Moleculesthatarenotsuperposable
ontheirmirrorimagearecalled
enan4omers.
Stereoisomersthatarenotmirror
imagesandarenotsuperposable
arecalleddiastereomers.
Objectsthatarenotsuperposableon
theirmirrorimagesaresaidtobechiral.
NotesonConfigura4onalStereoisomerism
Achiralmoleculeisonethatisnotsuperimposableonitsmirrorimagestructure.Thepairof
structuresiscalledapairofenan(omers.Enan8omershaveiden8calphysicalproper8esexcept
fortheinterac8onwithplanepolarizedlight(staytuned).Enan8omershaveiden8calchemical
proper8esexceptfortheirinterac8onwithotherchiralmolecules.
Themaximumnumberofstereoisomerstheore8callypossible=2n(wherenisthenumberof
chiralitycentersinthemolecule).Theremaybefewerduetotheexistenceofmesocompounds,
orduetotherigidityimposedbyringsorothergeometricconstraints.
Amesocompoundisacompoundthathastwoormorechiralitycentersbutisnevertheless
achiralbecauseofaplaneofsymmetry.
Diastereomersarestereoisomersthatarenotenan8omers.Thatis,theyarenotmirrorimages
andtheyarenotsuperposable.Diastereomershavedifferentchemicalandphysicalproper8es
(butoEennotverydifferent).
Aracemicmixtureisa1:1ra8oofthe(two)enan8omers.Racemiza(on(toracemize)isthe
processofconver8ngasolu8onofonepureenan8omertowardsaracemicmixture.
Specificrota4on[α]Becauseop4calrota4on
dependsonwavelength,temperature,concentra4on,path
lengthandsolvent,weneedtohaveawaytonormalize
orstandardizethemeasurementsmadeindifferentlabs.
α
[α] =
c×ℓ
T
D
Forspecificrota4on[α]
-Thesuperscript(T)isthetemperatureof
themeasurementin°C.
-Thesubscriptisthewavelengthoflightused,usuallythe
so-calledsodiumD-line,whichis589nm.
-αistheexperimentallymeasuredop4calrota4onofyour
sampleindegrees.
-cistheconcentra4oning/mL(notetheunusualunits).
-l is the cell path length in dm (usually = 1 dm = 10 cm)
- The solvent is usually reported separately.
Adrenaline(epinephrine)ischiral.
Theenan4omershownattheleUabove,the
naturallyoccurringoneinhumans,
rotatestheplaneofpolarizedlighttothe
leU(an4clockwise),soitiscalledthe
levorotatoryor(l)or(-)enan4omer.
[α]D25≈-53°
ThereisNOcorrela8onbetweentheabsolute
stereochemistrydesigna0on(R)or(S)andthe
signofitsop8calrota8on.Toknowtheop8cal
rota8onofamoleculewemustmeasureit.
ThereisNOKNOWNcorrela8onbetweenthe
absolutestereochemistryofamoleculeandthe
sign(andmagnitude)ofitsop8calrota8on.To
knowtheop8calrota8onofamoleculewemust
measureit.Ofcoursetheremustbesucha
correla8on.Wejustdon’tyetknowwhatitisnor
howtocalculateit.
Achemicalreac4onthatcreatesanewchiralitycentre.Thisisvery
common.Theresultisaracemicmixture.
Notethatthereactants(andcatalyst)areachiralandop4cally
inac4ve,andthattheproduct,aracemicmixture,isalsoop4cally
inac4veeventhoughitcontainschiralmolecules.
Itisageneralrulethatintheabsenceofachiralcatalystorsolventitis
impossibletomakeanop4callyac4veproductmixturefroman
op4callyinac4vereactantmixture.
Therulesforassigningpriori8esinvolveassigningthehighest
prioritytotheatomwiththehighestatomicnumber(Z),andgoing
outwardindefinitelyfromthechiralitycentreun8ladecisionis
possible.
But:Addi4onalRulesforAssigningPriori4estoMul4ple
BondedSubs4tuents
vinyl
isopropyl
Butlet’sgobackforamomenttomoleculeswithonlyonechiralitycentre.
FromOrganicChemistrybyCareyetal,McGraw-Hillpublishing.
WhenusingFischerprojec4onsasaconvenient
stereochemicalnota4on,theques4onbecomes:“What
opera4onscanIgetawaywithwhileretainingthesame
stereochemistry?”
ThereareonlyTWOsuchopera4ons(thoughyoucan
repeatthemasmany4mesasyouwish):
1)Rotatetheen4restructureby180°.
2)Holdonesubs4tuentinafixedposi4onandrotate
theremainingthree.
*
*
*
*
*
* *
*
Thereare8chiralitycentres,so,inprinciplethereare28
stereoisomers.
That’s256!Inprac4ce,manyarenotpossiblebecause
ofgeometricalconstraints.Natureproducesonlyone.
chiralitywithoutchiralitycentres