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UNIT-4

ORGANIC REACTIONS& INTRODUCTION TO POLYMERS:

Types of Organic reactions: Addition - electrophilic, nucleophilic and free radical -

Substitution - electrophilic, nucleophilic (SN1 and SN

2) and free radical – Elimination(E1 and

E2) (ECB- Examples ) – Rearrangement Reactions (Claisen, Pinacol pinacolone

rearrangement) – Diels-Alder reaction - Isomerism (Cis- Trans)

Definition of Polymer - Polymerisation(Addition and Condensation) – Functionality – Degree

of Polymerisation–Classification of Polymers – Zeiglar Natta Catalysis.

Types of Organic reactions:

Organic reactions are accompanied by the breaking and the making of covalent bonds which

involve the exchange of electrons. The functional groups of organic compounds also play a

very significant role in the exchange of electrons. Based on all such fundamental

considerations, organic reactions are classified into the following types:

1) Addition reactions

2) Substitution reactions

3) Elimination reactions

4) Rearrangement Reactions

Addition reactions: These are the reactions in which products are formed by the addition of

some reagent to an unsaturated organic compound.

Types of Addition Reactions:

Depending upon the nature of the attacking reagent, the addition reactions are classified as

follows:

a) Electrophilic addition Reactions.

b) Nucleophilic addition Reactions.

c) Free Radical Addition Reactions.

a) Electrophilic addition reactions: Such reactions are shown by alkenes and alkynes.

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Mechanism: In this type of addition, an electrophile attacks C = C bond to form an

intermediate carbonium ion. In the second step, the carbonium ion forms the final product.

Consider the addition of HBr to Ethene.

b) Nucleophilic Addition Reactions: Such reactions are mainly shown by carbonyl

compounds. These reactions are initiated by nucleophiles i.e., the nucleophile (-ve part of

molecule) adds in the first step. Consider th e addition of HCN to Acetaldehyde.

c) Free Radical Addition Reactions: Such reactions in alkenes are initiated by free

radicals. The free radicals can be generated in presence of organic peroxide. Consider the

addition of HBr to propene in presence of organic peroxide (ROOR).

The addition occurs according to anti markownikoff’s rule or Khrasch effect.

Mechanism: The addition reaction proceeds with the generation of free radical.

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2) Substitution (or) Displacement Reactions: These are the reactions in which an

atom or a group of atoms attached to a carbon atom in a molecule is replaced by some other

atom or group of atoms without any change in the structure of the remaining part of the

molecule. The product formed is known as the substitution product. The new atom or group

which enters the molecule is called the substituent. Some examples of substitution reactions

are:-

Types of Substitution Reactions:

Depending upon the nature of the attacking reagents, substitution reactions can be classified

as below:

a) Electrophilic Substitution Reactions.

b) Nucleophilic Substitution Reactions.

c) Free Radical Substitution Reactions.

a) Electrophilic Substitution Reactions: These are the reaction in which an atom or

group in a molecule is replaced by an electrophile. Such reactions are shown by aromatic

compounds.

Mechanism: Consider the chlorination of benzene in presence of halogen carrier (FeCl3)

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b) Nucleophilic Substitution Reactions: These are the reactions in which an atom or a

group in a molecule is replaced by a nucleophile. Such reactions are shown by alkyl halides.

Consider the action of aqueous KOH on methyl chloride.

Nucleophilic substitution reactions are further classified as:

i) SN2 Reactions (Nucleophilic Substitution Bimolecular). Such reactions are shown

by primary alkyl halides and involve a single step. The breaking of C – X bond and the

making of C – OH bond takes place simultaneously. The nucleophile approaches the C –

atom (of C – X part) from the side opposite to that carrying the halongen. In such reactions,

an inverted product is formed. The reactions involves the formation of a transition state.

Consider the reaction between Ethyl bromide and aqaueous KOH.

The reaction follows the rate law;

Rate = k [Alkyl halide] [OH-].

