11

Basic Concepts of

Organic Chemistry

22

CarbocationsCarbocations Carbocation:Carbocation:

• a species in which a carbon atom has only six electrons in its valence shell and bears positive charge

Carbocations are:• classified as 1°, 2°, or 3° depending on the number of carbons bonded to the carbon bearing the positive charge

• electrophiles; that is, they are electron-loving

• Lewis acids

33

Carbocation StabilityCarbocation Stability• relative stability

• methyl and primary carbocations are so unstable that they are never observed in solution

M ethyl cation(m ethyl)

Ethyl cation(1°)

Isopropyl cation(2°)

tert-Butyl cation(3°)

Increasing carbocation stability

+ + + +CH

HCH3 CCH3

CH3

HCCH3

CH 3

CH3CHH

H

44

Carbocation StabilityCarbocation Stability

• we can account for the relative stability of carbocations if we assume that alkyl groups bonded to the positively charged carbon are electron releasing and thereby delocalize the positive charge of the cation

• we account for this electron-releasing ability of alkyl groups by (1) the inductive effect, and (2) hyperconjugation

55

HC

H H

CH 3C

H H

CH 3C

H3C H

CH 3C

H3C CH 3

MethylCarbocation

PrimaryCarbocation

SecondaryCarbocation

TertiaryCarbocation

LEASTSTABLE MOST

STABLE

The methyl groups have +I inductive effects.

Carbon atom is electron deficient (only has 6 electrons in its outer valence).

Thus, extra electron density is ‘pushed’ onto the carbocation, which stabilises the carbocation.

CH 3

CH3C CH 3

66

The Inductive EffectThe Inductive EffectThe polarization (polarity) of a bond INDUCED by by

adjacent polar bond is known as the Inductive effect.inductive effect of an atom or functional group is a function of that groups

1). Electronegativity 2). Charge 3). Position within a structure. Inductive effects refer to those electronic effects of an atom or functional group can contribute through single bonds such as saturated (sp3) carbon atoms

77

The Inductive EffectThe Inductive EffectIt involves σ electrons. The σ electrons which form a covalent bond are seldom shared equally between the two atoms. This is because different atoms have different electronegativity values, i.e., different powers of attracting the electrons in the bond. Consequently, electrons are displaced towards the more electronegative atom introducing a certain degree of polarity in the bond. The more electronegative atom acquires a small negative charge (δ-). The less electronegative atom acquires a small positive charge (δ+).

88

Atoms or functional groups that are electronegative relative to hydrogen such as the halogens, oxygen, nitrogen, etc. may have a negative inductive effect (-I). Thus these atoms withdraw electron density through the single bond structure of a compound. Consider the case of acetic acid,chloroacetic acid and trichloroacetic acid shown below. All three of these compounds can ionize (loss of proton from the carboxyl OH). The only difference between these three structures in the degree of chloro group substitution. Chlorine atoms are electronegativeand thus have a -I effect. Thus they can help stabilize a negative charge, and enhance the ionization of an acid.

ELECTRONEGATIVITY

99

Bonding order and charge: As mentioned above, it is important to consider both the electronegativity and bonding order when analyzing the inductive potential of an atom. For example, oxygen in a hydroxyl group (OH) is electron withdrawing by induction (-I) because the oxygen atom is relatively electronegative and is uncharged in that bonding arrangement. However, oxygen in an "alkoxide" (O-) structure is electron donating (+I) by inductionbecause in this bonding order (a single bond to oxygen) it has an "excess" of electron density.

Bonding order and charge

1010

The strength of the inductive effect produced by a particular atom or functional group is dependent on it's position within a structure. For example, the further from the site of ionization, the lower the inductive effect. This is illustrated in the example below where the acid with the chlorine atom positioned on a carbon atom nearer the reaction site (OH) is more acidic that the acid where the chlorine atom is positioned further away.

Bonding position

1111

The Inductive EffectThe Inductive EffectConsider the carbon-chlorine bond. As chlorine is more electronegative, it will become negatively charged with respect to the carbon atom.

