Organic chemistry 9.docx

Organic chemistry 9.docx

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Organicchemistry9

Chapter9AldehydesandKetones:

NucleophilicAdditionReaction.

It'susefultoclassifycarbonylcompoundsintotwogeneralcategoriesbasedonthekindsofchemistrytheyundergo.

1.Structure

aldehydesketonesaldehydegroup

2.ClassifacationandNomenclature

Mthylketone

3.NamingAldehydesandKetones

Aldehydesarenamedbyreplacingtheterminal-eofthecorrespondingalkanenamewith-al.

Theparentchainmustcontainthe-CHOgroup,andthe-CHOcarbonisalwaysnumberedascarbon1.

Formorecomplexaldehydesinwhichthe-CHOgroupisattachedtoaring,thesuffix-carbaldehydeisused:

Cyclohexanecarbaldehyde2-Naphthalenecarbaldehyde

Ketonesarenamedbyreplacingtheterminal-eofthecorrespondingalkanenamewith-one.

Theparentchainisthelongestonethatcontainstheketonegroup,andnumberingbeginsattheendnearerthecarbonylcarbon.

Afewketonesalsohavecommonnames:

Whenit'snecessarytorefertothe-CORgroupasasubstituent,thegeneraltermacylisused.

4.SynthesisofAldehydesandKetones

We'vealreadydiscussedoneofthebestmethodsofpreparingaldehydesandketones-theoxidationofalcohols（Section8.5）.

Oxidizingagents:

PCC,CrO3,Na2Cr2O7

Othermethodsforpreparingketonesincludethehydrationofaterminalalkynetoyieldamethylketone（Section4.13）andtheFriedel-Craftsacylationofanaromaticringtoyieldanalkylarylketone（Section5.6）.

6.OxidatlonofAldehydes

Aldehydesareeasilyoxidizedtoyieldcarboxylicacids,butketonesareunreactivetowardoxidation.RCHO→RCO2H

Oneofthesimplestmethodsforoxidizinganaldehydeistousesilverion,Ag+,indiluteaqueousammonia,amixturecalledTollens'reagent.

Oxitionreation

Mildoxidant:

TollensreagentFehlingsreagent

Tollensreagent:

oxidizesanyofaldehydes.Fehlingsreagent:

oxidizesonlyaliphaticaldehydes

ChemicalProperties

7.NucleophilicAddition.ReactionsofAldehydesandKetones:

Reduction

Themostcommonreactionofaldehydesandketonesisthenucleophilicadditionreaction.

Additionofhydrideionnucleophiletoacetone:

Generalmechanismofanucleophilicadditionreaction.

ThisisexactlywhathappenswhenaketoneoraldehydeisreducedtoanalcoholbyNaBH4.

Thereactingnucleophilecanbeeithernegativelycharged（:

Nu-）orneutral（:

Nu）.

Nucleophilicaddition

Nucleophilicreagent:

NuA

Analdehydehasagreaterpartialpositivechargeonitscarbonylcarbonthandoesaketone

Stericfactorscontributetothereactivityofanaldehyde

Thecarbonylcarbonofanaldehydeismoreaccessibletothenucleophile

Ketoneshavegreaterstericcrowdingintheirtransitionstates,sotheyhavelessstabletransitionstates

Additionofhydrogencyanide

OH-Cyanohydrins

Cyanohydrins:

anyorganiccompoundinwhichthecyanoradical-CNandthehydroxylradical-OHareattachedtothesamecarbonatom

mechanism

syntheticglass

8.NucleophilicAdditionofWater:

Hydration

Aldehydesandketonesundergoanucleophilicadditionreactionwithwatertoyield1,1-diols,calledgeminal（gem）diols.Thereactionisreversible,andthediolproductcaneliminatewatertoregenerateaketoneoraldehyde

Thepositionoftheequilibriumbetweengemdiolsandaldehydes/ketonesdependsonthestructureofthecarbonylcompound.

