Diazonium Salts - Methods of Preparation

The diazonium salts have the general formula RN⁺₂X⁻ where R stands for an aryl group and X^- ion may be Cl⁻, Br⁻, HSO⁻₄, BF⁻₄ etc.

Primary aliphatic amines form highly unstable alkyldiazonium salts.

Primary aromatic amines form arenediazonium salts which are stable for a short time in solution at low temperatures (273 - 278 K).

Preparation: Benzenediazonium chloride is prepared by the reaction of aniline with nitrous acid at 273-278K.

Nitrous acid is produced in the reaction mixture by the reaction of sodium nitrite with hydrochloric acid. The conversion of primary aromatic amines into diazonium salts is known as diazotisation.

Due to its instability, the diazonium salt is not generally stored and is used immediately after its preparation.

Preparation of Amines

1. Reduction of nitro compounds:

Reactants: Nitro compounds

Reagent: H₂ in the presence of finely divided nickel, palladium or platinum

Product: Amines

Reactants: Nitro compounds

Reagent: Metals in acidic medium. Reduction with iron scrap and hydrochloric acid is preferred because FeCl₂ formed gets hydrolysed to release hydrochloric acid during the reaction. Thus, only a small amount of hydrochloric acid is required to initiate the reaction.

Product: Amines

This method cannot be used when the molecule also contains some other easily hydrogenated group, such as a Carbon carbon double bond.

2. Ammonolysis of alkyl halides:

Reactants: alkyl or benzyl halide

Reagent: ethanolic solution of ammonia

Mechanism: nucleophilic substitution reaction

Product: Amines

CH₃CH₂CH₂Br + NH₃ → CH₃CH₂CH₂NH₂ + HBr

This process of cleavage of the C–X bond by ammonia molecule is known as ammonolysis.

The primary amine thus obtained behaves as a nucleophile and can further react with alkyl halide to form secondary and tertiary amines, and finally quaternary ammonium salt.

The free amine can be obtained from the ammonium salt by treatment with a strong base:

R - NH⁺₃X⁻ + NaOH → R - NH₂ + H₂O + NaX⁻

The order of reactivity of halides with amines is RI > RBr >RCl.

3. Reduction of nitriles: This reaction is used for ascent of amine series, i.e. for preparation of amines containing one carbon atom more than the starting nitrile.

Reactants: Nitriles

Reagent: lithium aluminium hydride (LiAlH⁴)

Reaction: Reduction

Products: Amines

Reactants: Nitriles

Reagent: H² in presence of catalyst Ni

Reaction: Reduction

Product: Amines

4. Reduction of amides:

Reactants: Amides

Reagent: Lithium aluminium hydride (LiAlH₄)

Reaction: Reduction

Product: Amines

Reactants: Amides

Reagent: mixture of base and bromine (KOH + Br₂)

Product: Hofmann Bromamide reaction.

Product: Amines

The reaction is as follows, RCONH₂ + Br₂ + 4KOH → RNH₂ + K₂CO₃ + 2KBr + 2H₂O

Here, the amine formed has one carbon less than that of the corresponding amide. Due to the loss of carbon atom, this reaction is also called as Hofmann degradation of amides.

Base abstracts an acidic N-H proton, yielding an anion.

The anion reacts with bromine in an α-substitution reaction to give an N-bromoamide.

Base abstraction of the remaining amide proton gives a bromoamide anion.

The bromoamide anion rearranges as the R group attached to the carbonyl carbon migrates to nitrogen at the same time the bromide ion leaves, giving n isocyanate.

The isocyanate adds water in a nucleophilic addition step to yield a carbamic acid (aka urethane).

The carbamic acid spontaneously loses CO₂, yielding the amine product.

Apart from this, amides can be dehydrated by P₂O₅ to their corresponding nitriles and nitriles can then be reduced.

By this method you are retaining the number of carbon atoms in both amide and the amine

5. Gabriel phthalimide synthesis:

Reactants: Pthalimide and alkyl halide

Reagent: Ethanolic potassium hydroxide  

Product: Primary amine

Phthalimide on treatment with ethanolic potassium hydroxide forms potassium salt of phthalimide

Phthalimide on heating with alkyl halide followed by alkaline hydrolysis produces the corresponding primary amine.

