enolization

               Assignment  No  2            Topic:                             Enolization       Course code:                                    CHEM-522      Submitted By:                             Zunaira Arshad                            17101707-043                            M.Phil B       Submitted To:                       Dr.Sajjad Sumerra                                         EnolisationDefinition:              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 compoundvNature of carbonylcompound and Reactivity;Carbonyl compound is very versatileand it contains partial positive charge on carbon and partial negative chargeon oxygen.                   Important points in its chemistryare;1.Carbon and Oxygen are sp2 hybridized2.Carbon form single bond with other R  groups3.Two lone pairs are present on oxygen atomThe geometery or framework ofcarbonyl  compound is  TrigonalPlanner.      While the reactivity of carbonyl compound  is  dueto the presence of  acidic ?-hydrogen.                          Since in alkanes hydrogen is bondedto carbon by sp3 hybridization but the electronegativity differencebetween carbon and hydrogen is  so smalldue to which the attatched hydrogen is less acidic.While in the case ofaldehyde or ketone due to the presence of carbonyl group to the ?-carbon the?-hydrogen becomes  more acidic.

But stillthis ?-hydrogen is less acidic then hydrogen of H2O.   CH3CH3                                                          Actually two main factorsresponsible for acidity of carbonyl compounds are;1. Inductive effect ofcarbonyl compound  increases the pocitivecharacter of ?-hydrogen2.

 Resonancestabilization of conjugate baseSince above values shows thataldehyde and ketones are more acidic then sp3 hybridized Carbon as alkanes.BecauseAfter the removal of ?-hydrogen the conjugate base formed is more stable incarbonyl containing compounds due to resonance than sp3 hybridizedcarbon.                                          Similarly aldehyde and ketone aremore acidic than ethers for the same reason which is described above and ethersare less acidic because after removal of ?-hydrogen in ethers the non-bondingelectrons of OR and O compete for resonance.                When ?-hydrogen is present betweentwo 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 intermediatestructure 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 ionformed is called  Enolate anion                               Enolization‘When carbonyl compound afterlossing alpha-hydrogen changed into enol’                        enolEnolization in neutral media is aslow process  so we do enolization inacidic or Basic media. Typesof Enolization:1.               Acid catalysed Enolization:                            Step  1:           Transferof proton  from acid to oxygen of carbonylcompound.

Protonation increases acidity of alpha-hydrogen nowa neutral or weak Basic solvent easily can accept proton from acid.                 Step 2:                Proton is removed  from alpha-C of carbonyl compound by abronsted     Base H2O.                                       2.                Base catalysed Enolization:              Step1:        Alpha-hydrogen of  carbonyl compd removed by base               Step  2:          Proton transfer to carbonyl oxygen bybronsted acid as  H2O              Difference between above two types is that thesteps involve are reverse.

As proton abstraction is first step in base catalysedenolization while it is second step in acid catalysed enolization.But thesimilarity in both steps is that the rate determining step is that where  abstraction occur.Second similarity is thatboth consist of two steps.Mobile’    Carbonyl compd behaves electrophile as well as nucleophile as they exist in twoFormsone is carbonyl containing i.e electrophile while other is enol form thatbehaves as Nucleophile.

                 Experimental proof ofketo-enol form;           ItsIUPAC name is DIMEDONEKetoform has  4 peaks for OH, keto gp,c-c,c=c.but above structures can be studied in these ways.1.IR2.1HNMR3.

13CNMRIN IR Functional group region:      Forketo form peaks for C-H,C-H(out of ring),C-C,C=O(sharp) form but in enol formin addition to above four  C=C,OH (broad)will also appear.IR:     Tellsthat in practice peaks for C-H(in ring),C-H(out of ring),C-C,C=O observe but inactual practice C=C,OH(broad) are found (finger print region).it means bothcompounds are present C=O keta and OH enol form.1HNMR:         Gives 3 1HNMR peaks for keto form and 5 in case of enolform.Total 7 peaks  appear because bothpeaks for CH3 combine and their intensity increases.1HNMRshows it is not a pure compound but both keto and enol forms exists.                               13CNMR:        INketo form it gives 5 peaks while in enol form it gives 7 peaks.But 5 carbon ofketo and 7 carbon of enol overlap and gives 10 peaks total.

               We cannot purify orseparate these  compounds  but which product form depends on predominantform of compound. Sinceenolization is transfer of proton from carbon to oxygen still it has no effecton pH.Why enolizationshifted towards carbonyl compounds?Sinceequilibrium shifts towards more stable reactant.e.

g simple carbonyl compoundsas acetone have traces of enol under ordinary set of conditions so equilibriumlies well  towards carbonyl compound.Thisis due to C-H,C=C,C=O combination is less stable than               Combinationof C=O,C-H bondhere balance between bond energies is quite fine .On one hand OHbond in enol is stronger than C-H.But on other hand C=O is more stable than C=C.So under normalconditions in a solution one part of enol is present in hundred parts ofketone.Ketone is more stable than enolby some45-60kJ/mol the reason behind isthe C=O stronger bond than C=C bond.                 Stabilized  enols:                                     Some  structural features stabilizes the enol formmore stable.

e.g phenolic form of 2,4-cyclohexadienone  than keto form by gaining aromatic ring  structure.                         Similarly1,3 arrangement of two carbonyl group in diketone makes the enol form morestable due to1.conjugationof double bond with carbonyl group                                                                                                    2.intermolecular hydrogen bonding                                                                     Requirement of Enolization( on  basis oforbital):       Any organic compound  with 1 electronwithdrawing functional groupwith at least 1 pi-bond joined to such saturatedcarbon atom and also having at least 1 H-atom can form enol in acidic orneutral media.