Structure (1) indicates the relative charges on the two atoms. In (2), the arrow head placed in the middle of the bond indicates the direction in which the electrons are drawn. In (3), the more heavily shaded part shows the region in which the electron density is greatest.

(1) (2) (3)

1212

The Inductive EffectThe Inductive EffectThe inductive effect (I Effect) refers to the polarity produced in a molecule as a result of higher electro negativity of one atom compared to another.

The carbon-hydrogen bond is used as a standard. Zero effect is assumed in this case. Atoms or groups which lose electrons toward a carbon atom are said to have a +I Effect. Such groups will be referred to as electron-releasing. Those atoms or groups which draw electrons away from a carbon atom are said to have a –I Effect. Such groups will be referred to as electron-attracting.

1313

The Inductive EffectThe Inductive Effect

Some common atoms or groups which cause –I Effect Groups (Electron-attracting) are:

NO2> -CN > COOH > F > Cl > Br > I > OH > C6H5–

& atoms or groups which cause +I Effect Groups (Electron-releasing) :

(CH3)3C– > (CH3)2CH– > CH3CH2– > CH3–

1414

The Inductive EffectThe Inductive Effect Tertiary alkyl groups exert greater +I effect

than secondary which in turn exert a greater effect than primary.

An inductive effect is not confined to the polarization of one bond. It is transmitted along a chain of carbon atoms, although it tends to be insignificant beyond the second carbon.

The inductive effect of C1 upon C2 is significantly less than the effect of the chlorine atom on C1. The inductive effect results in a permanent state of the molecule and can be observed practically

1515

Applications of inductive Applications of inductive effecteffectIE have the following applications1.Strength of an acidCommonly the strength of an acid cab be calcualted for the pKa value. Greater the pKa less the strong will be the acid. The pKa is related to the electron donating and electron drawing group. For example

1616

Strength of an acidStrength of an acid

Formic acid Acetic acid

In the above acids formic acid is more strong than acetic acid. Because in acetic acid, with carbonly carbon EDG (CH3) is attached. In case of electron donation by methyl their will be abundance of electron (-ve charge) on Corbonly carbon. For the stability of Corbonly carbon EWG is required but CH3 is EDG. In acetic acid due to electron abundance on carbonly carbon it will not easily give H+ ions. Less the potential of releasing H+ ions less stong will be the acid.

1717

Strength of an acidStrength of an acidin case of FORMIC ACID there is attached EWG (H) with

carbonyl carbon. In case of EWG their will be electron deficiency on the next carbon. This electron deficient carbon (+ve charge) will stabilize by sharing of electron from oxygen and so H ions will be easily released.

Other examples Acetic acid (CH3 is EDG) Chloroacetic acid (Cl is EWG) Di-chloroacetic acid (2 Cl are EWG) Tri chloracetic acid (3 Cl are EWG)ALL chloro acetic acids are stronger than acetic acid.

1818

Substitution of electrophile in Substitution of electrophile in BenzeneBenzeneElectrophile are electronloving groups it will easily move 2ward nucleuphile (+ve charge). In case of reaction between nitro group and Toluene, due to the presence of methyl with benzene ring, which is electron releasing group will increase the electronic density on ortho and para position.

1919

The Mesomeric EffectThe Mesomeric EffectIt involves π electrons of double and triple bonds.The Mesomeric effect (M effect) refers to the polarity produced in a molecule as a result of interaction between two π bonds or a π bond and lone pair of electrons. The effect is transmitted along a chain in a similar way as are inductive effects. The Mesomeric effect is of great importance in conjugated compounds. (in which the carbon atoms are linked alternately by single and double bonds). In such systems, the π electrons get delocalized as a consequence of Mesomeric effect, giving a number of resonance structures of the molecule.

