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lecture 1- carbonyls and
tautomerisation
Date: @January 11, 2022
Topic:
key words
Notes
carbonyl=electrophile
C=O carbonyl
sp2 hybridised
sp2 hybridisation at C carbonyl compound's are planar giving
120 degrees bond angle
lecture 1- carbonyls and tautomerisation
1
since the carbonyls are polarised
oxygen is the electronegative atom and pulls electrons to itself
making it electrophilic
nucleophilic attack
the nucleophile attacks the carbonyl (electrophilic) carbon
order of reactivity
reactivity
aldehydes> ketones are most reactive to NU attack
then in decreasing order f reactivity it is> ester~ carboxylic acid
> amide
lecture 1- carbonyls and tautomerisation
2
this is due to the dipolar nature which reduces the delta +
carbon through resonance
Carbonyl Group as Electrophile: Two recurring mechanisms
(Nucleophile = e
...
H-, HO-, Cl-, RO-, RMgBr, RLi)
Nuclophilic addition
due to absense of
leaving group and
acidic work up to
form alcohols
1
...
Nucleophilic Addition: In the absence of an appropriate
leaving group, the oxyanionic intermediate can be worked
up with
acid to yield an alcohol
leaving groups
good LG are neutral molecules or stable anions
nucleophilic attack
by Grignard and
the acidic workup
(protonation)
mechanism to
produce alcohol
the weaker the basic strength the better the LG and a low pKa
value
But alkoxy groups are BETTER LG than hydroxy groups
Remember, a hydroxy group would be deprotonated with a
strong base (HOthe strong base does NOT deprotonate the alkoxy group
...
e Nuc attack of
hydride on carbonyl (use Sodium borohydride, NaBH4)
B: Reduction of an ester (or carboxylic acid or amide) (use
Lithium aluminium hydride, LiAlH4)
lecture 1- carbonyls and tautomerisation
5
tautomerisation
In addition to existing as an electrophile (keto form), most
carbonyl compounds with an alpha-proton, can exist as a
nucleophile (enol form)
carbon adjacent to C=O
and that has a proton
shifted double bond
enol form
keto-enol carbonyl compound have two potential sites for
reactions
keto form
dominates with
exceptions
Tautomerisation; process of intramolecular proton transfer
Keto-Enol Tautomerisation: shift of double bond and a proton
transfer
Enolisation: tautomerisation reaction to form enol from keto
compound
Keto-Enol tautomerisationn Example
exceptions include
loss of aromaticity
lecture 1- carbonyls and tautomerisation
6
and in 1,3-carbonyl
compounds enol is
stabilised due to
conjugation and
intramolecular H
bonds
Dimedone exists as a mixture of both the keto and enol
tautomers
Enol form favoured : stabilised by conjugation (more later)
Keto-enol equilibrium
Main exceptions where the enol form dominates:
phenol
keto form means loss of aromaticity &
enol form is stabilised by aromaticity
lecture 1- carbonyls and tautomerisation
7
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SUMMARY: Tautomerisation; process of intramolecular proton transfer
...
keto
compunds favoured with exceptions of aromaticity and 1,3-dicarbonyl
compunds due to conjugation and intramolecular H bonding
...