Tropone & Tropolones Aromaticity

Tropone & Tropolones

1. Tropone& Tropolone

2. Aromaticity is a Suspicious Concept
CSE 2021
❖Aromatic Hydrocarbon with 16 πElectron Periphery

3. Tropone, Tropolone & Cycloheptanone
Tropone Tropolone Cycloheptanone
Hydrogenation Readily hydrogenated over PdNot hydrogenated over Pd --
C=O stretching frequency (cm
-1
) 1638 1615 1699
Dipole Moment (D) 4.17 3.71 3.04
Resonance Energy (Kcal/mol) 29 36 --
Stability Less stable More stable --
Aromaticity Weakly aromatic Marked aromatic character
(Reason: Intramolecular H-
bonding + tautomerism)
--

4. Aromaticity of Tropone
❖Troponedisplaylimitedaromaticcharacter
❖Troponeisaresonancehybridoftheclassicalstructureandtropyliumcationstructure
❖ReactlikeTriene:readilyhydrogenated,rapidlydecolorizesaqueouspermanganate
❖Ionicstructurecontribution:highDipolemoment(4.17D)indicatingcontributionfromthepolarizedform,C=Ostretching
frequency(1638cm
-1
)
❖Classicalstructurecontribution:Showsbondlengthalteration
❖Basicity:morebasicthannormalketone

5. Aromaticity of Tropolone
❖TropolonenucleusisnotreducedbyhydrogeninthepresenceofPdcatalystsbutRaneynickelorAdams’platinumoxide
usedascatalysts
❖BothH-bondingandresonancewiththeionicform(C)helptoincreasethestabilityoftropolonecomparednotonlyto
troponebutalsototheisomerichydroxytropones
❖Inisomerichydroxytropones:IntramolecularH-bondingmustbeabsentandcorrespondingionicformsinvolvegreater
separationofcharge

6. Reaction with Electrophiles
CSE 2012

7. Tropolone both fairly acidic and basic
❖Itisacidicbecausecapableofloosingaprotontoformrelativestableanionwhileitisbasicbecauseofitsabilitytoreceive
aprotontoformcation.
❖IntramolecularH-bondingissignificantlydiminishedin3comparisonto1

8. ❖Tropolonecanberegardedasextendedorvinylogouscarboxylicacidandenolformof1,3-diketones.
❖onthesimplebasisofvinylogy,acidityoftropolonefallsslightlybelowexpectation
❖acidityiscertainlynotenhancedbyaromaticresonanceoftheion(a).
❖Resonatingionsofthecomparisonsubstancesarelikewisefullyequivalentandanyadditional“aromatic”resonancewould
havetocomefromstructures(b)whichmustberatherlessfavoredthanthecorrespondingstructures(c)ofthefreeacid.
❖Basicity:Thispropertyisadirectresultofthearomaticity,Inthissenseitcontrastswiththeacidityoftropolones,whichis
notaidedbythataromaticity.
a b c
Is Tropolone stable in acidic or basic medium?

9. IR Spectrum
❖emphasisonthecarbonylandhydroxylbands
❖carbonylgroup:exceptionallylowwavenumber
❖Reasons:RingSize,Highdegreeofconjugation,Highpolarity,Hydrogen-BondingEffects
❖RingSize:
IFoS2006

10. IR Spectrum
❖Conjugationeffect:IncreasesinglebondcharacterodC=O,hencelowersforceconstant
❖Hydrogen-BondingEffects:HydrogenbondingtoacarbonylgrouplengthenstheC=Obondandlowersthestretchingforce
constantK,resultinginaloweringoftheabsorptionfrequency.
❖1,3-Diketones:Thecarbonylgroupintheenolformappearingatabout1622cm−1issubstantiallyshiftedandintensifiedin
comparisontothenormalketonevalue,1715cm−1.Theshiftisaresultofinternalhydrogenbonding

11. TroponeUmpolung
❖Conversionofthecarbonylgroupintoahydrazoneionreversesthepolarityoftheexocyclicdoublebond,increasesthe
[4n]ringπ-antiaromaticity.
IFoS2006
Diazocycloheptatriene: antiaromaticity effects in this derivative
are completely attenuated.

12. Synthesis & Reaction
❖Synthesisoftropone
❖Synthesisoftropolone
❖Rearrangementtobenzoicacid