As the slow rate determining step involves two molecules (alkyl halide and alkali); therefore,

the reaction is known as bimolecular substitution reaction.

ii) SN1 Reactions (Nucleophilic Substitution Unimolecular). This mechanism is

generally followed by tertiary alkylhalides. In the first step, tertiary alkyl halide breaks

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heterolytically to form intermediate carbonium ion. The formation of carbonium ion is the

slow rate determining step.

In the second step, the nucleophile (say OH- ion from aqueous KOH) attacks the

carbonium ion to form an alcohol. Consider the action of aqueous KOH or tertiary butyl

bromide.

In the rate determining step, only one molecule of alkylhalide (30) is involved. Thus, the rate

of formation of alcohol depends upon the molar concentration of only alkyl halide;

Rate = k [alkyl halide]

c) Free radical Substitution reactions: These are the reactions in which an atom or

group of atoms in a molecule is replaced by a free radical. The replacement of H-atom by a

halogen atom is an example of free radical substitution.

Mechanism: The mechanism of free radical substitution involves three steps.

Initiation: In this step, halogen molecule breaks homolytically to form free radicals.

Consider the action of Br2 on ethane in presence of sunlight.

Propagation Step: The Br formed in the first step reacts with alkane molecule to form a

new free radical which inturn reacts with bromine molecule and the chain reaction starts and

so on.

Termination: In this step, the free radicals combine and the reaction stops.

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Similarly, consider action of Cl2 on propene.

3) Elimination Reactions: These are the reactions in which products get formed by

the loss of simple molecules like H2O, HX etc., from the reactant molecules. As a result of

this loss, a multiple bond is introduced in the product molecule. Some examples are:

i) When alkyl halide is treated with alcoholic KOH, an alkene is formed. For example,

consider the action of alc. KOH on Ethyl bormide.

ii) Consider the action of Conc. Sulphyric acid on Ethyl alcohol at 1700C.

iii) When an acid amide is heated with P2O5, alkyl cyanide results.

Elimination Reactions are further classified as EN2 or E2 and EN1 or E1 reactions.

a) EN2 or E2 Mechanism. (Elimination Nucleophilic bimolecular). This type of

reaction is generally followed by primary alkyl halides. The attacking base, OH- removes a

proton from the -Carbon. Simultaneously, the halide ion is lost from the α-Carbon and

(C=C) double bond is formed. For example:

Rate of reaction = k [Alkyl halide][OH-]

Since the slow rate determining step involves two molecules, (i.e., alkyl halide and alkali),

the reaction is known as bimolecular reaction.

b) EN1 or E1 Mechanism. (Elimination Nucleophilic Unimolecular). This type of

reaction is generally followed by tertiary alkyl halides and involve two step mechanism. In

the first step, the C – X bond break heterolytically forming the intermediate carbonium ion.

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The formation of the intermediate carbocations is the slow rate determining step. In

the second step, the nucleophile abstracts a proton from the -Carbon to form the alkene.

For example, consider the action of alcoholic KOH on tertiary butyl bromide.

The reaction follows the rate law:

Rate of reaction = k [Alkyl halide]

E1CB-Elimination Reaction:

The E1cB elimination reaction is a type of elimination reaction which occurs under basic

conditions, where a particularly poor leaving group (such as -OH or -OR) and an acidic

hydrogen eliminate to form an additional bond.

Mechanism:

First, a base abstracts the most acidic proton to generate a stabilized anion. The lone

pair of electrons on the anion then moves to the neighboring atom, thus expelling the leaving

group and forming double or triple bond.

The name of the mechanism - E1CB - stands

for Elimination Unimolecular conjugate Base. Elimination refers to the fact that the

mechanism is an elimination reaction and will lose two substituents. Unimolecular refers to

the fact that the rate-determining step of this reaction only involves one molecular entity.

Finally, conjugate base refers to the formation of the carbanion intermediate, which is the

conjugate base of the starting material.

