reactions of benzene diazonium salt

Replacement reaction of benzene diazonium chloride by  cyanide ion

  Benzene diazonium salt with cuprous cyanides in presence of potassium cyanide gives cyanobenzene (benzene nitrile)Cyanobenzene formed can be converted to benzoic acid by hydrolysis. This method is used to introduce-COOH group in aromatic ring.

Replacement reaction of benzene diazonium chloride by  OH - ion

Aromatic diazonium salts are unstable and hence easily decompose when warned with water forming phenol and nitrogen gas.

Replacement reaction of benzene diazonium chloride by  hydrogen

Reducing agents like hypo phosphorous acid can replace diazonium group with hydrogen atom producing arene.

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Sandmaeyer's Reaction - benzene diazonium salt

Replacement reaction of benzene diazonium chloride by halide ion

When the solution of freshly prepared benzene diazonium salt is trated with a mixture of cuprous chloride or cuprous bromide in presence of concentrated HCL or HBr, we get chlorobenzene or bromobenzene. This reaction is known as Sandmaeyer's reaction.

Alternatively the same reaction can be done by copper powder and cone. HX instead of cuprous halide and this modified reaction is called Gatterman reaction.

Indobenzene can be prepared from benzene diazonium salt by reacting with KI solution.
Replacement with fluoride is known as Balz Schiemann reaction.In this benzene diazonium salt is reacted with fluoroboric acid heated to get fluorobenzene.
Preparation of aryl halides from diazonium salt by replacement gives pure aryl halides and provides important synthetic route for preparation of aryl iodides and aryl fluorides.

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Reactions of benzene diazonium chloride

 Chemical reactions of benzene diazonium chloride

Due to unstable nature diazonium group ( -N2- X+ ) is a good leaving group. In most of the reactions of diazonium salt, -N2-X+  group is replaced by other groups. Such reactions are usually grouped under replacement reactions.

In the second type, due to electron deficiency (+ve charge) on diazonium ion, it can behave like an electrophile and couple with electron rich aromatic compounds and undergo coupling reactions.

Replacement reactions of benzene diazonium chloride

Depending on replacing groups these reactions can be mentioned under following heads. In this way we can easily introduce_F, -CI, -I, -OH and -CN groups in the aromatic ring.

Diazonium salt

Diazonium salt reactions

Diazonium salts have N2 X functional group attached to an aryl group. They are highly unstable and are mainly used as a synthetic intermediate for various compounds. Alkyl diazonium salt do not exist even below 00C.


Nomenclature of Diazonium salt
The name is obtained by adding diazonium chloride, sulphate etc. to the name of parent hydrocarbon.

Methods of Preparation

Aromatic diazonium salts are prepared from aromatic primary amines. aromatic primary amine is taken in the ice-cold solution of HCL and adding sodium nitrite at 00C we get aromatic diazonium salt. This reaction is known as diazotisation.

Diazonium salt are highly unstable and they are not stored and they are not stored and are used immediately after their preparation.

Cyanides and Isocyanides

Cyanides are the compounds with functional group -CN and can be regarded as the derivatives of hydrogen cyanide (HCN). The cyanide group is an ambident group. This means cyanide group can attach to alkyl or aryl group through both carbon atoms as well as nitrogen atom. Thus the compounds obtained by replacement of the H-atom of hydrogen cyanides by alkyl or aryl group are called organic cyanides or nitriles.

Isocyanides are the compounds isomeric with cyanides and can be regarded as derived from Hydrogen isocyanide by replacement of H by alkyl or aryl group.

Reaction of Primary Amines with Nitrous Acids

Reaction of nitrous acid with Aromatic Primary amines

At 273 - 278 K, atomic primary amines react with sodium nitrite and dilute HCl, to give aromatic diazonium salts. For example, aniline forms benzene diazonium chloride.

Aromatic diazonium salts are stable only at low temperature. Hence at higher temperature, aromatic primary amine gives phenols.

