Chemistry

Session

GENERAL ORGANIC CHEMISTRY - 5

Session Objectives

Session Objective

1. Fission of a covalent bond: homolysis and heterolysis

2. Reaction intermediates: carbocations, carbanions and free radicals

3. Different reagents: electrophiles, nucleophiles and carbene

4. Brief idea about type of organic reactions: substitution, addition, elimination,condensation, rearrangement, isomerisation

Reaction Mechanism

• Detailed description of sequence of steps involved in group from reactantsto products.

• Reactant intermediate product

Bond Cleavage

B:AB:A A : B A– : B+

+vely charged ion – carbocation

-vely charged ion – carbaanion

Heterolytic Cleavage

Homolytic Cleavage

BAB:A

Free radicals.

Carbonium ion

• Planar – sp2 hybridised bond angle 120o

• Has six electrons

• Stabilized by resonance or inductive effort or hyperconjugation

C

Empty unhybridisedp-orbital

sp2 Hybridisation ofcarbon

Planar Strucutre of carbnion

+

Examples of Carbonium ion

Benzyl cation

CH2 CH2 CH2

etc

+

+ +

CH 2 CH CH 2 CH 2 CH CH 2 + +

Stabilised through resonance

Allyl cation

H2C CH+

Vinyl cation

no resonance hence unstable.

Stability of Cabocation

(i) By inductive effect

The resonance effect is always more predominant than the inductive effect in stabilizing an ion.

CH3 C

CH3

CH3

CH3 C

CH3

H

> > H C

CH3

H

> >

3° 2°

+ +

1°

+

Stability of Cabocation

(ii) By hyperconjugation

H3C — C

CH3

CH2 — H

H3C — C

CH3

CH2H

H3C — C

CH2H

C

CH3

CH3CH3

CH2H

+

+

+

etc. +

Thus, tertiary carbocation is more stable than secondary and so on.

Carbanion

• Pyramidal - sp3 hybridised bond angle 109.28

• Has eight electrons

• Stabilized by resonance or by inductive effect.

. . sp3 hybrid orbitalcontaining lone pair

Tetrahedral structure of carboanion

Stability of Carbanion

(i) By resonance

H

-

H

-

H

Cyclopentadienyl carbanion

Stability of Carbanion

(ii) By inductive

CH3C H

CH3

CH3

CH3 C

CH3

H

C

CH3

H

3° 2° 1°

Stability of Carbanion

(iii) Electron-donating groups destabilize a carbanion while electron-withdrawing groups stabilize it.

N O 2 3OCH

>

2CH

2CH

Free Radical

• Planer or Pyramidal

• Has seven electrons

• Stabilized by resonance or by inductive effect.

• Order of stability of free radical 3o >2o> 1o

C(

Unhybridised orbitalcontaining odd electron

120oC

sp2 hybridised carbon

Planar Sturcutre

+

Classification of Reagents

Nucleophilic Reagents (Nucleophiles)

• Attacks the positive end of a polar bond or nucleus-loving is known as nucleophile.

• Generally, negatively charged or electron rich species are nucleophilic.

3 3 2 3e.g. OH , OCH , CN , I , CH COO , NH , CH

2 3 3 2H O, NH , NH — NH

N..

..N H 3,CH 3 — O — CH 3,

..

. ...

C 2H 5 — O H,. ...H 2 O,

• All nucleophiles are in general Lewis bases.

Classification of Reagents

Electrophilic Reagents (Electrophiles)

• Attacks a region of high electron density or electron-loving is known as electrophile.

• All positively charged or electron deficient species are electrophilic.

3 2H , CH , NO , Cl , Br , Ag

Classification of Reagents

• Neutral reagents which contain an electron-deficient atom are also electrophiles.

AlCl3, SO3, BF3, SOCl2, POCl3, FeCl3, ZnCl2

• All electrophiles are in general Lewis acids.

Carbenes

• Divalent carbon compound.

• Carbon atom is linked to two adjacent groups by covalent bonding.

• A carbene is neutral and possesses two free electrons, i.e. a total of six electrons.

• Electron deficient.

Carbenes

Carbene is of two types

(i) Singlet carbene:

(ii) Triplet carbene:

Triplet carbene is more stable than single carbene.

CH2 hybridisation sp2

it is v-shaped

CH2 hybridisation spit is linear shaped

Types of Organic Reaction

Substitution Addition

Elimination Rearrangement

Condensation Isomerisation

Types of Organic Reaction

Substitution Reaction

Replacement of an atom or group byother atom of group

Nucleophilic substitution:

R X OH R OH X

SN1 Reaction: Unimolecular nucleophilic substitution reaction.

Types of Organic Reaction - SN1 Reaction

CH3 — C — CH2Cl

CH3

CH3

OH–

slow

CH3 — C — CH2

CH3

CH3SN1 +

CH3 — C — CH2

CH3

CH3

CH3 — C — CH2 – CH3

CH3

+

+

1, 2-Methyl anionshift

Fast OH–CH3 — C — CH2CH3

OH

CH3

(1)

(2)

Types of Organic Reaction - SN2 Reaction

SN2 Reaction: This is called bimolecular nucleophilic substitution and it is one-step process.

