Term
| If you want self condensation to occur what should you do? |
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Definition
| then use a milder base (this is best when the pka of carbonyl group species and conjugate acid are similar! |
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Term
| If you want self condensation to be avoided |
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Definition
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Term
| Less basic to more basic basses |
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Definition
NaOH, K2CO3
NaOMe/MeOH
NaH
NaNH2, KNH2,
LDA, LiHMDS
nBuLI
Ph3C- Na+ |
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Term
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Definition
| used in hydroxylic or non-hydroxylic solvents |
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Term
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Definition
| Don't match with base you would have OMe- and OEt-, sometimes can use non-hydroxylic solvents |
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Term
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Definition
| good base, poor nucleophile, insoluble in most organic solvents, deprotonation is not instantenous, self condensation issues |
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Term
|
Definition
| good solubility, not nucleophilic |
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Term
|
Definition
| very strong base can be nucleophilic |
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Term
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Definition
| hindered bases, soluble in organic solvents |
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Term
| Name 6 factors that favor kinetic enolates: |
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Definition
aprotic solvents (THF, Et2O)- no acidic proton to encourage reverse reaction
strong bases (LDA) generate weak conjugate acid
Low temperatures (-78C)
short reaction time
oxophilic cation (Li+)
ensure that reaction is irreversible |
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Term
| Name 5 factors that favor thermodynamic enolate: |
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Definition
Protic solvents (ROH)- favor formation of enol and thus tautomerization to ketone
Weak bases- generate relatively strong conjugate acids
High temperatures
long reaction time
ensure that reaction is reversible |
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Term
| Silyl and phosphorus electrophiles have high affinity for _____ and thus undergo _ reaction |
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Definition
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Term
| Factors affecting C vs. O alkylation rate: five things |
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Definition
metals and solvents
solvents affect dissociation
metals affect clustering of ions
leaving group
orbital overlap
conjugation/unsaturation |
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|
Term
What metals reduce the reactivity of O
when is o alkylation preferred?
when is c alkylation preferred? |
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Definition
Li>Na>K
O alkylation preferred with dissociated enolates
C alkylation preffered with tight ion clusters |
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Term
| Polar aprotic solvents (HMPA) promote what and favors _ |
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Definition
| promote ion dissociation favors O |
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Term
| Apolar aprotic solvents (THF, DME) promote what and favor _ |
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Definition
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Term
| protic solvents (MeOH, H2O): can do what and promote what |
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Definition
| can H bond with enonlate oxygen, promote C |
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Term
Leaving group effects:
O-alkylation |
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Definition
| predominant with hard leaving groups and conditions with dissociated and more reactive enolates |
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Term
Leaving group effects:
C-alkylation |
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Definition
| predominant with soft leaving groups |
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Term
| Characteristics of hard acid/base |
|
Definition
| small species, high oxidation state, high charge to atomic radii ratio, low polarizability, high electronegativity |
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Term
|
Definition
| H+, Li+, Na+, K+, Ti4+, Cr3+, Co6+, R3C+, BF3 |
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Term
|
Definition
| OH-, OR-, F-, CL-, CO3(2-), CH3C=O(-), SO3R-, Br- |
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Term
|
Definition
| M0, Ag+, Pd2+, Mg2+, CH3-Mg+ |
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Term
| Charcteristics of a soft acid/base |
|
Definition
large atomic radius
low or zero oxidation state
high polarizability
low electronegativity |
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Term
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Definition
| H-, SR-, SCN-, PR3-, CO, I- |
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Term
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Definition
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Term
| What atom can stabilize carboanions? |
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Definition
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Term
| Origin of sulfur carboanion stabilization?(4 things) |
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Definition
resonance
inductive effect
polarizability
hyperconjugation |
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|
Term
stabilization of carboanion with sulfur:
Resonance |
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Definition
| d-p pi resonance is minor because d-orbital is too diffused and too high in energy |
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|
Term
stabilization of carboanion with sulfur:
inductive effect |
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Definition
| some contribution for the electronegative elements (Cl and S) |
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|
Term
stabilization of carboanion with sulfur:
polarizability |
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Definition
| of the sulfur stabilizes it |
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|
Term
stabilization of carboanion with sulfur:
hyperconjugation |
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Definition
| MAJOR contribution of sigma-hyperconjugation for S-R, PR2, SiR3 delocalaztion of the charge into the X-R sigma* orbital |
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