Assignment No 2
in which an aldehyde or ketone is converted into its
respective Enole is called enolization or keto-enol tautomerism.
Chemistry of Carbonyl compound
Nature of carbonyl
compound and Reactivity;
Carbonyl compound is very versatile
and it contains partial positive charge on carbon and partial negative charge
Important points in its chemistry
1.Carbon and Oxygen are sp2 hybridized
2.Carbon form single bond with other R groups
3.Two lone pairs are present on oxygen atom
The geometery or framework of
carbonyl compound is Trigonal
While the reactivity of carbonyl compound is due
to the presence of acidic ?-hydrogen.
Since in alkanes hydrogen is bonded
to carbon by sp3 hybridization but the electronegativity difference
between carbon and hydrogen is so small
due to which the attatched hydrogen is less acidic.While in the case of
aldehyde or ketone due to the presence of carbonyl group to the ?-carbon the
?-hydrogen becomes more acidic.But still
this ?-hydrogen is less acidic then hydrogen of H2O.
Actually two main factors
responsible for acidity of carbonyl compounds are;
Inductive effect of
carbonyl compound increases the pocitive
character of ?-hydrogen
stabilization of conjugate base
Since above values shows that
aldehyde and ketones are more acidic
then sp3 hybridized Carbon
After the removal of ?-hydrogen the conjugate base formed is more stable in
carbonyl containing compounds due to resonance than sp3 hybridized
Similarly aldehyde and ketone are
more acidic than ethers for the same reason which is described above and ethers
are less acidic because after removal of ?-hydrogen in ethers the non-bonding
electrons of OR and O compete for resonance.
When ?-hydrogen is present between
two carbonyl compounds then pka of ?-diketoester is more then pka value of ?-Diketones i.e
?-diketoester is less acidic then ?-dietones.
Enol and Enolate anion;
An enol is such intermediate
structure which has alkene attatched with
en for alkene and ol for alcohol.
While when carbonyl compound losses its alpha-proton the resulting ion
formed is called Enolate anion
‘When carbonyl compound after
lossing alpha-hydrogen changed into enol’
Enolization in neutral media is a
slow process so we do enolization in
Acid catalysed Enolization:
of proton from acid to oxygen of carbonyl
Protonation increases acidity of alpha-hydrogen now
a neutral or weak
Basic solvent easily
can accept proton from acid.
Proton is removed from alpha-C of carbonyl compound by a
Base catalysed Enolization:
Alpha-hydrogen of carbonyl compd removed by base
Proton transfer to carbonyl oxygen by
bronsted acid as H2O
Difference between above two types is that the
steps involve are reverse.As proton abstraction is first step in base catalysed
enolization while it is second step in acid catalysed enolization.But the
similarity in both steps is that the rate determining step is that where abstraction occur.Second similarity is that
both consist of two steps.
compd behaves electrophile as well as nucleophile as they exist in two
one is carbonyl containing i.e electrophile while other is enol form that
Experimental proof of
IUPAC name is DIMEDONE
form has 4 peaks for OH, keto gp,
c-c,c=c.but above structures can be studied in these ways.
IN IR Functional group region:
keto form peaks for C-H,C-H(out of ring),C-C,C=O(sharp) form but in enol form
in addition to above four C=C,OH (broad)
will also appear.
that in practice peaks for C-H(in ring),C-H(out of ring),C-C,C=O observe but in
actual practice C=C,OH(broad) are found (finger print region).it means both
compounds are present C=O keta and OH enol form.
Gives 3 1HNMR peaks for keto form and 5 in case of enol
form.Total 7 peaks appear because both
peaks for CH3 combine and their intensity increases.1HNMR
shows it is not a pure compound but both keto and enol forms exists.
keto form it gives 5 peaks while in enol form it gives 7 peaks.But 5 carbon of
keto and 7 carbon of enol overlap and gives 10 peaks total.
We cannot purify or
separate these compounds but which product form depends on predominant
form of compound. Since
enolization is transfer of proton from carbon to oxygen still it has no effect
shifted towards carbonyl compounds?
equilibrium shifts towards more stable reactant.e.g simple carbonyl compounds
as acetone have traces of enol under ordinary set of conditions so equilibrium
lies well towards carbonyl compound.This
is due to C-H,C=C,C=O combination is less stable than
of C=O,C-H bondhere balance between bond energies is quite fine .On one hand OH
bond in enol is stronger than C-H.But on other hand C=O is more stable than
C=C.So under normal
conditions in a solution one part of enol is present in hundred parts of
ketone.Ketone is more stable than enolby some45-60kJ/mol the reason behind is
the C=O stronger bond than C=C bond.