H — C — Br + OH

CH2CH3

CH3

–OH C Br

H CH3

.

CH2CH3

FastHO — C — H

CH3

CH2CH3

d– d–

Transition state unstable

slow

Addition

The reagent often adds to bond

and the bond is converted into bond.

Can be electrophilic addition or nucleophilic addition.

C C , C O or C N C C ,

Cl22 2 2 2

CCl4CH CH Cl CH CH Cl

OH2H

OH

+

(Hydration)

Elimination Reaction

Two groups on adjacent atoms are lost as a double bond is formed.

CH3 – CH – CH – CH3

OH H

Conc. H2SO4

– H2OCH3 — CH CH – CH3

We divide elimination reactions into three classes.

(1) E1 (2) E1 CB (3) E2

Rearrangement

• Migration of a group takes place within the same molecule.

C6H5

C6H5

C = N

OH

C6H5 — C — N — C6H5

O

H

H

ether

OHH+

(Beckmann rearrangement)

(Dehydration and rearrangement)

Condensation

H3C — C — CH3 + H3C — C — CH3

O

H3C — C — CH — C — CH3

CH3O O

dil.

NaOH,

Two molecules of same or different reactants combine to give a new product with the elimination of simple byproducts like H2O, NH3, etc,

Isomerisation

C

H

H3C

C

H

CH3

C

H

H3C

C

CH3

H

h

Class Test

Class Exercise – 1

Select the most stable carbocation among the following.

CH3

CH3HC

6 5 3(C H ) C

3 2 2CH CH CH

3 3(CH ) C

(a) (b)

(c) (d)

Solution

C — C6H5

C6H5

C6H5

C+ +

This carbocation is highly stabilised through resonance with three benzene rings.

Hence answer is (b).

Class Exercise - 2

Which of the following is an addition reaction?

3 3CH CH CH

|

Br

OH2 3

AlcoholCH CH CH

h3 3 2 3 2CH CH Cl CH CH Cl

(a)

(b)

3 2 3 2CH CH Br CN CH CH CN Br(c)

3 2 3 3CH CH CH H CH CH CH

|

Br

Br(d)

Solution

H3C — CH — CH3

Br

AlcoholH2C CH — CH3

H3C — CH3 + Cl2 CH3CH2Cl + HCl

H3C — CH2Br + CN H3C — CH2CN + Br

H3C CH CH2 + HBr H3C CH CH3

Br

OH–

(Elimination)

h

–

(Substitution)

Substitution

–

AdditionHence answer is (d).

Class Exercise - 3

Which of the following is the most effective group in stabilizing a free radical inductively?

(a) F (b) I

(c) Br (d) Cl

Solution

Since free radical is electron deficient, any substituent with more electron releasing and less electron withdrawing ability will stabilize the radical inductively.

The decreasing order of electronegativity of halogens is: F > Cl > Br > I

Hence answer is (b).

Class Exercise - 4

Which of the following is not a nucleophile?

(a) CN– (b) BF3

(c) RNH2 (d) OH–

Solution

Among the following, BF3 is only electron deficient. Hence, it will not act as a nucleophile.

Hence answer is (b).

Class Exercise - 5Which of the following is the correct order regarding –I effect of the substituents?

(a) –NR2 > –OR > –F

(b) –NR2 > –OR < –F

(c) –NR2 < –OR < –F

(d) –OR > –NR2 > –F

Solution

–I effect increases with electronegativity of atom. The decreasing order of electronegativity is

F > O > N

The correct order for –I effect is

–NR2 < –OR < –F

Hence answer is (c).

Class Exercise - 6The least stable carbonium ion is

(a) (b)

(c) (d)

3 2H CCH

6 5 2 2C H — CH — CH

6 5 2C H — CH

6 5 6 5C H — CH — C H

Solution

Among the following, (a) is stabilised through +I effect and (b) is destabilized through –I effect of phenyl ring. Other two are stabilised through resonance.

Hence answer is (c).

Class Exercise - 7Arrange the following ions in the decreasing order of stability.

2HC

CH3 CH3CH3

++

+

(a) (b) (c) (d)

Solution

CH2

+

. It is a primary cation. Hence, minimum stability.

CH3CH3

+and

+(c) (b)

are secondary cations.

Hence, stabilised through +I effect of –CH3 group which decreases with distance. (c) is more stable as compared to (b).

(d) is most stable as it is tertiary cation and stabilised through +I effect of –CH3 group and hyperconjugation.

The order is (d) > (c) > (b) > (a)

Class Exercise - 8

Arrange the following radicals in order of their decreasing stability

3 2 3 3 6 5 2 2 2CH CH , (CH ) C, C H CH , CH CH CH

Solution

Radicals are stabilised through electron releasing resonance and inductive effect.

CH2CH2etc.

More resonating structure

H2C CH — CH2 H2C — CH CH2

Solution

One resonating structure, although

both are primary radicals.

Among and , later is a tertiary radical. Hence, more stable.

The decreasing order of stability is

H3C — CH2 (CH3)3C

C6H5CH2 > H2C CH — CH2 > (CH3)3C > H3CCH2

Thank you