Aromatic primary amines cannot be prepared by this method because aryl halides do not undergo nucleophilic substitution with the anion formed by phthalimide.

6. Schmidt reaction (IIT):

Reactants: Hydrozoic acid and carboxylic acid

Reagent: Sulphuric acid

Product: Amines

7. Curtius Reaction (IIT):

Reactants: Acid chloride and sodium azides

Reagent: Heat and water

Product: Amines

The isocyanate formed by reaction of acid chloride with sodium azides is decomposed with treatment of water and amines are obtained.

Nucleic Acids

Definition:

⇒ The particles in nucleus of a cell, responsible for heredity, are called chromosomes which are made up of proteins and nucleic acids.

⇒ Nucleic acids are long chain polymers of nucleotides, so they are also called polynucleotides.

⇒ Examples: Deoxyribonucleic acid (DNA) & Ribonucleic acid (RNA).

Chemical Composition of Nucleic Acids:

⇒ Each nucleotide has three components: a 5 - carbon sugar, a phosphate group, and a nitrogenous base.

⇒ In DNA molecules, the sugar moiety is β - D - 2 - deoxyribose where as in RNA molecule, it is β -  D - ribose.

⇒ DNA contains four bases viz. adenine (A), guanine (G), cytosine (C) and thymine (T).

⇒ RNA also contains four bases, the first three bases are same as in DNA but the fourth one is uracil (U).

Structure of Nucleic Acids:

⇒ A unit formed by the attachment of base to 1’ position of sugar is known as nucleoside, the sugar carbons are numbered as 1’, 2’, 3’, etc. in order to distinguish these from bases. When nucleoside is linked to phosphoric acid at 5’ - position of sugar moiety we get a nucleotide.

⇒ Nucleotides are joined together by phosphodiester linkage between 5’ and 3’ carbon atoms of pentose sugar.

⇒ A simplified version of nucleic acid chain is as shown below.

⇒ Information regarding the sequence of nucleotides in the chain of nucleic acid is called primary structure. Nucleic acids have a secondary structure also.

⇒ James Watson and Francis Crick gave a double strand helix structure for DNA. Two nucleic acid chains are wound about each other and held together by hydrogen bond between pairs of bases.

⇒ These are complementary to each other because the hydrogen bonds are formed between specific pairs of bases.

DNA Fingerprinting:

⇒ Every individual has unique fingerprints.

⇒ A sequence of bases on DNA is also unique for a person and information regarding this is called DNA fingerprinting.

⇒ It is same for every cell and cannot be altered by any known treatment.

Uses of DNA finger printing:

i) In forensic laboratories for identification of criminals.

ii) To determine paternity of individual

iii) To identify racial groups to rewrite biological evolution.

Biological functions of Nucleic acids:

⇒ DNA is the chemical basis of heredity and may be regarded as the reserve of genetic information.

⇒ DNA is responsible for maintaining the identity of different species of organisms over millions of years.

⇒ A DNA is capable of self-duplications during cell division and identical DNA strands are transferred to daughter cells.

⇒ Nucleic acids are responsible for protein synthesis in the cell.

Types of Polymerization Reactions

1. Addition polymerization (or) chain growth polymerization:

⇒ The molecules of the same monomer or different monomers add together on a large scale to form a polymer.

⇒ The monomers used are unsaturated compounds.

Eg: Alkenes, alkadienes and their derivatives.

⇒ Can take place through the formation of either free radicals or ionic species.

Free radical mechanism: A variety of alkenes or dienes and their derivatives are polymerized in the presence of a free radical generating initiator (catalyst) like benzoyl peroxide, acetyl peroxide, tert-butyl peroxide etc.,

Eg: Polymerisation of ethene to polythene.

The free radical mechanism involves following steps.

Chain initiation step:

Ċ₆H₅ - CH₂ = CH₂ → 2C₆H₅ - CH₂ - ĊH₂

Chain propagation step:

C₆H₅ - CH₂ - ĊH₂ + CH₂ = CH₂ → C₆H₅ - CH₂ - CH₂ - CH₂ - ĊH₂ → C₆H₅ (- CH₂ - CH₂-)_n – CH₂ - ĊH₂

Chain terminating step: For termination of the long chain, these free radicals can combine in different ways to form polythene.