Some structural features stabilizes the enol form
more stable.e.g phenolic form of 2,4-cyclohexadienone than keto form by gaining aromatic ring structure.
1,3 arrangement of two carbonyl group in diketone makes the enol form more
stable due to
of double bond with carbonyl group
molecular hydrogen bonding
of Enolization( on basis of
organic compound with 1 electron
withdrawing functional groupwith at least 1 pi-bond joined to such saturated
carbon atom and also having at least 1 H-atom can form enol in acidic or
neutral media.Some form enol in basic media with exceptions beingcarboxylic acid,
primary and secondary amines.
The enolate ion is just
like an alkoxide ion but more stable than corresponding saturated one because
it is conjugated.
One important difference between enol and enolates is that th delocalization
in later.This delocalization actually cause of its stability.
and carbanion structures are just two ways of representing
same thing and this thing can be visualized
on the basis of orbitals.
pi-orbitalof allyl anion and enolate ion.
Populated pi orbital of an allyl
anion Populate pi
orbital of an enolate
Allyl anion is more stable and in enolate ion 1 carbon of allyl anion replaced by an
oxygen.This replacement result in 2 important changes.
to more electronegativity of O2
both orbitals goes down in energy.
orbitals are distorted.
The lower energy
atomic orbitals of more electronegative O2 contributes to the more energetic orbitals less to .
Charge distribution comes
from both populated orbitals so negative charge
spread over all the 3 atoms but mostly on ends.The important reactive orbital is HOMO which has
large orbital on terminal carbon.
enolates the oxygen atom has the more of
negative charge but carbon atom has more
of the HOMO.In other words we can say oxygen is hard nucleophilic centre while
carbon is soft centre.Here the reactions
which dominated by charge and
electrostatic interactions will occur on
oxygen and those dominated by orbital interactions occur on carbon.
Thus acyl chloride tend to react on
oxygen to give ester while allyl halide tend to react on carbon.
Compounds other than
carbonyl group can aiso similar reactions
called enol equivalents.e.g
imines,enamine also show similar tautomerism.
Some are thermodynamic enolates which
predominates in thermodynamic control conditions.
enolates are more substituted and more stable while kinetic enolates are those
These occur in aprotic solvent and they remove the less hindered alpha-hydrogen
In primary amine (aniline) stable imine
is formed but in secondary amine(cyclic structure) iminium salt formed which is
less stable than enamine.
As enamine is nitrogen
analog of enol azo enolates made by
deprotonation of enamine with strong base.
Similarly nitroalkane form enolate like anion which are quite weak bases.Here base
catalysed enolization by transfer of
proton stable oxyanion is formed.In CH3NO2
Strong base is needed here.The negative charge is delocalized b/w nitrogen and carbon of nitroalkane .The resulting anion is 3 atoms 4 electron system just as ketone or
Cyclic ketones can also form enol and
Similarly cyclic aldehydes also form
enols and enolates anions
Enolization and synthetic
1.The carbonyl group is accepting proton in enolization and nucleophilic attack.
2.The same compd which is
electrophilic are easily ionizable.Hence acyl halides are easily ionizable.
3.To avoid nucleophilic
attack we cannot use Cl- ion
as base.So nucleophilic base like tri amine should used.
4.The resulting enolate is
unstable and eliminate Cl- or halide ion, agood leaving group for
“This is best method for
preparation of dichloroketone from dichloroacetylchloride as the proton removed
is very acidic”
In basic medium the carboxylic acid do not
form the enolate anion easily.Here in basic media 1st proton is
removed .This thing protects the acids from the attack bynucleophile. E.g
In acidic solution no such problem occur and
“ene-diole” formed.The original OH group
from acid and new OH group of the enols are equivalent
REACTIONS THAT PROCEED VIA ENOL OR ENOLATES:
These important reactions are given by
halogenations of carbonyl compounds:
It is type of substitution
reaction.The hydrogen of carbonyl is not
take part in reaction only alpha- hydrogen is take part in reaction.the
solvents used are water chloroform,acetic acid,diethyl ether etc.
In acidic media
In Basic media:
The difference between acidic and basic media
is that in acidic media electron withdrawing halogen decreases the basicity of
oxygen of carbonyl compound by protonation while in basic media halogen increases the acidity of alpha-hydrogen
that’s why halogenations is faster and favourable in basic media than acidic
Methyl ketone reaction in presence of excess halogen and
base Haloform prepared .
this reaction enolate ion involve.
Enolate form give nucleophilic
Enolates can also give alkylated products.Second reaction
is useful in regioselective using LDA with ketone.
BIOLOGICAL IMPORTANCE OF ENOLATES: