CHEM 2021 Chapter Notes - Chapter 3: Halogenation, Racemic Mixture, Benzene

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7 Feb 2016
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SummaryofCCBondFormingReactions
Organometallics (Grignards,organolithiums,alkynides)deliveralkyl,alkenyl,aryloralkynyl groupsasnucleophilic carbanions
inreactionswithelectrophiles toformnewCCbonds:
• epoxides,carbonylcompounds,esters(0.5equiv.)oracidchlorides(0.5 equiv.)withanyorganometallic,producingalcohols
ofvarioussorts(afterworkup),
•CO
2withGrignards,producingcarboxylicacids(afterworkup),
• nitriles withGrignards,producingketones (afterhydrolyticworkup),
• carboxylicacidswithorganolithiums,producingketones,
Esters andacidchloridesreactwithtwoequivalentsofanorganometallic togivetertiaryalcoholswhichhavetwoofthesame
substituent.Thesereactionscannotbestoppedattheintermediateketone stage.
Cyanide (CNˉ)formsnewCCbondswith
•alkylhalides(SN2),
• carbonylcompounds(nucleophilic addition)givingcyanohydrins,and
• epoxides (ringopeningatleastcongestedsitewithinversion).
Thenitrile groupthatresultscanbefurthertransformedtoCH2NH2groupsbyreduction, toCONH2byacidcatalyzed
hydration,orto COOHgroupsbyhydrolysisortoketonesbyalkylation.
FriedelCrafts alkylationandacylation deliverelectrophilic carbocations oracylium ionsinreactionswitharomaticsactingas
nucleophiles,toformnewCCbonds.
Theketones (commonlycalledphenones)thatresultfromFriedelCraftsacylation canbereducedtoprimaryalkylgroupsby
Clemmensen reduction,orcanbefurthertransformedjustaswithotherketones.
Thebenzylic positionsthatresultfromalkylations oracylation/Clemmensen reductionarethepreferredsitesforfreeradical
halogenation,whichopensthedoorforsubsequenttransformations.
Thosereagentsdeliveringonecarbonunits are
•CNˉ&CH3MgIasnucleophiles,or
•CH
3I/CH3Br/CH3Cl/CH3OTs(withnucleophiles), HCHO(formaldehyde,togive1alcohols), HCOOR(formate esters,togive2
alcohols),ROCOOR (carbonateesters,,togive3alcohols)&CO
2(togivecarboxylates)as electrophiles.
•Aswell,themixedanhydrideHCOOCOCH3(aceticformicanhydride–seeChapter21)canservetoformformateestersorin
FriedelCraftsacylations insteadoftheunstableHCOCl (formyl chloride)toformbenzaldehydes.
Twocarbon orlongerunits withfunctionalityatthesecondcarbon canoftenbedeliveredbyepoxides undergoingring
openingwithanorganometallic orwithCNˉ,andtheOHgroupresulting(afterworkup)cansubsequentlybetransformedto
variousotherfunctionalities.
Examples
(2)
(1) NaNH2
CCHCH3CH2O
CH3
CCCH3CH2
OH
CH3
dil. H3O+
H2 /Pt
OH
H2CrO4
NaCN
HOAc
OH
NC
ThenewCCbondsarecircled.
Inthisexample,therearetwonewCCbond
formingreactionsemployed.Thealkyne starting
materialisimposedandofferstheopportunity
toformanewCCbondasanalkynide.The
presenceoffunctionalitytwocarbonsaway
dictatestheuseofanepoxide.Thecyanohydrin
functionalityismadebyHCNadditiontothe
derivedketone.
(a)
Cl KOtBu
Ph
OH
(racemic)
O
MCPBA
(1) PhMgBr
(2) dil. H3O+
Inthisexample,tworoutestothesameproductareprovided,oneusinganti epoxide ringopeningandtheotherFriedel
Craftsalkylation,followedbysynhydration(antiMarkovnikov).
(b)
benzene
AlCl3
Cl Ph Br2Ph
Br
light
KOtBu
Ph
(1) BH3
(2) H2O2, NaOH
Ph
OH
(racemic)
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