2020

The Mesomeric EffectThe Mesomeric EffectConsider a carbonyl group (>C=O). The oxygen atom is more electronegative than the carbon atom. As a result, the π electrons of the carbon-oxygen double bond get displaced toward the oxygen atom. This gives the following resonance structures :

2121

The Mesomeric EffectThe Mesomeric EffectThe mesomeric effect is represented by a curved arrow. The head of the arrow indicates the movement of a pair of π electrons. If the carbonyl group is conjugated with a carbon-carbon double bond, the above polarization will be transmitted further via the π electrons.

2222

In a system involving resonance the distribution of electron density is different from the system that does not involve resonance. For example, in ammonia where resonance is absent, the unshared pair of electrons is located on the nitrogen atom, however if one of the H atom is replaced by benzene ring the electron pain of N is delocalized over the ring and resulting the decrease of electron density on the N atom and the corresponding increase f electron density on the ring.

2323

2424

This decrease in electron density at one position and the corresponding increase elsewhere is called the RESONACE EFFECT OR MESOMERIC EFFECT. Thus –NH2 group in aniline donated electrons to the ring by the resonance effect or mesomeric effect.

2525

The Mesomeric EffectThe Mesomeric EffectThe mesomeric effect like the inductive effect may be positive or negative. Atoms or groups which lose electrons toward a carbon atom are said to have a +M Effect. Atoms or groups which draw electrons away from a carbon atom are said to have a –M Effect Some common atoms or groups which cause(a) +M Effect are:Cl, Br, I, NH2, NR2, OH, OCH3

&(b) –M Effect are:NO2, CN, >C=O

2626

The Mesomeric EffectThe Mesomeric EffectThe +M effect of the bromine atom is:

The -M effect of the Nitro group is:

2727

Significance of Mesomeric EffectsSignificance of Mesomeric EffectsThe mesomeric effect has an appreciable influence

on the physical properties and the chemical reactivity of the organic compounds. For example compare the acidity if phenol with that of ethanol, the acidity of both is the result of the dissociation of O-H bond yet phenol (pKa = 10) is more acidic that ethanol (pKa =17).

CH3CH2OH + H2O CH3CH2O- + H3O+ Ph-OH + H2O Ph-O- + H3O+ The enhanced acidity of phenol can be attributed to

the –ve charge distribution which is not possible in ethoxide ion. Phenol therfore has much tendency to lose proton and behave as an acid.

2828

Nitro group further enhances the acidity of phenol particularly in the ortho and para position because it further delocalizes the negative charge over to the nitro group and increasing the number of contributing structures.

2929

Nitro groups stabilize the phenolate ion by resonance electron withdrawal that allows the negative charge to be moved to an electronegative oxygen atom in the nitro group when the nitro group is ortho- or para- to the -OH group. The more nitro groups there are in these positions, the greater the stabilization of the phenolate and the more acidic the phenol.

3030

The basicity of anime is very sensitive to resonance effect. For example aniline is a weaker base than aliphatic amines because the electron pair on the N atom which is responsible for the basic strength of the amines is delocalized over the aromatic ring in aniline and is not available for protonation to the same extent as in the case of aliphatic amines where such delocalization is not possible.

3131

HyperconjugationHyperconjugation

The relative stability of various classes of carbonium ions may be explained by the number of no-bond resonance structures that can be written for them. Such structures are arrived at by shifting the bonding electrons from an adjacent C–H bond to the electron-deficient carbon. In this way, the positive charge originally on carbon is dispersed to the hydrogen. This manner of electron release by assuming no-bond character in the adjacent C–H bond is called Hyperconjugation or No-Bond Resonance.

3232

HyperconjugationHyperconjugation

H

HH H

HHH

HH H

ethyl carbocation

3333

HyperconjugationHyperconjugationThe more hyperconjugation structures (no-bond resonance structures) that can be written for a species, the more stable is the species. For example,

(1) Ethyl carbonium ion is stabilized by three hyperconjugation structures:

(2) Isopropyl carbonium ion is stabilized by six hyperconjugation structures.

