15046 Organic Chemistry

CHE101 (B.Tech. Chemistry Course)

Lecture - 23

Basic of Electron Displacement Effect

Prepared By

Dr. ASHISH KUMAR

Department of Chemistry Lovely Professional University, Phagwara, Punjab, India.

E-mail: [email protected]

Prepared by:Dr. Ashish Kumar 1

Inductive effect


TYPES OF INDUCTIVE EFFECTSTYPES OF INDUCTIVE EFFECTSELECTRONWITHDRAWINGGROUPS

ELECTRONDONATINGGROUPS

Cl-

C CH3

-

C

F, Cl, Br, N, O R, CH3, B, Sielectronegative elements take electron densityfrom cabon

alkyl groups and elements less electronegative than carbon donate electron density to carbon

These electron withdrawing and donating groups work throughthe sigma bond system, unlike the similarly named resonance

groups that work through the system.

Inductive effect


Inductive EffectsElectronic effects that are transmitted through

space and through the bonds of a moleculeThe effect gets weaker with increasing distance

Permanent displacement of electron along the chain of carbon atoms due to presence of atom or group of atom having different electronegativity at the end of the carbon chain.

Electron withdrawing inductive effect or –I effect

-NO2 > -SO3H> -CN > -COOH > -F > -Cl > -Br > -I > -OC6H5 > -COOR > -OR > - OH > -C6H5> -H

Electron donating inductive effect or +I effect

-C(CH3)3 > -HC(CH3)2 > -C2H5 > -CH2CH3 > -CH3 > -H

Inductive effect

Cl C C C-

- + - +

O

O

APPLICATION OF INDUCTIVE EFFECTS APPLICATION OF INDUCTIVE EFFECTS

The effect diminishes with distance - it carries for about 3 bonds.

Cl C

O-

Chlorine helps

to stabilize -CO2-

by withdrawingelectrons

O


Influence on dipole momentInfluence on dipole moment

H-F H-Cl H-Brµ = 1.90D µ= 1.04D µ = 0.78D

Comparison of relative acidic Comparison of relative acidic strength of alkynesstrength of alkynes

HC ≡ CH > CH3C ≡ CH > C2H5C ≡ CH

Inductive effect

Factors that Determine Acid Strength—Inductive Effects

• When electron density is pulled away from the negative charge through bonds by very electronegative atoms, it is referred to as an electron withdrawing inductive effect.

• More electronegative atoms stabilize regions of high electron density by an electron withdrawing inductive effect.

• The more electronegative the atom and the closer it is to the site of the negative charge, the greater the effect.

• The acidity of H—A increases with the presence of electron withdrawing groups in A.


Electromeric effect

Electromeric effect

This is a temporary effect and takes place between two atoms joined by a multiple bond, i.e., a double or triple bond. It occurs at the requirements of the attacking reagent, and involves instantaneous transfer of a shared pair of electrons of the multiple bond to one of the linked atoms.

It is temporary in nature because the molecule acquires its original electronic condition upon removal of the attacking reagent.

For example, consider the carbonyl group, >C=O, present in aldehydes and ketones. When a negatively charged reagent say approaches the molecule seeking positive site, it causes instantaneous shift of electron pair of carbonyl group to oxygen (more electronegative than carbon). The carbon thus becomes deprived of its share in this transferred-pair of electrons and acquires positive charge. In the meanwhile oxygen takes complete control of the electron pair and becomes negatively charged. Therefore, in the presence of attacking reagent, one bond is lost and this negatively charged attacking reagent links to the carbon having positive charge.


Electromeric effect

This phenomenon of movement of electrons from one atom to another at the demand of attacking reagent in multibonded atoms is called electromeric effect, denoted as E effect. The electromeric shift of electrons takes place only at the moment of reaction. Like the inductive effect, the electromeric effect is also classified as +E and E:

When the transfer of electrons takes place towards the attacking reagent, it is called + E (positive electromeric) effect. For example,

When the transfer of electrons takes place away from the attacking reagent, it is called, -E (negative electromeric) effect. For example,


Electromeric effect


Ex.of application of electromeric effect: Additions to Alkenes

Generally the reaction is exothermic because one and one bond are converted to two bonds

The electrons of the double bond are loosely held and are a source of electron density, i.e. they are nucleophilicAlkenes react with electrophiles such as H+ from a hydrogen halide to form a carbocation

Inductive effect

The inductive effectthe electron-deficient carbon bearing the

positive charge polarizes electrons of the adjacent sigma bonds toward it

the positive charge on the cation is not localized on the trivalent carbon, but delocalized over nearby atoms

the larger the volume over which the positive charge is delocalized, the greater the stability of the cation