Althoughtheequilibriumstronglyfavorsthecarbonylcompoundinmostcases,thegemdiolisfavoredforafewsimplealdehydes.

Thenucleophilicadditionofwatertoaldehydesandketonesisslowinpurewaterbutiscatalyzedbybothacidandbase.Asalways,thecatalystdoesn'tchangethepositionoftheequilibrium;itaffectsonlytherateatwhichthehydrationreactionoccurs.

9.NucleophilicAdditionofAlcohols:

AcetalFormation

Aldehydesandketonesreactwithalcoholsinthepresenceofanacidcatalysttoyieldacetals：

Acetalformationinvolvestheacid-catalyzednucleophilicadditionofanalcoholtothecarbonylgroupinamannersimilartothatoftheacid-catalyzedhydration.

Theinitialnucleophilicadditionstepyieldsahydroxyethercalledahemiacetal,whichreactsfurtherwithasecondequivalentofalcoholtoyieldtheacetalpluswater.

Acetalsarevaluabletoorganicchemistsbecausetheycanserveasprotectinggroupsforaldehydesandketones.

Aldehydesandketonescanbeprotectedbyconvertingthemintoacetals.

Acetals,likeotherethers,arestabletobases,reducingagents,andvariousnucleophiles,buttheycanbecleavedbytreatmentwithacid.

Additionofalcohols:

Semiacetalacetal

mechanism

Application:

Protectingaldehydegroup

Acetalsarequitestabletoalkali,Oxidizingandreducingagents

ketonecanreactwithethanediol:

Ethanediol:

protectingaldehydegroup

Somecyclicsemiacetalsareverystable

89%

94%

>99.%

10.NucleophilicAdditionofAmines:

ImineFormation

Ammoniaandprimparyamines,RNH2,addtoaldehydesandketonestoyieldimines,R2C=NR'.

Reactionwithderivativesofammonia

H2N-Y：

carbonylreagent

Examples:

11.NucleophilicAdditionofGrignardReagents:

AlcoholFormation

WesawinSection7.3thatorganichalidesreactwithmagnesiummetalinethersolutiontogiveGrignardrengents,RMgX.

Grignardreagentsthereforereactasthoughtheywerecarbonanions,orcarbanions,:

R-,andtheyundergonucleophilicadditiontoaldehydesandketonesjustaswaterandalcoholsdo.

AgreatmanyalcoholscanbeobtainedfromGrignardreactions,dependingonthereagentsused.

Althoughuseful,theGrignardreactionalsohaslimitations.Forexample,aGrignardreagentcan'tbepreparedfromanorganohalidethathasotherreactivefunctionalgroupsinthesamemolecule.

Grignardreagentscan'tbepreparedfromcompoundsthathavethefollowingfunctionalgroupsinthemolecule:

12.Unsaturatedaldehydeandketone

Electrophilicaddition

Nucleophilicaddtion

13.SomeBiologicalNucleophilic:

AdditionReactions

Nucleophilicadditionstepsareanimportantpartofthebiologicalsynthesisofmanyvitalmolecules.

ThebacteriumBacillussubtilissynthesizesalaninefrompyruvicacidandammonia.Thekeystepisthenucleophilicadditionofammoniatotheketonecarbonylgroupofpyruvicacidtogiveananiminethatisfurtherreducedbyenzymes.

Otherexamplesofnucleophiliccarbonyladditionsoccurfrequentlyincarbohydratechemistry.

Glucoseexistsasacyclichemiacetal.Thehydroxylgroupatcarbon5addstothealdehydeatcarbon1inaninternalnucleophilicadditionstep.

Celluloseconsistssimplyofglucoseunitsjoinedbyacetalbondsbetweencarbon1ofoneglucoseandthe-OHgroupatcarbon4ofanotherglucose:

14.Redutiontoalkanes

Clemmensenreduction

Wolff-kishner-Huang-Minglongreaction

HOOC（CH2）9COOH

Cannizzaroreacti

CrossedCannizzaroreaction

application