(3) t-Butyl carbonium ion is stabilized by nine hyperconjugation structures.

3434

HyperconjugationHyperconjugationThus, the following order of stability holds :

In general, resonance effects (mesomeric effects) are more important than hyperconjugation effects.

The allyl and benzyl carbonium ions are more stable than most alkyl carbonium ions because the former are stabilized by resonance while the latter are stabilized by hyperconjugation.

3535

ConjugationConjugationA diene is said to be conjugated when its double bonds are not directly next to each other, but rather separated by a single bond in between them (CH2=CH-CH=CH2). Conjugated dienes are particularly stable due to the delocalization of the pi electrons along sp2 hybridized orbitals, and they also tend to undergo reactions atypical of double bond chemistry. For instance, chlorine can add to 1,3-butadiene (CH2=CH-CH=CH2) to yield a mixture of 3,4-dichloro-1-butene (ClCH2-CHCl-CH=CH2) and 1,4-dichloro-2-butene (ClCH2-CH=CH-CH2Cl). These are known as 1,2 addition and 1,4 addition, respectively. 1,2-addition is favored in mild reaction (irreversible) conditions (the kinetically preferred product) and 1,4-addition is favored in harsher reaction (reversible) conditions (which results in the thermodynamically preferred product).

3636

The ResonanceThe ResonanceA number of organic compounds cannot be accurately represented by one structure. For example, benzene is ordinarily represented as :

This structure has three carbon-carbon single bonds and three carbon-carbon double bonds. However, it has been determined experimentally that all carbon-carbon bonds in benzene are identical and have the same bond length (1 .42Å). Furthermore, the carbon-carbon bond length of (1.42 Å) is intermediate between the normal carbon-carbon double-bond length (1.33 Å) and the normal carbon-carbon single-bond length (1.52 Å). Actually two alternative structures (1 and 2) can be written for benzene :

1 2

3737

1.33 Å 1.52 Å

1.42 Å 1.42 Å

3838

The ResonanceThe ResonanceThese two structures differ only in the position of electrons. Neither (1) nor (2) is a correct representation of benzene. The actual structure of benzene lies somewhere between these two structures.This phenomenon in which two or more structures can be written for a compound which involve identical positions of atoms is called Resonance.

3939

The ResonanceThe Resonance The actual structure of the molecule is said to be a

Resonance Hybrid of various possible alternative structures. The alternative structures are referred to as the Resonance Structures or Canonical Forms. A double headed arrow (↔) between the resonance structures is used to represent the resonance hybrid. Thus in the case of benzene, (1) and (2) represent the resonance structures. Actual structure of the molecule may be represented as hybrid of these two resonance structures, or by the single structural formula (3).

It should be clearly understood that the resonance structures (1) and (2) are not actual structures of the benzene molecule. They exist only in theory. None of these structures adequately represents the molecule. In resonance theory, we view the benzene molecule (which is of course a real entity) as being hybrid of these two hypothetical resonance structures.

4040

The Resonance EnergyThe Resonance EnergyThe resonance hybrid is more stable than any one of the various resonance structures. The difference in energy between the hybrid and the most stable resonance structure is known as the Resonance Energy. Resonance energy can be determined by the difference between the calculated and experimental heats of combustion (energy given off as heat when one mole of compound is burned) of the compound. For example, it has been calculated that the hypothetical structure (1) or (2) would have a heat of combustion of 797 Kcal/mole. The measured value for the heat of combustion of benzene is 759 Kcal/mole. Therefore, the resonance energy of benzene is (797–759) Kcal/mol. The benzene is said to be "stabilised" by a resonance energy of 38 Kcal/mole

4141

The ResonanceThe ResonanceAnother species that is not correctly represented by a single structure is the acetate ion. As in the case of benzene, acetate ion is a hybrid of two resonance structures. Both carbon-oxygen bonds in the acetate ion are identical and have the same bond length (1.26 Å). The carbon-oxygen bond length of 1.26 Å is intermediate between the normal carbon-oxygen double-bond length (1.20 Å) and the normal carbon-oxygen single-bond length (1.43 Å).

