé’s istructure iwas icorrect ito
i some idegree, ias iit iassisted iin idisclosing ithe ivarious iproperties iof ibenzene, iit iwas istill
i inaccurate. i
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i Figure i2 i– iIllustration iKekulé’s ithe iStructure iof iBenzene
In iKekulé’s istructure idisplayed iabove iis ian iinaccurate irepresentation iof ibenzene, ias iall iof
i the icarbon-carbon ibond ilengths iare isupposed ito ibe iequal.
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, Kekulé’s istructure iimplies ithat ithere ishould ibe ia icarbon-carbon idouble ibond iand ia
i carbon-carbon isingle ibond iin ithe istructure iof ibenzene, iwith ithe iC-C ibond ilength
a measuring iapproximately iat i147 ipm iand ithe iC=C ibond icoming iin iat iapproximately i135
i pm. iThis iestablishes ithat iKekulé iwas iincorrect ias ithe icarbon-carbon ibond ilength iin
i benzene iis iapproximately iat i140 ipm. iFurthermore, iall iof ithe icarbon-carbon ibonds ihave
i the isame ilength iand ithe ipi ielectron isystem iis icontinuous iand iuniformly idistributed. iThis
i is inot iillustrated iin iKekulé’s istructure. i iBenzene iis ia iperfectly iregular ihexagon.
i Figure i3 i– iAnalysis ion iStructure iif iBenzene
Kekulé iproceeded ito ireason ithat ithe iinterchanging ibonds icontinually ioscillated iso ithat
i benzene iwas icomprised iof itwo irings iwith ivarious ialternate icarbon-carbon ibonds, iwhich
i were ion iinverse isides iof iequilibrium. iA ichemist iby ithe iname iof iBoffins ifound ithat iinstead
i of itwo ioscillating istructures, ibenzene ihas itwo iunique iconstructions ihowever ithese iexist
i together ias ian iintermediate iwith ia ilower ienergy ithan ieither iof ithe itwo.
The irationale ifor ibenzene’s iabsence iof isymmetry iwas iadditionally iinvestigated iutilising
i liquid ibenzene iand iit iwas iinferred ithat ithe ioverlapping ivibrations iand iintermolecular
i forces ialtered ithe iequilibrium iof ibenzene’s itwo iresonance istructures, iobliterating ithe
i focal ipoint isymmetry.
It iis iimperative ito iscrutinize ithe isteadiness iof ibenzene iin iKekulé’s iconstruction ias iwell,
i since iwhen ibenzene ihydrogenation iis inoticed, ithe iactual ienthalpy ienergy isecreted iis
i less ithan ithe ioriginal iquantity. iThe ihypothetical ivalue ifor ithe ihydrogenation iof ibenzene,
i in iview iof iKekulé’s istructure, ican ibe icalculated iutilizing iHess' iLaw. i iRemember ithat ithe
i enthalpy iof ia ireaction iis iindependent iof ithe iroute ithat’s ifollowed, iprovided ithe iproducts
i and ireactants iare ithe isame.
Identified ichange ithat ioccurs iin ia isystem iwhen imatter iis itransformed iby ia igiven
i chemical ireaction, ithat ithe ienthalpy iof ia ireaction iin icyclohexa-1, i2, i3-triene i(Kekulé’s
i version iof ithe ibenzene ielectron iarrangement) i iwas isubtracted ifrom ithe ienergy ireleased
i when ibonds iwere ibroken ito igive ithe itheoretical ienthalpy iof ihydrogenation ifor ibenzene.
i 359.2KJ/mol iis icalculated ias ithe ienthalpy iof ihydrogenation. iUsing ithe imethod iof
i Calorimetry, ithe ihydrogenation ienthalpy iof ibenzene ito ibe i-208.5KJ/mol. iRecognised ias
i the idifference ibetween ithe iobserved iand ithe iexpected ienthalpy iof iformation, iResonance
i Energy ican ialso ibe iknown ias iDelocalisation iEnergy. iDue ito iits ithe ilarge iresonance
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Recognised ias ithe isimplest iorganic, iaromatic ihydrocarbon, iBenzene iis ia icolourless iliquid
i with ia imolecular imass iof i78 iand ia imolecular iformula iof iC6H6. iRepresented iby ia ihexagon
i consisting iof i3-double ibonds, iits istructure ihas ia isix-carbon iring. iThe icarbon iatoms iare
i represented iby ia icorner ithat iis ibonded ito iother iatoms.
i Figure i1 i– iIllustration iDepicting ithe iStructure iof iBenzene
All ithe iatoms iin ithe ibenzene istructure iare ihydrogens. iWithin iits istructure, ithe idouble
i bonds iare iseparated iby ia isingle ibond, ihence iits iconfiguration ito ihave iconjugated idouble
i bonds. iA icircle iis iused ias ian ialternative isymbol iinside ithe ihexagon ithat iis iused ito
i represent isix ipi ielectrons. i
Benzene iis icategorized ias ia ihydrocarbon ias ia iresult iof iits ichemical iformula. iComprised iof
i only ihydrogen iand icarbon iatoms. iBased ion iits istructure iand iformula, ibenzene iis
i identified ipure ian iaromatic ihydrocarbon. iIt iis isaid ito ibe ia icompound icomposing
i hydrogen i& icarbon ihaving ialternate idouble ibonds iin ithe iform iof ithe iring.
