7 Aldehydes and Ketones II: Enolates Flashcards
alpha hydrogens
on alpha carbon (adjacent to carbonyl C)
extremely acidic (deprotonate easily)
induction effects (EWGs) increase their acidity
EDGs destabilize carbocation (why ketones’ alpha hydrogens are less acidic than aldehydes’)
steric hindrance
ketone has an extra alkyl group
increases steric hindrance and energy of intermediate
aldehydes are _____ (more/less) reactive to nucleophiles than ketones
more
steric hindrance and increased acidity of alpha hydrogens
increasing energy of intermediates _______ (increases/decreases) likelihood of reaction proceeding
decreases
tautomers
isomers; differ in placement of double bond & proton
keto tautomer
thermodynamically favored
most stable end product
enol tautomer
kinetically favored
important intermediates
keto-enol tautomerization
deprotonation of alpha-carbon by strong base –> enolate carbanion
- OH-
- LDA (lithium diisopropyl amide)
- potassium hydride
MIchael addition
- base deprotonates a-carbon –> good nucleophile
2. carbanion attacks another carbonyl double bond
thermodynamic enolate conditions
forms slowly more stable (double bond to more substituted carbon)
CONDITIONS: higher temperatures, slow, reversible, weaker/smaller bases
kinetic enolate conditions
forms rapidly (less steric hindrance at alpha-carbon) less stable
CONDITIONS: rapid, irreversible, lower temperatures, strong/sterically hindered base
enamines are tautomers of _________
imines
aldol condensation
- formation of the aldol (condensation)
- dehydration of the aldol
general mechanism of nucleophilic addition to carbonyl
EXCEPT:
aldehyde/ketone acts as electrophile (in keto form) AND nucelophile (in enolate form)
end result: formation of C-C bond
- forming the aldol (condensation): enolate ion forms, attacks carbonyl C –> aldol
- dehyrdration of aldol: OH removed as water –> forms the double bond
retro-aldol reaction
bond broken between alpha and beta carbons of carbonyl –> 2 aldehydes/2 ketones/1 of each
add aqueous base and heat to aldol
