Background:Turmericpowderisaninexpensiveyellow-orangespicethatisusedtogivesome
brandsofmustardabrightyellowcolor.Thiscolorderivesprincipallyfromcurcumin(1,2).
HO
OH
H
OH
H H
H 3CO
OCH 3
O
HO
OH
1
calculated λmax = 417 nm
H 3CO
OCH 3
O
O
2
calculated λmax = 389 nm
CalculatedλmaxvaluesarereportedinL.Shen,H.-F.Ji“Theoreticalstudyonthephysiochemical
propertiesofcurcumin.”SpectrochimicaActaPartA.2007,67,619–623.
ReviewQuestions:
1.Applywhatyouknowaboutconjugatedπ-bondingsystemstoexplainwhythemaximum
absorbancewavelengthfortheketo-enoltautomer1shouldbelongerthanthatfordiketone2.
2.Therehasbeensomedebateinthescientificliteratureaboutwhethertheketo-enoltautomer1
ordiketone2predominatesatneutralpH~7.Ifthecalculationsforthemaximumabsorbance
wavelengtharecorrect,whichofthesecompoundsdoyouseeontheinitialyellowpaper?Why
mightthatcompoundbemorestable?
Recall:Whenlightisabsorbed,youreyedetectsthecomplementarycolor.
Wavelengthoflight
ComplementaryColor
• Violet:400-420nm
Yellow
• Indigo:420-440nm
Yellow
• Blue:440-490nm Orange
• Green:490-570nm
Red
• Yellow:570-585nm
Indigo&Violet
• Orange:585-620nm
Blue
• Red:620-780nm Green
3.Inprincipal,watercanaffecttheinterconversionofketo-enoltautomer1anddiketone2.Draw
arrow-pushingmechanismsusingwatertointerconvert1and2.
Watchthevideooftheprocedure(https://www.youtube.com/watch?v=0afSRiCzBco)and
carefullyobservewhathappens.Then,respondtothebelowquestions,whichrequireyouto
drawonyourobservationsandyourknowledgeoforganicsynthesis.
Inthevideo,aseriesofsolutionswerepreparedbyaddingtowateroneofthefollowing:
(a)Ateaspoonofbakingsoda(sodiumbicarbonate,NaHCO3).AssumeasolutionpH~9.
(b)Vinegar,whichcontainsaceticacid(CH3CO2H).AssumethissolutionhasapH~2.
(c)Concentratedhouseholdbleach,whichcontainsNaOCl.AssumeasolutionpH~13.
(d)Water.AssumethissolutionhasapH~7.
4.Ifthecalculationsandexperimentalvaluesforthemaximumabsorbancewavelengthare
correct,whichcompoundscouldberesponsiblefortheredcolorinthepaper?Seebelowfor
possiblestructures.
HO
OH
H
H 3CO
OCH 3
O–
O
HO
calculated
λmax = 417 nm
H 3CO
H 3CO
OCH 3
O–
O
5
OH
4
calculated λmax = 531 nm
OH
H
OCH 3
O
3
calculated λmax = 429 nm
–O
O–
H
1
pK a~10.7
in H 2O
–O
O–
H
H 3CO
OCH 3
O
O–
6
experimental λmax = 490–560 nm
(depends on counter cation, solvent)
5.Drawanarrow-pushingmechanismtoexplainwhythebakingsodasolutiontransformedthe
yellowpapertoredpaper.Whatistheroleofbakingsodainthisreaction?
6.Drawanarrow-pushingmechanismtoexplainwhythevinegarsolutiontransformsthered
papertoyellowpaper.Whatistheroleofvinegarinthisreaction?
7.Whydoesn’tthevinegarsolutioncausetheyellowpapertochangecolors?
8.Afterexposuretothebleachsolution,theinitiallyyellowpaperbecomesredandthen
eventuallybecomescolorless.Speculateaboutwhythepaperchangesfromredtocolorless.
Chemically,whatmightthisprocessentail?
Hints:
8a.Allofthedepictedstructuresareindynamicequilibrium,soifoneofthesestructuresis
present,thereislikelytobesomesmallamountofalloftheothers(whichwouldmeanthatifany
ofthesecompoundswaspresent,thesheetofpapermightremaincolored).
8b.Pretendthatreactivitystartswithcompound2forthisquestion.
9.Write1-2sentencestoexplainwhatyouhavelearnedfromthisworksheet.
10.Wasthisexerciseuseful?Whyorwhynot?Provide1-2suggestionstoimprovethisworksheet.
TheinitialdraftofthisworksheetwaspreparedbyTanyaThomas,MirandaAllen,GilbertBrooks,
III,BashirAdeed(2015)toaccompanytheirproceduralvideo.IthasbeenadjustedbyProf.
JenniferRoizenandMr.MinaShehata(2016).