August iKekulé ivon iStradonitz, ia ipronounced iGerman iorganic ichemist iwas ithe ifirst ito
i propose ia ireasonable iconstruction ifor ibenzene. iAligned iin ia ihexagon, iKekulé iproposed
i single ibonds iand iinterchanging idouble ibonds ibetween ithe icarbon iatoms i(C-C---C=C).
i Conjoined ito ievery icarbon iatom iwas ia ihydrogen. iWhile iKekulé’s istructure iwas icorrect ito
i some idegree, ias iit iassisted iin idisclosing ithe ivarious iproperties iof ibenzene, iit iwas istill
i inaccurate. i
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
i Figure i2 i– iIllustration iKekulé’s ithe iStructure iof iBenzene
In iKekulé’s istructure idisplayed iabove iis ian iinaccurate irepresentation iof ibenzene, ias iall iof
i the icarbon-carbon ibond ilengths iare isupposed ito ibe iequal.
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, Kekulé’s istructure iimplies ithat ithere ishould ibe ia icarbon-carbon idouble ibond iand ia
i carbon-carbon isingle ibond iin ithe istructure iof ibenzene, iwith ithe iC-C ibond ilength
a measuring iapproximately iat i147 ipm iand ithe iC=C ibond icoming iin iat iapproximately i135
i pm. iThis iestablishes ithat iKekulé iwas iincorrect ias ithe icarbon-carbon ibond ilength iin
i benzene iis iapproximately iat i140 ipm. iFurthermore, iall iof ithe icarbon-carbon ibonds ihave
i the isame ilength iand ithe ipi ielectron isystem iis icontinuous iand iuniformly idistributed. iThis
i is inot iillustrated iin iKekulé’s istructure. i iBenzene iis ia iperfectly iregular ihexagon.
i Figure i3 i– iAnalysis ion iStructure iif iBenzene
Kekulé iproceeded ito ireason ithat ithe iinterchanging ibonds icontinually ioscillated iso ithat
i benzene iwas icomprised iof itwo irings iwith ivarious ialternate icarbon-carbon ibonds, iwhich
i were ion iinverse isides iof iequilibrium. iA ichemist iby ithe iname iof iBoffins ifound ithat iinstead
i of itwo ioscillating istructures, ibenzene ihas itwo iunique iconstructions ihowever ithese iexist
i together ias ian iintermediate iwith ia ilower ienergy ithan ieither iof ithe itwo.
The irationale ifor ibenzene’s iabsence iof isymmetry iwas iadditionally iinvestigated iutilising
i liquid ibenzene iand iit iwas iinferred ithat ithe ioverlapping ivibrations iand iintermolecular
i forces ialtered ithe iequilibrium iof ibenzene’s itwo iresonance istructures, iobliterating ithe
i focal ipoint isymmetry.
It iis iimperative ito iscrutinize ithe isteadiness iof ibenzene iin iKekulé’s iconstruction ias iwell,
i since iwhen ibenzene ihydrogenation iis inoticed, ithe iactual ienthalpy ienergy isecreted iis
i less ithan ithe ioriginal iquantity. iThe ihypothetical ivalue ifor ithe ihydrogenation iof ibenzene,
i in iview iof iKekulé’s istructure, ican ibe icalculated iutilizing iHess' iLaw. i iRemember ithat ithe
i enthalpy iof ia ireaction iis iindependent iof ithe iroute ithat’s ifollowed, iprovided ithe iproducts
i and ireactants iare ithe isame.
Identified ichange ithat ioccurs iin ia isystem iwhen imatter iis itransformed iby ia igiven
i chemical ireaction, ithat ithe ienthalpy iof ia ireaction iin icyclohexa-1, i2, i3-triene i(Kekulé’s
i version iof ithe ibenzene ielectron iarrangement) i iwas isubtracted ifrom ithe ienergy ireleased
i when ibonds iwere ibroken ito igive ithe itheoretical ienthalpy iof ihydrogenation ifor ibenzene.
i 359.2KJ/mol iis icalculated ias ithe ienthalpy iof ihydrogenation. iUsing ithe imethod iof
i Calorimetry, ithe ihydrogenation ienthalpy iof ibenzene ito ibe i-208.5KJ/mol. iRecognised ias
i the idifference ibetween ithe iobserved iand ithe iexpected ienthalpy iof iformation, iResonance
i Energy ican ialso ibe iknown ias iDelocalisation iEnergy. iDue ito iits ithe ilarge iresonance
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