4242

The Resonance (Governing Rules)The Resonance (Governing Rules)1. Resonance occurs whenever a molecule can be

represented by two or more structures differing only in the arrangement of electrons, without shifting any atoms. Resonance only involves the delocalization of electrons.

2. Resonance structures are not actual structures for the molecule. They are nonexistent and hypothetical.

3. Resonance structures are interconvertible by one or a series of short electron-shifts. For example,

4343

The Resonance (Governing Rules)The Resonance (Governing Rules)4. Resonance hybrid represents the actual structure

of the molecule. The structure of the resonance hybrid is intermediate between the various resonance structures and is not a mixture of them.

5. Resonance hybrid is represented by a double headed arrow (↔). This should not be confused with the two arrows ( ) used to denote equilibrium between two different compounds.

6. Resonance hybrid is more stable than any of its contributing forms (resonance structures).

7. Resonance always increases the stability of a molecule and lessens its reactivity.

4444

The Hydrogen BondingThe Hydrogen BondingA bond formed between a functional group (H - A) and an other atom (B) or group within the same molecule or different molecule is called Hydrogen bonding. OR Hydrogen bonding is an attractive force which occurs in any compound whose molecules contain O–H or N–H bonds (as in water, alcohols, acids, amines, and amides) or any EN atom. The O–H bond, for example, is a highly polar bond. Oxygen is more electronegative than hydrogen and pulls the bonding electrons closer to it. As a result of this displacement, the oxygen atom acquires a small negative charge (δ–) and the hydrogen atom a small positive charge (δ+).

Adjacent molecules of the compound containing an O–H bond will be attracted to each other by means of these opposite charges. This force of attraction is known as the Hydrogen Bond. Usually a hydrogen bond is represented by a dotted line.

4545

The Hydrogen BondingThe Hydrogen Bonding Consider the following HB H2O – H20 NH3 - H20 In all the above examples we saw that in all cases the

two E.N atoms are linked due to H atoms. Besides this the strength of H-Bond will depend on the value of electronegativity for examples in H-F the H-Bond is most stronger as compared to HCl, HBr and HI. This is only due to EN. The order of Hydrogen bond strength in haloges atoms will be decreased from TOP to the BOTTOM.

H----F H----Cl H-----Br H-----I

4646

The Hydrogen BondingThe Hydrogen Bonding Types of Hydrogen bonding There are two type of Hydrogen bonding 1. Intermolecular HB 2. Intramolecular BH 1. INTERMOLECULAR HB Intermolecular HB exist between two SAME molecules or DIFFERENT molecules.

Examples HF-HF CH3-O-CH3 (Dimethly ether) and H20

4747

The Hydrogen BondingThe Hydrogen Bonding 2. INTRAMOLECULAR HBIntramolecular HB ocurres WITHIN THE SAME molecule and

sometime it is called as INTERNAL HB.For example SALICYLIC ACID

Some times the intramolecular HB results in the formation of a 2nd psudo ring like in salicyladehyde. The formation of an extra ring is know as CHELATION (holding of a H atom between two atoms of the same molecule). In case of chelation the H atom finds itself a member of a six member ring.

4848

The Hydrogen BondingThe Hydrogen Bonding Energy of HBThe HB is much weaker than ordinary covalent bond. The strength

of the HB are in the range of 8 – 42 kj/mol. In general the strength of HB increases with the acidity of hydrogen in H-A and the basicity of B. For example the HB energy for HF-HF, H2O- H2O, NH3-NH3 is 41.84, 29.29 and 8.37 kj/mol respectively. In case of Fluorine (strong base) a very low polarization occure because of its electrons being close to and tightly held by the nucleus, form a stronger HB.

If the H atom (present in strong acid) is too strong acidic and the acceptor atom is too strong basic, the H atom will shfit as a proton to form a covalent bond with the acceptor atom in a simple acid base reaction.

