organometallic compounds
TRANSCRIPT
Organometallic Organometallic CompoundsCompoundsCarbon-Metal BondsCarbon-Metal Bonds
RR XXδδ++ δδ––
RR MMδδ–– δδ++
Nucleophilic Nucleophilic carboncarbon
Organometallic Compounds
same for same for ArAr——XX
is an oxidation-reduction reaction: carbon is an oxidation-reduction reaction: carbon is reduced is reduced
Organolithium Organolithium CompoundsCompounds
RR XX + 2 + 2LiLi RR LiLi + + LiLiXX
normally prepared by reaction of alkyl halides normally prepared by reaction of alkyl halides with lithiumwith lithium
ExamplesExamples
(CH(CH33))33CCCl Cl + 2 + 2LiLi
diethyldiethyletherether
––1010°°CC(CH(CH33))33CCLi Li + + Li LiClCl
(75%)(75%)
BrBr + 2+ 2LiLi
diethyldiethyletherether
3535°°CC
LiLi ++ Li LiBrBr
(95-99%)(95-99%)
Preparation of OrganomagnesiumCompounds
same for same for ArAr——XX
Grignard Grignard ReagentsReagents
RR X X + + MgMg RRMgMgXX
prepared by reaction of alkyl halides prepared by reaction of alkyl halides with magnesiumwith magnesium
Diethyl ether is most often used solvent.Diethyl ether is most often used solvent.Tetrahydrofuran Tetrahydrofuran is also used.is also used.
ExamplesExamplesdiethyldiethyletherether
––1010°°CC(96%)(96%)
BrBr + + MgMg
diethyldiethyletherether
3535°°CC
MgMgBrBr
(95%)(95%)
ClCl + + MgMg
MgMgClCl
Alkyl halides, vinyl halides, and arylhalides can all be used to formorganolithium and organomagnesiumcompounds
However, these organometalliccompounds cannot be prepared fromcompounds containing acidic groups(OH, NH2, NHR, SH, C=CH, CO2H)
Cannot use HCannot use H22O, CHO, CH33OH, CHOH, CH33CHCH22OH,OH,etc. as solventsetc. as solvents
Cannot prepare from substances such asCannot prepare from substances such asHOCHHOCH22CHCH22Br, etc.Br, etc.
Coupling Reactions
Formation of carbon–carbon bonds
Coupling Reactions Coupling Reactions
Organolithium and OrganomagnesiumCompounds as Brønsted Bases
BrBrøønsted basicitynsted basicity
RR
MM
HH
OOR'R'••••
••••
RR HH
OOR'R'••••
••••••••––
MM++
δδ–– δδ++
Grignard Grignard reagents (reagents (MM = = MgX MgX) and) andorganolithium organolithium reagents (reagents (MM = Li) are strong = Li) are strongbases.bases.
ExampleExample
(100%)(100%)
+ H+ H22OO
++ LiLiOHOH
CHCH33CHCH22CHCH22CHCH22LiLi
CHCH33CHCH22CHCH22CHCH33
ExampleExample
MgMgBrBr
(100%)(100%)
+ CH+ CH33OHOH
++ CHCH33OOMgMgBrBr
Approximate AciditiesApproximate Acidities
CompoundCompound ppKKaa
(CH(CH33))33CCHH 7171
CHCH33CHCH33 6262
CHCH44 6060
EthyleneEthylene 4545
BenzeneBenzene 4343
AmmoniaAmmonia 3636
AcetyleneAcetylene 2626
WaterWater 1616
Hydrocarbons areHydrocarbons arevery weak acids.very weak acids.
Their conjugateTheir conjugatebases are verybases are verystrong bases.strong bases.
Grignard Grignard reagentsreagentsandand organolithium organolithiumreagents are strongreagents are strongbases.bases.
Acetylenic Grignard Acetylenic Grignard ReagentsReagents
are prepared by an acid-base reactionare prepared by an acid-base reaction
CHCH33CHCH22MgMgBrBr
++CHCH33CHCH33
stronger acidstronger acid
weaker acidweaker acid
HCHC CHCH
HCHC CCMgMgBrBr
++
Question
• Select the best method to prepareCH3CH2C≡CMgI.
• A) CH3CH2C≡C-I + Mg (in diethylether)
• B) I-CH2CH2C≡CH + Mg(OH)2 (indiethyl ether)
• C) CH3CH2C≡CH + CH3MgI (indiethyl ether)
• D) CH3CH2C≡CH + Mg (in diethylether)