ch1_Basic_concepts_Organic_sem1.ppt

1
Basic Concepts
of Organic
Chemistry

2
Carbocations
 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

3
Carbocation Stability
• relative stability
• methyl and primary carbocations are so unstable that
they are never observed in solution
Methyl
cation
(methyl)
Ethyl
cation
(1°)
Isopropyl
cation
(2°)
tert-Butyl
cation
(3°)
Increasing carbocation stability
+ + + +
C
H
H
CH3 C
CH3
CH3
H
C
CH3
CH3
CH3
C
H
H
H

4
Carbocation 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

5
H
C
H H
CH3
C
H H
CH3
C
H3C H
CH3
C
H3C CH3
Methyl
Carbocation
Primary
Carbocation
Secondary
Carbocation
Tertiary
Carbocation
LEAST
STABLE
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.
CH3
C
H3C CH3

6
The Inductive Effect
The 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

7
It 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+).

8
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

9
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 induction
because in this bonding order (a single bond to oxygen) it has an
"excess" of electron density.
Bonding order and charge

10
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

11
Consider 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)

12
The 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.

13
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–

14
 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

15
Applications of inductive effect
IE have the following applications
1. Strength of an acid
Commonly 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

16
Strength 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.

17
Strength of an acid
in 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.

18
Substitution of electrophile in Benzene
Electrophile 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.

19
The Mesomeric Effect
It 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.

20
Consider 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 :

21
The 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.

22
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.

24
 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.

25
The 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

26
The +M effect of the bromine atom is:
The -M effect of the Nitro group is:

27
Significance of Mesomeric Effects
The 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.

28
 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.

29
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.

30
 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.

31
Hyperconjugation
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.

32
Hyperconjugation
H
H
H H
H
H
H
H
H H
ethyl carbocation

33
Hyperconjugation
The 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.

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Hyperconjugation
Thus, 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.

35
Conjugation
A 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).

36
The Resonance
A 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

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1.33 Å 1.52 Å
1.42 Å 1.42 Å

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The Resonance
These 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.

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The 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.

40
The Resonance Energy
The 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

41
The Resonance
Another 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).

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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,

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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.

44
The Hydrogen Bonding
A 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.

45
 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

46
 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

47
 2. INTRAMOLECULAR HB
Intramolecular 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.

48
 Energy of HB
The 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.

49
The 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—OH
Propane 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.

50
Effect 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.

51
Effect 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.

52
 Effect on volatility
Volatility 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-nitrophenol
The 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.

53
 Effect on acidity
O-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.

54
The Steric Effect
the 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.

55
The 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).

56
The Steric Hindrance
 Nucleophile approaches from the back side.
 It must overlap the back lobe of the C-X sp3 orbital.

57
Examples
1. 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. Penicillin
Penicillin is an antibiotic with the following chemical formula,

58
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.

59
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.

60
Keto Enol Tautomerism
(As a general rule enols are unstable)
C C
O H
ol
ene
ENOLS :
( have -OH attached to a double bond)
Think of this combination as unstable.
OH
Phenols are not “enols” and they are
very stable (benzene resonance).
NOTE :

61
Nature 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.

62
C C
H
O
C C
O
H
K
keto enol
For most ketones, the keto form
predominates in the equilibrium

63
Acid-catalyzed Enol Formation
C
C
H
:O: H—A
Keto tautomer
C
C
H
+:O
H
:A-
C
C
:
:O
H
+ H
Protonation of the carbonyl oxygen
atom by an acid catalyst HA yield a
cation that can be represented by
two resonance structures.
+ HA
C
C
:O
H
:
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.

64
Base-catalyzed Enol Formation
C
C
H
:O:
Keto tautomer
+ OH-
C
C
:O
H
:
Enol tautomer
-:OH
:
:
C
C
: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.

ch1_Basic_concepts_Organic_sem1.ppt

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