Organic chemistry 67.docx

Organic chemistry 67.docx

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Organicchemistry67

Chapter6Stereochemistry

1.Opticalactivity

lightsourcepolarizerSamplecell

polarimeter

Plane-polarizedlight:

Abeamofordinarylightconsistsofelectromagneticwavesthatoscillateinaninfinitenumberofplanesatrightanglestothedirectionoflighttravel.Whenabeamofordinarylightpassesthroughadevicecalledapolarizerthough,onlythelightwavesoscillatinginasingleplanepassthroughandthelightissaidtobeplane-polarized.

Whenabeamofplane-polarizedlightpassesthroughasolutionofcertainorganicmolecules,suchassugarorcamphor,theplaneofpolarizationisrotated.sugarandcamphoraresaidtobeopticallyactive.

Theamountofrotationcanbemeasuredwithaninstrumentcalledapolarimeter.

Thelightthengoesthroughasecondpolarizercalledtheanalyzer.Byrotatingtheanalyzeruntillightpassesthroughit,wecanfindthenewplaneofpolarizationandcantelltowhatextentrotationhasoccurred.Theamountofrotationobservedisdenotedbyαandisexpressedindegrees.

Byconvention,rotationtotheleftisgivenaminussign（-）,androtationtotherightisgivenaplussign（+）.Forexample,（-）-morphineis,levorotatoryand（+）-sucroseisdextrorotatory

Inadditiontodeterminingtheextentofrotation,wecanalsofindthedirection.Fromthevantagepointoftheobserverlookingattheanalyzer,someopticallyactivemoleculesrotateplane-polarizedlighttotheleft（counterclockwise）andaresaidtobelevorotatory;othermoleculesrotatelighttotheright（clockwise）andaresaidtobedextrorotatory

opticalrotation（α）specificrotation[α]racemate（±）,dl

t：

temperature（℃）

D：

sodiumlightsource（589nm）

l：

lengthofopticaltube（dm）

C：

concentration（g·ml-1）

2.Enantiomers

①Amoleculethatisnotidenticaltoitsmirrorimageisakindofstereoisomercalledanenantiomer;Enantiomersarerelatedtoeachotherasarighthandisrelatedtoalefthandandresultwheneveratetrahedralcarbonatomisbondedtofourdifferentsubstituents（oneneednotbeH）.

②Chiral

Moleculesthatarenotidenticaltotheirmirrorimagesandthusexistintwoenantiomericformsaresaidtobechiral（Greekcheirmeaning"hand"）.Youcan'ttakeachiralmoleculeanditsmirrorimage（enantiomer）andplaceoneontopoftheothersothatallatomscoincide.

Themostcommon（althoughnottheonly）causeofchiralityinanorganicmoleculeisthepresenceofacarbonatombondedtofourdifferentgroups.

Suchcarbonsarereferredtoaschiralitycentersorstereocenters.

Notethatchiralityisapropertyoftheentiremolecule,whereasastereocenteristhecauseofchirality.

③Achiral（notchiral）

Amoleculeisnotchiralifitcontainsaplaneofsymmetry.Aplaneofsymmetryisaplanethatcutsthroughthemiddleofamoleculeorotherobjectsothatonehalfoftheobjectisanexactmirrorimageoftheotherhalf.

3.SequenceRulesforSpecifyingConfiguration.

Specifyingthethree-dimensionalarrangement,orconfiguration,ofsubstituentsaroundastereocenterisnecessary.

RULE1.Lookatthefouratomsdirectlyattachedtothestereocenterandassignprioritiesinorderofdecreasingatomicnumber.Theatomwiththehighestatomicnumberisrankedfirst;theatomwiththelowestatomicnumberisrankedfourth.

RULE2.Ifadecisioncan'tbereachedbyrankingthefirstatomsinthesubstituents,lookatthesecond,third,orfourthatomsoutwarduntilthefirstdifferenceisfound.