An example of the E1CB reaction mechanism in the degradation of a hemiacetal under basic

conditions.

Rate of reaction = k [ hemiacetal ]

4) Rearrangement Reactions: These are the reactions in which the products get

formed simply by the rearrangement of atoms and electrons in the reactant molecules. The

product formed is known as the rearranged product.

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CLAISEN REARRANGEMENT:

The claisen rearrangement is a powerful carbon-carbon bond forming chemical reaction

discovered by RAINER LUDWIG CLAISEN. It is the first Sigmatropic Rearrangement.

When O-allyl ether of phenol is heated to about 2000C in absence of any catalyst, it

rearrange to O-allyl phenol. This thermal rearrangement of Phenolic Allyl Ethers is

called CLAISEN REARRANGEMENT.

Mechanism:

In this type of reaction, the allyl group migrates almost invariable to the ortho position,

unless the ortho position is blocked. The cyclic mechanism accounts for the intramolecular

rearrangement of allyl ether to ortho ally l phenol.

PINACOL PINACOLONE REARRANGEMENT:

The conversion of pinacols (1,2-glycols) to ketones or aldehyde by means of mineral acids or

zinc chloride is known as pinacol-pinacolone rearrangement. It is a anionic rearrangement i.e.

migrating group moves from a carbon atom to an electron defecient carbon which has only

six electrons in it’s valence shell.

Pinacol-pinacolone rearrangement is general for 1,2-glycols & the migrating group may be

alkyl or aryl.

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CH3 C

CH3

OH OH

CH3

C CH3H

+

CH3 C

CH3

OH OH

CH3

C CH3

H

CH3 C

CH3

OH

CCH3

CH3

CH3 C

OH

CH3

C CH3

CH3

Mechanism:

Step-1:

Initiation:- Protonation of 1,2-Diol.

2,3-Dimethyl-2,3-butane diol

Step-2:

Formation of carbonium ion by loss of water from protonated diol.

Step-3:

Rearrangement of carbonium ion by 1,2-shift to give protonated ketone

Carbonium ion Protonated ketone

Step-4:

Formation of ketone by loss of proton from protonated ketone

DIELS-ALDER REACTION:

The Diels–Alder reaction is an organic chemical reaction (specifically, a

[4+2] cycloaddition) between a conjugated diene and a substituted alkene, commonly termed

the dienophile, to form a substituted cyclohexene derivative.

OH2

+CH3

CH3

CH3

OH

CH3

(3-hydroxy-2,3-dimethylbutan-2-yl)oxonium

C+

CH3

CH3

CH3

OH

CH3

3-hydroxy-2,3-dimethylbut-2-ylium

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Mechanism: The dienophile can be considered as simultaneously a nucleophile and

electrophile. The driving force is the conversion of two π bonds into two σ bonds. The

reaction will not happen without a Hückel number of electrons.

ISOMERISM: (Cis-Trans):

Cis–trans isomerism, also known as geometric isomerism or configurational isomerism, is

a term used in organic chemistry. The prefixes "cis" and "trans" are from Latin: "this side of"

and "the other side of", respectively and cis indicates that the functional groups are on the

same side of the carbon chain while trans conveys that functional groups are on opposing

sides of the carbon chain. Cis-trans isomers are stereoisomers, that is, pairs of molecules

which have the same formula but whose functional groups are rotated into a different

orientation in three-dimensional space. In general, stereoisomers contain double bonds that

do not rotate, or they may contain ring structures, where the rotation of bonds is restricted or

prevented. Cis and trans isomers occur both in organic molecules and in inorganic

coordination complexes.

Examples of Cis-Trans Isomers:

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INTRODUCTION TO POLYMERS

Definition Polymers:

Polymers‟ (Greek poly - many; mers - units or parts) is „macromolecules‟

built-up by the linking together of a large number of small molecules. The repeat units in a polymer chain are linked through strong covalent bonds.

Example: Polythene is a polymer formed by linking together of a large number of ethylene (C2H4) molecules.