Eg: C₆H₅ - (- CH₂ - CH₂ -)_n - CH₂ - ĊH₂ + C₆H₅ - (- CH₂ - CH₂ -)_n - CH₂  - ĊH₂ → C₆H₅ - (- CH₂ - CH₂ -)_n - CH₂ - CH₂ - CH₂ - CH₂ - (- CH₂ - CH₂ -)_n - C₆H₅

Preparation of some important addition polymers:

a) Polythene: These are of two types.

i) Low density polythene (LDP):-

⇒ By polymerization of ethene

⇒ Temperature: 350 K to 570 K in the presence of traces of dioxygen or peroxide initiator.

⇒ LDP obtained through the free radical addition and H atom abstractions has highly branched structure.

⇒ It is chemically inert and tough but flexible and a poor conductor of electricity.

ii) High density polythene (HDP):-

⇒ Addition polymerization of ethene in a hydrocarbon solvent in the presence of Ziegler - Natta Catalyst.

These are two sets of Zieglar - Natta catalyst/ co - catalyst systems. Either way, we have four chlorine atoms.

⇒ Temperature: 333 K to 343 K

⇒ Pressure of 6 - 7 atm.

⇒ HDP molecules consists linear molecules, its high density is due to close packing. It is also chemically inert and more tough and hard.

⇒ It is used for manufacturing of buckets, bottles, pipes, etc.,

b) Polytetrafluoro ethene (Teflon):

⇒ By heating tetrafluoroethene with a free radical or per sulphate catalyst at high pressure.

⇒ It is chemically inert and resistant to attack by corrosive reagents.

⇒ It is used in making oil seals and gaskets.

c) Polyacrylonitrile:

⇒ The addition polymerization of acrylonitrile in the presence of a peroxide catalyst leads to the formation of polyacryionitrile.

⇒ It is used as a substitute for wool in making commercial fibres as Orlon or acrilan.

2. Condensation polymerization or step Growth polymerization:

⇒ Involves a repetitive condensation reaction between two bifunctional monomers.

⇒ Results in the loss of simple molecules as water, alcohol, etc.

⇒ The product of each step is again a bi functional species and the sequence of condensation goes on.

⇒ So this process is also called as step growth polymerization.

Some important condensation polymerization reactions characterized by their linking units.

a. Polyamides:

⇒ Possess amide linkages.

⇒ Prepared by condensation polymerization of diamines with dicarboxylic acids and also of amino acids and their lactums.

Nylon 6, 6: Prepared by the condensation polymerization of hexamethylenediamine with adipic acid under high pressure and at high temperature.

Uses: It is used in making sheets, textile industry.

Nylon 6: Heating caprolactum with water at a high temperature.

Uses: It is used in the manufacture of tyre cords, fabrics and ropes.

b. Polyesters: Poly condensation products of dicarboxylic acids and diols.

Eg: Dacron or terylene.

It is manufactured by heating a mixture of ethylene glycol and terephthalic acid at 420 to 460 k in the presence of Zinc acetate antimony trioxide catalyst.

c. Phenol - formaldehyde polymer:

⇒ These are obtained by condensation reaction of phenol with formaldehyde in the presence of either acid or base catalyst.

⇒ The reaction starts with the initial formation of o – and/ or p – hydroxylmethylphenol derivatives, which further react with phenol to form compounds having rings joined to each other through – CH₂ groups.

⇒ The initial product could be a linear product- Novolac

⇒ Novolac is used in paints.

Bakelite: Novolac on heating with formaldehyde undergoes cross linking to form an infusible solid mass called Bakelite.

Uses: It is used in combs, phonograph records, etc.

Structure:

d. Melamine formaldehyde polymer: Melamine formaldehyde polymer is formed by the condensation polymerization of melamine and formaldehyde.

Uses: It is used in the manufacture of unbreakable crockery.

3. Copolymerization:

⇒ A mixture of more than one monomeric species is allowed to polymerize and form a copolymer.

⇒ The copolymer can be made not only by chain growth polymerization but by step growth polymerization also.

Example: Mixture of 1, 3 - butadiene and styrene can form a copolymer.

⇒ Copolymers have properties quite different from homopolymers.

⇒ For example, butaiene - styrene copolymer, is quite tough and is a good substitute of natural rubber. It is used in manufacture of auto tyres, footwear components, cable insulation etc.