Reaction of nitrous acid with Aromatic Primary amine

Aliphatic diazonium salts are highly unstable and hence alcohols are obtained when aliphatic primary amines are reacted with nitrous acid.

properties of amines

Chemical properties of amines

In most of its reaction, amines act as a nucleophile or a base due to lone pair on nitrogen. In aromatic amines, due to resonance, the -NH2 group make the ring more susceptible to electrophilic substitution at ortho and para position.

Important reactions of amines are classified as :

 1. Salt formation

Amines are soluble in mineral acids and carboxylic acids, due to the formation of soluble ammonium salts.

RNH2 + HCl  ------->  RNH3+ Cl-

C6H5NH2 --------> C6H5NH3+ Cl-

2. Alkylation

Alkyl halides when react with primary or secondary amines undergo nucleophilic substitution. When HX is removed, secondary or tertiary amine formed will again reacts with alkyl halide froming tertiary amine and finally quaternary ammonium salt.


3. Acylation

4. Reaction with carbonyl compounds

5. Carbylamine reaction

6. Reaction with nitrous acids

With nitrous  acid primary, secondary and tertiary amines react differently and give different product. Hence this reaction can be used to distinguish primary, secondary and tertiary amines from each other.

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Amine Carbonyl Reaction Mechanism

Reaction of Amine with Carbonyl compunds

Only primary amines react with aldehydes and ketones and condensation products are obtained. These condensation products are known as imines or Schiffs bases.

R-CHO (aldehyde) + RNH2  ---------> RCH = N-R (aldimine)

R2CO (ketone) + R-NH2 --------> R2C = N-R (ketimines)

Imines can be reduced to secondary amines. This reaction, in turn, can be used to convert a primary amine to secondary amine.

RCH = NR  ------(H2 / Ni)-----> R-CH2-NH-R  (Secondary amine)

R2C = N-R ------(H2 / Ni)-----> R2CH-NH-R

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acylation of amines

Acylation of amines mechanism

Both aliphatic and aromatic primary and secondary amines undergo N-acetylation. This acetylation can be effected using acid halides or anhydrides in presence of a base (pyridine) and the product obtained is an amide. Base remove HCl and prevent protonation of amine.

primary amine acylation
Acylation of primary amine

Secondary amine acylation
Acylation of Secondary amine

Since amides formed are not basic, they do not under go further actylation. Acid chlorides are stronger acetylating agent than acetic anhydride.
In tertiary amines there is no replaceable hydrogen and they do not undergo acylation. 

Amine Properties

Physical Properties of Amine

1. Physical state of Amine

Lower amines are gases while higher members are mostly liquids. Amines are almost unpleasant, having fishy smell. In the pure state amines are colourless, but in certain case they become coloured due to oxidation and mixing of impurities.

2. Boiling point

In amines, there are polar N-H bonds. Hence, in general they are polar. More over, in amines, inter molecular hydrogen bonds are present, leading to high boiling points in comparison to corresponding hydrocarbons of the same molar mass.

Primary and secondary amines can form hydrogen bonding hence they higher boiling points than isomeric tertiary amines.

3. Solubility in water

All the three classes of amines can form hydrogen bond with water. Hence they are soluble in water. But higher amines are least soluble due to the non-polar effect of big alkyl groups. Due to weaker hydrogen bonds solubility of amines in water is less than that of alcohols.

Carbylamine Reaction - Test for Amines

Distinction between primary, secondary and tertiary amine Using Carbylamine Reaction

Primary amine (both aliphatic and aromatic) when warmed with chloroform and alcoholic KOH, gives isocyanides (carbylamines). This is called carbylamine reaction. Carbylamines has an offensive smell. This reaction is answered only by primary amine and hence to distinguish primary amine from other classes of amines.

In general

R-NH2  +  CHCl3  +  3KOH  --------->  RNC (carbylamine) +  3KCl + 3H2O