Inductive effect


ResonanceOften a single Lewis structure does not accurately

represent the true structure of a moleculeThe real carbonate ion is not represented by any of

the structures 1,2 or 3

Experimentally carbonate is known not to have two carbon-oxygen single bonds and one double bond; all bonds are equal in length and the charge is spread equally over all three oxygens

Resonance


The real carbonate ion can be represented by a drawing in which partial double bonds to the oxygens are shown and partial negative charge exists on each oxygen

The real structure is a resonance hybrid or mixture of all three Lewis structures

Double headed arrows are used to show that the three Lewis structures are resonance contributors to the true structureThe use of equilibrium arrows is incorrect since

the three structures do not equilibrate; the true structure is a hybrid (average) of all three Lewis structures

Resonance


One resonance contributor is converted to another by the use of curved arrows which show the movement of electronsThe use of these arrows serves as a bookkeeping

device to assure all structures differ only in position of electrons

A calculated electrostatic potential map of carbonate clearly shows the electron density is spread equally among the three oxygensAreas which are red are more negatively charged;

areas of blue have relatively less electron density

Resonance


Rules for Resonance: Individual resonance structures exist only on paper

The real molecule is a hybrid (average) of all contributing forms Resonance forms are indicated by the use of double-headed arrows

Only electrons are allowed to move between resonance structures The position of nuclei must remain the same Only electrons in multiple bonds and nonbonding electrons can be moved

Example: 3 is not a resonance form because an atom has moved

All structures must be proper Lewis structures

Resonance


The energy of the actual molecule is lower than the energy of any single contributing formThe lowering of energy is called resonance stabilization

Equivalent resonance forms make equal contributions to the structure of the real moleculeStructures with equivalent resonance forms tend to be

greatly stabilizedExample: The two resonance forms of benzene

contribute equally and greatly stabilize it

Unequal resonance structures contribute based on their relative stabilities More stable resonance forms contribute more to the

structure of the real molecule

Resonance


Rules to Assign Relative Importance of Resonance FormsA resonance form with more covalent bonds is more

important than one with lessExample: 6 is more stable and more important

because it has more total covalent bonds

Resonance forms in which all atoms have a complete valence shell of electrons are more importantExample: 10 is more important because all atoms

(except hydrogen) have complete octets

Resonance


Resonance forms with separation of charge are less importantSeparation of charge cost energy and results in a less

stable resonance contributorExample: 12 is less important because it has charge

separation

Forms with negative charge on highly electronegative atoms are more importantThose with positive charge on less electronegative

atoms are also more important

Resonance


Resonance


Chapter 1

ExampleThe nitrate ion is known to have all three nitrogen-

oxygen bond lengths the same and the negative charge spread over all three atoms equally

Resonance theory can be used to produce three equivalent resonance forms Curved arrows show the movement of electrons between forms When these forms are hybridized (averaged) the true structure of the

nitrate ion is obtained

Resonance


CH3 C O

O

H-H+

CH3 C

O

O

CH3 CO

O

base

_

_

acetate ion

RESONANCE IN THE ACETATE IONRESONANCE IN THE ACETATE ION

acetic acid

equivalent structurescharge on oxygens


PHENOLATE ION RESONANCEPHENOLATE ION RESONANCE

O

-

_

_

_

_

_

O O

OOO

Non-equivalent structurescharge on carbon and oxygen

More structures,but not betterthan acetate.


Resonance


Factors that Determine Acid Strength—Resonance Effects

• Resonance is a factor that influences acidity.

• In the example below, when we compare the acidities of ethanol and acetic acid, we note that the latter is more acidic than the former.

• When the conjugate bases of the two species are compared, it is evident that the conjugate base of acetic acid enjoys resonance stabilization, whereas that of ethanol does not.

Resonance


• Resonance delocalization makes CH3COO¯ more stable than CH3CH2O¯, so CH3COOH is a stronger acid than CH3CH2OH.

• The acidity of H—A increases when the conjugate base A:¯ is resonance stabilized.

Resonance


• Electrostatic potential plots of CH3CH2O¯ and CH3COO¯ below indicate that

the negative charge is concentrated on a single O in CH3CH2O¯, but

delocalized over both of the O atoms in CH3COO¯.C

Hyperconjugation


• Hyperconjugation: The spreading out of charge by the overlap of an empty p orbital with an adjacent bond. This overlap (hyperconjugation) delocalizes the positive charge on the carbocation, spreading it over a larger volume, and this stabilizes the carbocation.

works for any sigma bond on the adjacent Csecondary can do 2x, tertiary can do 3x

Hyperconjugation


• The order of carbocation stability is also a result of hyperconjugation.

• Example: CH3+ cannot be stabilized by hyperconjugation, but

(CH3)2CH+ can.