4949

The Hydrogen BondingThe Hydrogen BondingThe strengths of hydrogen bonds (5 to 10 Kcal per bond) are much less than the strengths of ordinary covalent bonds. However, they have a very significant effect on the physical properties (boiling points, solubility) of organic compounds.Effect on Boiling Points:

It is understandable that substances having nearly the same molecular weights, have the same boiling point. The boiling points of alkanes and ethers of comparable molecular weights are not far apart, but the boiling points of alcohols having almost equal molecular weights are considerably higher.CH3--CH2---CH3 CH3—O— CH3 CH3—CH2—OHPropane Dimethyl ether Ethanol(MW 44 ; bp –45°C) (MW 46 ; bp –25°C) (MW 46 ; bp +78°C)

This can be explained on the basis of hydrogen bonding. Ethanol forms hydrogen bonds. Extra energy in the form of heat is required to break the hydrogen bonds holding the molecules together before it can be volatilized. Propane and dimethyl ether do not form hydrogen bonds and, therefore, have low boiling points.

5050

The Hydrogen BondingThe Hydrogen BondingEffect on Water Solubility:

A hydrogen-bonded substance is usually soluble in another hydrogen-bonded substance. For example, alcohols are soluble in water but alkanes are not. This is because a nonpolar alkane molecule cannot break into the hydrogen-bonded sequence in water. It cannot replace the hydrogen bonds that would have to be broken to let it in.

An alcohol molecule is capable of hydrogen bonding. It can slip into the hydrogen bonded sequence in water. It can replace the hydrogen bonds that must be broken to let it in.

5151

The Hydrogen BondingThe Hydrogen BondingEffect on Water Solubility:

Thus alcohols of low molecular weight are water soluble. However, when the alkyl group (R–) is four or more carbons in length the alkane nature of the molecule predominates, and water solubility falls off sharply. Alcohols containing more than seven carbons are insoluble in water.

5252

The Hydrogen BondingThe Hydrogen Bonding Effect on volatilityVolatility increase by increasing intramolecular HB. In

case of chelation in salicylaldehyde the BP is lower expectedly because in this case the molecule behaves as monomer and is therefore easy to volatilize. The chelated salicylaldehyde boils at 196oC and can easily vaporize while its para or mata isomor boils above 240oC and not vaporize through steam distillation.

In case of o-nitrophenolThe solubility of o-nitrophenol is water is lower as

compared to its para and mata isomer because in case of ortho isomor the volatiliy increases and solubility decreased. While in case of para isomor the volitily decreases and solubility in water increases due to fromtion of intermolecular HB.

5353

The Hydrogen BondingThe Hydrogen Bonding Effect on acidityO-Hydroxybenzoic acid (salicylic acid) is more acidic than para position. Because in ortho isomor the OH group is in a better position to sabilize the carboxylate ion, formed after ionization, by chelation.

5454

The Steric EffectThe Steric Effectthe effect of the structure of molecules on the STABILITY and REACTION of the compound is called steric effect.

Steric effects arise from the fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds (Pauli or Born repulsion), and this may affect the molecule's preferred shape (conformation) and reactivity. The size as well as the electronic properties (i.e. inductive and mesomeric effects) of the surrounding groups affects the stability of carbocations, carbanions and radicals. When bulky substituents surround a cation the reactivity of the cation to nucleophilic attack is reduced by steric effects. This is because the bulky groups hinder the approach of a nucleophile.

5555

The Steric EffectThe Steric Effect When the size of groups is responsible for

reducing the reactivity at a site within a molecule, this is attributed to steric hindrance. When the size of groups is responsible for increasing the reactivity at a site within a molecule, this is attributed to steric acceleration. Steric hindrance or steric resistance occurs when the size of groups within a molecule prevents chemical reactions that are observed in related smaller molecules. Although steric hindrance is sometimes a problem, it can also be a very useful tool, and is often exploited by chemists to change the reactivity pattern of a molecule by stopping unwanted side-reactions (steric protection).

