enolization

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                             Enolization

       Course 
code:

                                    CHEM-522

 
     Submitted 
By:

                             Zunaira Arshad

                            17101707-043

                            M.Phil 
B

       Submitted To:

                       Dr.Sajjad 
Sumerra 

      

 

             

                  Enolisation

Definition:

              A   process 
in which an aldehyde or ketone is converted  into its 
respective Enole  is called  enolization or keto-enol  tautomerism.[1]

           Chemistry  of Carbonyl compound

v
Nature of carbonyl
compound and Reactivity;

Carbonyl compound is very versatile
and it contains partial positive charge on carbon and partial negative charge
on oxygen.

                   

Important points in its chemistry
are;

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
Planner.

      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.

   
CH3CH3            

                        

                    

Actually two main factors
responsible for acidity of carbonyl compounds are;

1. 
Inductive effect of
carbonyl compound  increases the pocitive
character of ?-hydrogen

2. 
Resonance
stabilization of conjugate base

Since above values shows that
aldehyde and ketones are more acidic 
then sp3 hybridized Carbon 
as alkanes.Because

After the removal of ?-hydrogen the conjugate base formed is more stable in
carbonyl containing compounds due to resonance than sp3 hybridized
carbon.

                        

                

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 
OH group

As  
en  for alkene and ol for alcohol.

 
While when carbonyl compound losses its alpha-proton the resulting ion
formed is called  Enolate anion

                               Enolization

‘When carbonyl compound after
lossing alpha-hydrogen changed into enol’

                        enol

Enolization in neutral media is a
slow process  so we do enolization in
acidic or

Basic media.

 Types
of Enolization:

1.               
Acid catalysed Enolization:

                           

Step  1:

           Transfer
of proton  from acid to oxygen of carbonyl
compound.

Protonation increases acidity of alpha-hydrogen now
a neutral or weak

Basic solvent easily 
can accept proton from acid.

          

       Step 2:

                Proton is removed  from alpha-C of carbonyl compound by a
bronsted

    Base H2O.

         

                           

2.               
 Base catalysed Enolization:

             

Step
1:

        Alpha-hydrogen of  carbonyl compd removed by base

              

 

Step  2:

          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.

Mobile’

    Carbonyl 
compd behaves electrophile as well as nucleophile as they exist in two

Forms
one is carbonyl containing i.e electrophile while other is enol form that
behaves as

Nucleophile.

                 Experimental proof of
keto-enol form;

           

Its
IUPAC name is DIMEDONE

Keto
form has  4 peaks for OH, keto gp,
c-c,c=c.but above structures can be studied in these ways.

1.IR

2.1HNMR

3.13CNMR

IN IR Functional group region:

      For
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.

IR:

    Tells
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.

1HNMR:

         
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.

                              

13CNMR:

        IN
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
on pH.

Why enolization
shifted towards carbonyl compounds?

Since
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              

Combination
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.

                 Stabilized  enols:

                       
              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.

                        

Similarly
1,3 arrangement of two carbonyl group in diketone makes the enol form more
stable due to

1.conjugation
of double bond with carbonyl group

                    

                                                        
                       

2.inter
molecular hydrogen bonding

               

                                    

                Requirement 
of Enolization( on  basis of
orbital):

       Any 
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.

 

                         

     The  oxyanion
and  carbanion  structures are just two ways of representing
same thing and this thing can be visualized 
on the basis of orbitals.

                                  Consider the
pi-orbitalof allyl anion and enolate ion.

 
Populated  pi orbital of an allyl
anion                       Populate pi
orbital of an enolate

     
                                                                                                                                         

 Reactive 
HOMO                                                        
Reactive  HOMO

                                                                                  

                           

     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.

1.Due
to  more electronegativity of O2
both orbitals goes down in energy.

2.The
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.

                                 In the
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.

                  

                            

   Enol
equivalents:

                       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.

Thes
enolates are more substituted and more stable while kinetic enolates are those
formed faster

 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.

(b)
Azo enolates:

                      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

                          

Nitriles(cyanides):

               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
allene.

              

  (d) cyclic 
compounds:

        Cyclic ketones can also form enol and
enolates

                                 

        Similarly cyclic aldehydes also form
enols and enolates anions

                                                                     Enolization and synthetic
routes:

                   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
“ketone” formation.

                             COMMENT:

                     “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

1.Alpha
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

                 

            Mechanism

                 

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
media.

 Haloform Reaction:

       Methyl ketone  reaction in presence of excess halogen and
base  Haloform  prepared .

In
this reaction enolate ion involve.

         

Nucleophilic
substitution:

          Enolate form give nucleophilic
substituted product.

                             

Alkylation:

   Enolates  can also give alkylated products.Second reaction
is useful in regioselective  using  LDA with ketone.

                      

                          

   BIOLOGICAL IMPORTANCE OF  ENOLATES:

                                   

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