RULE3.Multiple-bondedatomsareequivalenttothesamenumberofsingle-bondedatoms.

4.（R）,（S）-Convention（IUPAC）

（1）Assignaprioritytoeachofthefouratomsorgroupsattachedtotheasymmetriccarbonatom.

（2）Thegroupoflowestpriorityisplacedinthepositionmostremotefromus.Theremaininggroupsareassignedpositionsinaclockwiseorderofdecreasingpriorityforthe（R）-isomerandinacounterclockwiseorderofdecreasingpriorityforthe（S）-isomer.

Fischer投影式：

将一手性碳原子的四面体球棍模型投影在纸面上得到的，投影时是假定把手性碳原子放在纸平面上，四面体的两个顶点，即两个原子或基团指向前方，用横线表示；四面体的另两个顶点，即另两个原子或基团指向后方，用竖线表示，通常简写成十字形。

一般总把含有碳原子的基团放在竖线相连的位置上。

对于Fischer投影式：

当手性碳原子连有四个不同基团时，可表示为C（a,b,c,d）,按次序规则其优先次序假定为a>b>c>d,由于规定在竖线上的基团在纸平面的后面，若优先次序最小的集团d在竖线上，则符合上述观察条件，这时由a→b→c若是按顺时针方向排列，则手性碳原子构型为R构型，由a→b→c按逆时针方向排列，则为S构型。

若优先次序最小的基团d在横线上，则与上述观察条件相对，这时由a→b→c若是按顺时针方向排列，则手性碳原子构型为S构型，由a→b→c若按逆时针方向排列，则为R构型。

Enantiomersincycloalkanes

Enantiomersinconformationalformula

5.Diastereomers

Notecarefullythedifferencebetweenenantiomersanddiastereomers:

Enantiomershaveoppositeconfigurationsatallstereocenters;diastereomershaveoppositeconfigurationsatsome（oneormore）stereocentersbutthesameconfigurationatothers.

6.MesoCompounds.

The2R,3Sand2S,3Rstructuresareidenticalbecausethemoleculehasaplaneofsymmetryandisthereforeachiral.

BecauseoftheplaneofsymmetrythetartaricacidstereoisomershowninFigure6.11isachiral,despitethefactthatithastwostereocenters.Suchcompoundsthatareachiral,yetcontainstereocenters,arecalledmesocompounds

7.MoleculeswithMoreThanTwoStereocenters

Amoleculewithnstereocentershasamaximumof2nstereoisomers（2n-1pairsofenantiomers）.

8.RacemicMixtures

Racemicmixturesareoftendenotedbythesymbol（±）orbytheprefixd,ltoindicatethattheycontainequalamountsofdextrorotatoryandlevorotatoryenantiomers.

9.Enantiomerswithnochiralcarbonatoms

ABriefReviewofIsomerism

10.Biologicaleffectsofchiralcompounds

interactionbetweenchiralmoleculeandbiologicalacceptor

Chapter7AlkylHalides

1.ClassificationandNomenclatureofhalohydrocarbon

Classification

Nomenclature

Commonname

Manysimplealkylhalidesarenamedbyidentifyingfirstthealkylgroupandthenthehalogen.

IUPACname

step1Findthelongestchain,andnameitastheparent.

step2Numberthecarbonsoftheparentchainbeginningattheendnearerthefirstsubstituent,regardlessofwhetheritisalkylorhalo.

3-bromo-4-methylhexane2-chloro-3,4-dimethyl-3-hexene

6-bromo-3-chloro-4-methylcyclohexene

3.Chemicalproperties

ThemoreimportantnucleophilicsubstitutionsofalkylhalidesareillustratedbythefollowingGereralequations

1.replacementofthehalogenatombyhydroxy（-OH）

reactivity：

R—I>R—Br>R—ClHydrolysisofalkylhalides

2.replacementofthehalogenatombycyano（-CN）

3.replacementofthehalogenatombyalkoxy（-OR）

4.Replacementofthehalogenatombyamino（-NH2）

5.Replacementofthehalogenatombymercapto（-SH）

6.Replacementofthehalogenatomby（-）

7.Replacementofthehalogenatomby-ONO2

Identificationofalkylhalides

Generalequation:

nucleophilic-substitution（SN）

nucleophilicreagent：

OH-、CN-、OR-、NH3、SH-、ONO2-

4.Mechanismofnucleophilicsubstitution.

SN1Mechanism

Formationofcarbocation

SN1EnergyDiagram

CharacteristicofSN1

•Unimolecularnucleophilicsubstitution.

•Twostepreactionwithcarbocationintermediate.

•Rateisfirstorderinthealkylhalide,zeroorderinthenucleophile.

•Racemizationoccurs.

StereochemistryofSN1Racemization:

inversionandretention

RatesofSN1Reactions

①3°>2°>1°>>CH3X

②Orderfollowsstabilityofcarbocations

③Morestableionrequireslessenergytoform

④Betterleavinggroup,fasterreaction

⑤Polarproticsolventbest:

Itsolvatesionsstronglywithhydrogenbonding.

SN2Mechanism

SN2EnergyDiagram

CharacteristicofSN2

•Bimolecularnucleophilicsubstitution.

•Concertedreaction:

newbondformingandoldbondbreakingatsametime.

•Rateisfirstorderineachreactant.

•Waldeninversion.

StereochemistryofSN2Waldeninversion

SN2:

ReactivityofSubstrate

①Carbonmustbepartiallypositive.

②Musthaveagoodleavinggroup.

③Carbonmustnotbestericallyhindered.

④CH3X>1°>2°>>3°TertiaryhalidesdonotreactviatheSN2mechanism,duetosterichindrance

5.FactorsaffectingSN1andSN2

（1）thestructureofthesubstrate.

（2）theeffectofthenucleophile（forSN2only）.

（3）thenatureoftheleavinggroup.

（4）theeffectofthesolvent.

（1）thestructureofthesubstrate

Primaryormethyl（SN2）Tertiary（SN1）Secondary（SN2、SN1）

Allylicandbenzyl（SN2、SN1）

Vinylicandphenyl（noSN）

allylichalideandbenzylhalide

Vinylicandphenyl

（2）theeffectofthenucleophile（forSN2only）.

Strongernucleophilesreactfaster.

Strongbasesarestrongnucleophiles,butnotallstrongnucleophilesarebasic

RS->CN->I->NH3（RNH2）>RO->OH->Br->CH3CO2->Cl->H2O>F-

（3）thenatureoftheleavinggroup.

①Electron-withdrawing

②Stableonceithasleft（notastrongbase）

Reactivity:

RI>RBr>RCl>RF

Badleavinggroup:

－OH，－OR，NH2－，CN－

（4）theeffectofthesolvent.

①Polarproticsolvents（O-HorN-H）reducethestrengthofthenucleophile.

②Hydrogenbondsmustbebrokenbeforenucleophilecanattackthecarbon.

③Polaraproticsolvents（noO-HorN-H）donotformhydrogenbondswithnucleophile

6.EliminationReactions

①Thealkylhalideloseshalogenasahalideion,andalsolosesH+ontheadjacentcarbontoabase.

②Apibondisformed.Productisalkene.

Alsocalleddehydrohalogenation（-HX）.

examples

Saytzeff’sRule：

Ifmorethanoneeliminationproductispossible,themost-substitutedalkeneisthemajorproduct（moststable）

E1Reaction

•Unimolecularelimination

•Twogroupslost（usuallyX-andH+）

•Nucleophileactsasbase

•AlsohaveSN1products（mixture）

•Halideionleaves,formingcarbocation.

•BaseremovesH+fromadjacentcarbon.

•Pibondforms.

E2Re