5656

The Steric HindranceThe Steric Hindrance

Nucleophile approaches from the back side. It must overlap the back lobe of the C-X sp3 orbital.

5757

ExamplesExamples1. The methylation of 2,6-Ditertiary butyl pyridine

under high pressure is not possible as compare to the methylation of 2,6-Dimethyl pyridine. Because the possibility of the 2nd reaction is that methyl groups are less bulkier than tertiary butyl. in case of SN2 reaction the rate of reaction is inversely proportional to bulkeness of the attached groups.

2. PenicillinPenicillin is an antibiotic with the following chemical

formula,

5858

In the above structure R is related to the chemical activity of penicillin. If in place of R there is Benzyl group so this penicillin will be called benzyl penicillin. Now Benzyl group is not bulky so the enzyme produced by the bacteria know as beta lactam or penicillinase will attack on this penicillin and will rupture the beta lactam ring.

5959

In case of Cloxacillin, the R ihas been replaced by the bulky group and due to steric hinderance the beta lactamase enzyme can’t rupture the beta lactam ring.

6060

Keto Enol TautomerismKeto Enol Tautomerism(As a general rule enols are unstable)

C CO H

ol

ene

ENOLS :( have -OH attached to a double bond)Think of this combination as unstable.

OH

Phenols are not “enols” and they arevery stable (benzene resonance).

NOTE :

6161

Keto Enol TautomerismKeto Enol TautomerismNature of tautomerism:1. Carbonyl compounds with hydrogens bonded to their α carbons

equilibrate with their corresponding enols.2. This rapid equilibration is called tautomerism, and the individual

isomers are tautomers.3. Unlike resonance forms, tautomers are isomers.4. Despite the fact that very little of the enol isomer is present at

room temperature, enols are very important because they are reactive. For example, ethyl acetoacetate is an equilibrium mixture of the keto and enol form. At room temperature, the mixture contains 93% of keto-form plus 6% of the enol-form.

Mechanism of tautomerism:

1. In acid-catalyzed enolization, the carbonyl α carbon is protonated to form an intermediate that can lose a hydrogen from its carbon to yield a neutral enol.

2. In base-catalyzed enol formation, an acid-base reaction occurs between a base and an α hydrogen.i. The resultant enolate is potonated to yield an enol. ii. Protonation can occur either on carbon or on oxygen.iii. Only hydrogen on the α positions of carbonyl compounds are

acidic.

6262

Keto Enol TautomerismKeto Enol Tautomerism

C CH

OC C

OH

K

keto enolFor most ketones, the keto formpredominates in the equilibrium

6363

Acid-catalyzed Enol FormationAcid-catalyzed Enol Formation

CC

H:O:

H—A

Keto tautomer

CC

H

+:OH

:A-CC

:

:OH

+ H

Protonation of the carbonyl oxygen atom by an acid catalyst HA yield a cation that can be represented by two resonance structures.

+ HACC

:OH:

Enol tautomer

Loss of H+ from the α position by reaction with a base A- then yields the enol tautomer and regenerates HA catalyst.

Acid-catalyzed enol formation. The protonated intermediate can lose H+, either from the oxygen atom to regenerate keto tautomer or from the α carbon atom to yield an enol.

6464

Base-catalyzed Enol FormationBase-catalyzed Enol Formation

CC

H:O:

Keto tautomer

+ OH-

CC

:OH:

Enol tautomer

-:OH

::

CC

:O: I: C

C

:

:O:- H—O—H

::

Base removes an acidic hydrogen from the α position of the carbonyl compound, yielding an enolate anion that has two resonance structures.

Protonation of the enolate anion on the oxygen atom yields an enol and regenerates the base catalyst.

Base-catalyzed enol formation. The intermediate enolate ion, a resonance hybrid of two forms, can be protonated either on carbon to regenerate the starting keto tautomer or on oxygen to give an enol.