Monomers:

Small molecules which combine with each other to form polymer molecules

are termed monomers; and the “repeat unit” in a polymer is called mer. Monomers

are often called building blocks of polymer chain. The essential requirement of a small molecule to qualify as a monomer is the possession of at least two bonding

sites (Bifunctional).

Polymerization:

The fundamental chemical process by which the low molecular weight

molecules are converted into high molecular weight molecule, with elimination of small molecules like water, ammonia, alcohol etc. or without such elimination is

called the polymerization.

Degree of Polymerization” (DP):

The number of repeating units (n) in polymer chain is known as the “degree

of polymerization” (DP). There may be hundreds or thousands or tens of thousands or more monomer molecules linked together in a polymer molecule. Most of the

polymers, usually, fall into the 5,000—200,000 molecular mass range.

Functionality: The total number of bonding sites or reaction sites present in a

monomer molecule is called the functionality of the monomer. Depending on the number of bonding sites, the monomers are called bifunctional, trifunctional and

polyfunctional.

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S.No Name of the monomer Molecular formula functionality

1 Vinyl chloride CH2= CHCl 02

2 Ethylene glycol HO-CH2-CH2-OH 02

3 Phenol C6H5OH 03

4 Formaldehyde CH2OH 02

5 Adipic acid HOOC-(CH2)4-COOH 02

6 Ethylene H2C= CH2 02

7 Hexamethylene diamine 2HN-(CH2)6-NH2 02

TYPES OF POLYMERIZATION: On the basis of type of chemical reaction polymerization reactions are classified into three types.

1. Addition or chain polymerization: The fundamental chemical process by which the low molecular weight

molecules are converted into high molecular weight molecule, without the elimination any small molecules like water, ammonia, ethyl alcohol etc. is called

the addition polymerization. A chain polymerization is a reaction that yields a polymer product which is the exact multiple of monomers.

2. Condensation or step growth polymerization:

The fundamental chemical process by which the low molecular weight molecules are converted into high molecular weight molecule with the elimination

any small molecules like water, ammonia, ethyl alcohol etc. is called condensation

or step growth polymerization.

Example: Nylon 6:6: It is prepared by polymerization of Hexamethylene diamine

and Adipic acid.

Difference between Addition and Condensation Polymerization:

S.NO Addition Polymerization Condensation Polymerization

1 It is also known as chain growth

polymerization

It is also known as step growth

Polymerization

2 The polymerization takes place by self addition of monomers

It is due to slow step wise condensation of the functional groups

2 It takes place only in monomers

having multiple bonds

It takes place in monomers having

reactive functional groups

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3 It takes place without elimination of simpler molecule

It takes place with elimination of simple molecule like H2O, NH3, HCl etc

4 High molecular weight polymer is

formed soon.

The molecular weight of polymer

increases steadily throughout the reaction

5 The product obtained by this

Polymerization is thermoplastic.

The product obtained in the

polymerization may be either thermoplastic or thermosetting

6 Reaction initiated by

initiator(catalyst)

Reaction initiated by heat

7 Reaction proceeds in fast manner Reaction proceeds comparatively slow.

8 Polyethylene, polystyrene, polypropylene, polyvinyl chloride etc.

are its examples

Bakelite, urea formaldehyde resin, epoxy resins, etc. are its examples.

CLASSIFICATION OF POLYMERS:

1. Classification On The Basis Of Source: (i) Natural polymers: The polymers which are obtained from natural sources

such as plants and animals are called natural polymers. Example: starch,

cellulose, proteins, nucleic acids, natural rubber cotton, wool and silk. (ii) Synthetic polymers The polymers which are synthesized from simple

molecules are called synthetic polymers. Example: polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinylchloride (PVC), nylon, Terylene,

bakelite.

2. Classification on the basis of struct ure:

(i) Linear polymers: in which monomeric units are joined in the form of long straight chains. For example, high density polythene (HDPE), nylons,

polyester, etc. These polymers possess high m.p., density and tensile strength, due to close packing of polymer chains.