Some form enol in basic media with exceptions beingcarboxylic acid,primary and secondary amines.                      The enolate ion is justlike an alkoxide ion but more stable than corresponding saturated one becauseit is conjugated.            One important difference  between enol and enolates is that th delocalizationin later.This delocalization actually cause of its stability.

                                The  oxyanionand  carbanion  structures are just two ways of representingsame thing and this thing can be visualized on the basis of orbitals.                                  Consider thepi-orbitalof allyl anion and enolate ion. Populated  pi orbital of an allylanion                       Populate piorbital of an enolate                                                                                                                                               Reactive HOMO                                                        Reactive  HOMO                                                                                                                  Allyl anion  is more stable and in enolate ion  1 carbon of allyl anion replaced by anoxygen.This replacement result in 2 important changes.1.Dueto  more electronegativity of O2both orbitals goes down in energy.

2.Theorbitals are distorted.                             The lower energyatomic orbitals of more electronegative O2 contributes to the  more energetic orbitals    less to .                      Charge distribution comesfrom 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 theenolates the oxygen atom has the more  ofnegative charge but carbon atom  has moreof the HOMO.In other words we can say oxygen is hard nucleophilic centre whilecarbon is soft centre.Here  the reactionswhich dominated  by charge andelectrostatic interactions will  occur onoxygen and those dominated by orbital interactions  occur on carbon.          Thus acyl chloride tend to react onoxygen to give ester while allyl halide tend to react on carbon.                                                   Enolequivalents:                       Compounds other thancarbonyl group can aiso similar reactions called enol equivalents.

e.g imines,enamine also show similar tautomerism.   Some are thermodynamic enolates whichpredominates in thermodynamic control conditions.

Thesenolates are more substituted and more stable while kinetic enolates are thoseformed faster These occur in aprotic solvent and they  remove the less hindered alpha-hydrogen                                                                                        In primary amine (aniline) stable imineis formed but in secondary amine(cyclic structure) iminium salt formed which isless stable than enamine.(b)Azo enolates:                      As enamine is nitrogenanalog of enol  azo enolates made bydeprotonation of enamine with strong base.                           Similarly  nitroalkane form  enolate like anion  which are quite weak bases.Here basecatalysed enolization  by transfer ofproton 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 orallene.                (d) cyclic compounds:        Cyclic ketones can also form enol andenolates                                          Similarly cyclic aldehydes also formenols and enolates anions                                                                     Enolization and syntheticroutes:                   1.

The carbonyl group is  accepting proton  in enolization and nucleophilic attack.                   2.The same compd which iselectrophilic are easily ionizable.Hence acyl halides are      easily ionizable.                   3.To avoid nucleophilicattack we cannot use Cl-  ionas base.So nucleophilic base like tri amine should used.                   4.

The resulting enolate isunstable and eliminate Cl- or halide ion, agood leaving group for“ketone” formation.                             COMMENT:                     “This is best method forpreparation of dichloroketone from dichloroacetylchloride as the proton removedis very acidic”                                 In basic medium the carboxylic acid do notform the enolate anion easily.Here in basic media 1st proton isremoved .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 groupfrom acid and new OH group of the enols are equivalent                      REACTIONS  THAT PROCEED VIA  ENOL OR ENOLATES:              These important reactions are given by1.Alphahalogenations of carbonyl compounds:                    It is type of substitutionreaction.The  hydrogen of carbonyl is nottake part in reaction only alpha- hydrogen is take part in reaction.thesolvents used are water chloroform,acetic acid,diethyl ether etc. In acidic media                              Mechanism                 In  Basic media:                        The difference between acidic and basic mediais that in acidic media electron withdrawing halogen decreases the basicity ofoxygen of carbonyl compound by protonation while in basic media halogen  increases the acidity of alpha-hydrogenthat’s why halogenations is faster and favourable in basic media than acidicmedia. Haloform Reaction:       Methyl ketone  reaction in presence of excess halogen andbase  Haloform  prepared .

Inthis reaction enolate ion involve.          Nucleophilicsubstitution:          Enolate form give nucleophilicsubstituted product.                              Alkylation:   Enolates  can also give alkylated products.Second reactionis useful in regioselective  using  LDA with ketone.                                                    BIOLOGICAL IMPORTANCE OF  ENOLATES:                                   

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