Hyperconjugation


Hyperconjugation stabilizes the carbocation by donation of electrons from an adjacent carbon-hydrogen or carbon-carbon bond into the empty p orbitalMore substitution provides more opportunity for

hyperconjugation

Hyperconjugation


Hyperconjugationpartial overlap of the bonding orbital of an

adjacent C-H bond with the vacant 2p orbital of the cationic carbon delocalizes the positive charge and also the electrons of the adjacent bond

replacing a C-H bond with a C-C bond increases the possibility for hyperconjugation

Carbocation


Cleavage of Covalent Bonds

Carbocation


• Radicals and carbocations are electrophiles because they contain an electron deficient carbon.

• Carbanions are nucleophiles because they contain a carbon with a lone pair.

Carbocation

Carbocations are classified as primary (1o), secondary (2o), or tertiary (3o), based on the number of R groups bonded to the charged carbon atom. As the number of R groups increases, carbocation stability increases.

Structure of carbocation:


Carbocation

More positive charge at C+ = a more unstable C+


Carbocation


Carbocation

Carbocation Stability

•The order of carbocation stability is also a consequence of hyperconjugation.

•Hyperconjugation is the spreading out of charge by the overlap of an empty p orbital with an adjacent bond. This overlap (hyperconjugation) delocalizes the positive charge on the carbocation, spreading it over a large volume, and this stabilizes the carbocation.

• Example: CH3+ cannot be stabilized by

hyperconjugation, but (CH3)2CH+ can.


Free radicals

Free Radicals:Homolytic bond cleavage leads to the formation of

radicals (also called free radicals)Radicals are highly reactive, short-lived species

Single-barbed arrows are used to show the movement of single electrons

Production of RadicalsHomolysis of relatively weak bonds such as O-O or

X-X bonds can occur with addition of energy in the form of heat or light

o or


Free radicals

• Homolytic Bond Dissociation Energies and the Relative Stabilities of Radicals:

•The formation of different radicals from the same starting compound offers a way to estimate relative radical stabilitiesExamples:•The propyl radical is less stable than the isopropyl radical

\•Likewise the tert-butyl radical is more stable than the isobutyl radical


Free radicals

•The relative stabilities of radicals follows the same trend as for carbocations•The most substituted radical is most stable •Radicals are electron deficient, as are carbocations, and are therefore also stabilized by hyperconjugation


Free radicals

Example of free radical Reaction:Chlorination of Methane: Mechanism of ReactionThe reaction mechanism has three distinct aspects: Chain initiation, chain propagation and chain termination

Mechanism:

Chain termination


Free radicals

2.Radical Addition to Alkenes: The anti-Markovnikov Addition of Hydrogen Bromide in presence of peroxide:

Mechanism:


Free radicals


Carbocation stability

Carbocations are classified as primary (1o), secondary (2o), or tertiary (3o), based on the number of R groups bonded to the charged carbon atom. As the number of R groups increases, carbocation stability increases.



Carbocation Stability

The inductive effectthe electron-deficient carbon bearing the

positive charge polarizes electrons of the adjacent sigma bonds toward it

the positive charge on the cation is not localized on the trivalent carbon, but delocalized over nearby atoms

the larger the volume over which the positive charge is delocalized, the greater the stability of the cation



Carbocation Stability•The order of carbocation stability is also a

consequence of hyperconjugation.

• Example: CH3+ cannot be stabilized by

hyperconjugation, but (CH3)2CH+ can.

Hyperconjugationpartial overlap of the bonding orbital of an adjacent C-H bond with the vacant 2p orbital of the cationic carbon delocalizes the positive charge and also the electrons of the adjacent bondreplacing a C-H bond with a C-C bond increases the possibility for hyperconjugation


Carbanion stability


Carbanions have 8 valence electrons and a negative charge

Carbanion stability


Kinds of Organic Reactions


Types of Organic Reactions:• A substitution is a reaction in which an atom or a group of atoms is replaced by

another atom or group of atoms.

• In a general substitution, Y replaces Z on a carbon atom.



• Substitution reactions involve bonds: one bond breaks and another forms at the same carbon atom.

• The most common examples of substitution occur when Z is a hydrogen or a heteroatom that is more electronegative than carbon.



• Elimination is a reaction in which elements of the starting material are “lost” and a bond is formed.



• In an elimination reaction, two groups X and Y are removed from a starting material.

• Two bonds are broken, and a bond is formed between adjacent atoms.

• The most common examples of elimination occur when X = H and Y is a heteroatom more electronegative than carbon.



• Addition is a reaction in which elements are added to the starting material.



• In an addition reaction, new groups X and Y are added to the starting material. A bond is broken and two bonds are formed.



• Addition and elimination reactions are exactly opposite. A bond is formed in elimination reactions, whereas a bond is broken in addition reactions.



Classify each of the following as either substitution, elimination or addition reactions.

a) OHBr

b)

c)

OH



Some other examples:

15046 Organic Chemistry

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