(ii) Branched chain polymer: In these polymers, the chain of the polymer contains branches of monomers, which hinders the close packing of

polymeric chains, and, hence, are less tightly packed in comparison to the linear chain polymers. They have low melting points and less density in

comparison to the linear chain

(iii) Three-dimensional network polymers: They are giant molecules in which movement of individual monomeric units prevented by strong cross-links. Due

to presence of strong cross-links, they are hard, rigid, brittle and do not melt, but burn on strong heating. Examples: are bakelite, urea-formaldehyde,

melamine formaldehyde, etc.

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3. Classification on the basis of their methods of synthesis:

(i) Addition polymers: The polymers which are formed by the addition polymerization reaction in which self addition of several olefinic monomers to

each other take place without elimination of by-product are called addition

polymers. Example: polyethylene, polypropylene, polystyrene, polyvinyl chloride, etc.

(ii) Condensation polymers: The polymers which are formed by intermolecular

condensation reaction through functional groups of monomers with continuous elimination of by- products are called condensation polymers.

Example: nylon-6, Bakelite, polyester, etc.

4. Classification on the basis of growth polymer chain:

(i) Chain growth polymers are formed by the successive addition of monomer units to the growing chain carrying a reactive intermediate. Example:

polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride

(PVC), etc.

(ii) Step-growth polymers are formed through a series of independent reactions involving bond formation (condensation) between two different monomers with

loss of small molecules like HCl, NH3 etc. Example: nylon-6, Bakelite, polyester, etc.

5. Classification based on molecular forces:

(i) Thermoplastic polymers: The polymers which become soft on heating and

hard on cooling are called thermoplastics. Examples: Polythene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), nylons,

polytetrafluoro ethylene (PTFE or Teflon), etc.

(ii) Thermosetting: The polymers which undergo chemical changes and cross

linking on heating and become permanently hard, rigid and infusible are called thermosetting. Examples: Polyester (Terylene), Bakelite, epoxy-resin,

urea-formaldehyde, etc.

6. Classification on the basis of structure and use; Polymers can also be

classified based on their structure and use: (i) Elastomer: The polymer which undergo a very large elongation when pulled

but return to the original length on release of force is called an elastomer. Example: Natural rubber, Buna-s, Buna-n, Neoprene rubber, Silicone rubber.

(ii) Fibers: These are long, thin and thread like polymer chains which do not

undergo stretching or deformation like elastomers. Example: Wool, silk cotton, jute, Nylon, Terylene.

(iii) Plastic: The polymer which can mould into desired articles by the application of heat and pressure are called plastics. Example: PVC, Teflon.

7. Classification on the basis of type of compound: Two types:

(i) Organic polymers: The polymers which are prepared from organic monomers are called organic polymers. Examples: PE, PS, PVC, Bakelite etc.

(ii) Inorganic polymers: The polymers which are prepared from inorganic monomers are called organic polymers. Examples: Silicone rubber.

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Coordination Polymerization or Ziegler-Natta Polymerization:

Ziegler (1953) and Natta (1955) discovered that in the presence of a combination of a transition-metal halide (TiCl4 or TiC13 ,ZrBr3 and TiC12, halides of V, Zr, Cr, Mo

and W) with an organo-metallic compounds (like triethyl aluminium, trimethyl

aluminium), steriospecific po1ymeriz can be carried out. Mechanism of co-ordination polymerization may be illustrated as follows:

Ziegler-Natta polymerization is used to prepare polypropylene, polyethylene,

polydiene, etc. The importance of this method lies in the fact that stereospecfic polymers are obtained. Thus, during polymerization of propylene, using the

conventional catalysts, normally random or a tactic polymer is obtained, by using suitable catalyst, solvent and temperature, it is possible to produce desired type (a

tactic or isotactic or syndiotactic) of sterioisomers. The atactic polymers are soft

and elastic; while the other two are dense