Preview:

This chapter deals with amines, their structure and various reactions that they undergo.

We shall begin by defining amines as derivatives of ammonia, obtained by replacement of one, two or all the three hydrogen atoms by alkyl or aryl groups. We will then discuss its structure. Further, we shall classify amines based on the number of hydrogen atoms replaced by alkyl or aryl groups in ammonia molecule into: primary ($1^o$), secondary ($2^o$) and tertiary ($3^o$). Next, we shall learn about its nomenclature.

Then, we will see how amines can be obtained through various reactions, under the head: Preparation of Amines. Physical and Chemical properties of amines shall then be dealt with, in detail. In chemical properties, our focus would be on reactions of amines with various substances and their effects on the stability of amine.

We shall then come to the study of Diazonium salts and; discuss its nomenclature and stability. Its physical and chemical properties shall also be dealt with in some detail.

We shall end this chapter with an interesting topic called, Coupling reactions which is a special type of reaction.

Lastly, we will study the significance of diazonium salts.

15.0 Introduction:

Amines are a group of organic compounds derived by replacing one or more hydrogen atoms of ammonia molecule by alkyl or aryl group(s).

Amines are present in proteins, vitamins, alkaloids and hormones. Synthetic products in which amines are present: polymers, dyestuffs and drugs.

Applications and uses of amines:

Two biologically active compounds, namely adrenaline and ephedrine, both containing secondary amino group, are used to increase blood pressure.
Novocain, a synthetic amino compound, is used as an anaesthetic in dentistry.
Benadryl, a well-known antihistaminic drug also contains tertiary amino group.
Quaternary ammonium salts are used as surfactants.
Diazonium salts are intermediates in the preparation of a variety of aromatic compounds including dyes.

Questions for the section:

1. What are amines? List its applications.

13.1 Structure of Amines

Definition box:
Amines: Amines can be defined as derivatives of ammonia, obtained by replacement of one, two or all the three hydrogen atoms by alkyl or aryl groups.

Structure of Amine:

Like ammonia, amines have a trivalent nitrogen atom with an unshared pair of electrons.
Nitrogen orbitals in amines are therefore, $sp^3$ hybridised. Thus, the geometry of amines is pyramidal.
Each of the three $sp^3$ hybridised orbitals of nitrogen overlap with orbitals of hydrogen or carbon depending upon the composition of the amine. The fourth orbital of nitrogen in all amines contains an unshared pair of electrons.
Due to the presence of unshared pair of electrons, the angle C–N–E, (where E is C or H) is less than $109.5^{\circ}$; for instance, it is $108^o$ in case of trimethylamine as shown in Figure 13.1.

Fig. 13.1 Pyramidal shape of trimethylamine

13.2 Classification of Amines:

Amines are classified as primary ($1^o$), secondary $(2^o)$ and tertiary $(3^o)$ depending upon the number of hydrogen atoms replaced by alkyl or aryl groups in ammonia molecule.

Primary amine: If one hydrogen atom of ammonia is replaced by R or Ar, we get a primary amine ($1^o$) like: $RNH_2$ or $ArNH_2$.
Secondary amine: If two hydrogen atoms of ammonia or if one hydrogen atom of “R-NH₂” is replaced by another alkyl/aryl (R’) group, we get secondary amine: R-NHR’. (The second alkyl or aryl group—R’ may be same or different).
Tertiary amine: Replacement of two hydrogen atoms by alkyl or aryl groups gives rise to the formation of tertiary amine.

Amines are said to be simple when all the alkyl or aryl groups are the same, and mixed when they are different.

Questions for section 13.1 and 13.2:

1. Explain the structure of Amine based on the number of hydrogen atoms replaced by alkyl or aryl groups.

2. When do we say that an amine is simple and when do we call it mixed?

3. Classify the following amines as primary, secondary or tertiary:

4. Write structures of different isomeric amines corresponding to the molecular formula,$ C_4H_{11}N$.

13.3 Nomenclature of Amines:

Nomenclature of Aliphatic amines:

In common system, an aliphatic amine is named by adding the prefix: “alkyl” to amine, that is: “alkylamine” (for example: methylamine).
In IUPAC system, amines are named as alkanamines , derived by replacing of ‘e’ of alkane group by the word, “amine”. For example, $CH_3NH_2$ is named as methanamine.
In naming an alkanamine, the longest chain of C atom is selected and the name of the amine is obtained by replacing the ending ‘e’ of alkane group by the word, “amine” as discussed in the previous point.

Nomenclature of Secondary and Tertiary amines:

The secondary and tertiary amines are named as N- alkyl pri amines. That is, alkanamines or N- alkyl amino alkanes. While naming these, the word, N-alkyl or N,N-dialkyl is prefixed to the word aminoalkane or alkanamine.
In these secondary and tertiary amines, when two or more groups are the same, the prefix “di” or “tri” is appended before the name of alkyl group.
In case, more than one amino group is present at different positions in the parent chain, their positions are specified by giving numbers to the carbon atoms bearing “–$NH_2$ groups” with a suitable prefix such as di, tri, and so on is attached to the amine. Furthermore, the letter, ‘e’ of the suffix of the hydrocarbon part is retained. For example, $H_2N–CH_2–CH_2–NH_2$ is named as ethane-1, 2-diamine.

Nomenclature of Aromatic amines:

In arylamines, “–$NH_2$ group” is directly attached to the benzene ring. $C_6H_5NH_2$ is the simplest example of arylamine. In common system, it is known as aniline. It is also an accepted IUPAC name.
While naming arylamines according to IUPAC system, suffix ‘e’ of arene group is replaced by ‘amine’. Thus in IUPAC system, $C_6H_5–NH_2$ is named as benzenamine.

Common and IUPAC names of some alkylamines and arylamines are given in Table 13.1.

Questions from section 13.3:

1. Explain the nomenclature of amines with examples.

2. Write IUPAC names of all the isomers.

3. What type of isomerism is exhibited by different pairs of amines?

4. How do we convert the following?

(i) Benzene into aniline.

(ii) Benzene into N, N-dimethylaniline.

(iii) $Cl–(CH_2)_4–Cl$ into hexan-1,6-diamine.

13.4 Preparation of Amines

Amines are prepared by the following methods:

1. Reduction of nitro compounds:

Nitro compounds are reduced to amines by passing hydrogen gas in the presence of finely divided nickel, palladium or platinum and also by reduction with metals in acidic medium. Thus, Nitroalkanes can be reduced to the corresponding alkanamines by this method.

Reduction with iron scrap and hydrochloric acid is preferred because FeCl₂ formed gets hydrolysed to release hydrochloric acid during the reaction; thus, reducing the amount of hydrochloric acid required to initiate the reaction.

2. Ammonolysis of alkyl halides:

A carbon – halogen bond in alkyl or benzyl halides can be easily cleaved by a nucleophile. Hence, an alkyl or benzyl halide on reaction with an ethanolic solution of ammonia undergoes nucleophilic substitution reaction in which the halogen atom is replaced by an amino (–NH₂) group.
This process of cleavage of the C–X bond by ammonia molecule is known as ammonolysis.
The reaction is carried out in a sealed tube at 373 K.
The primary amine thus obtained behaves as a nucleophile and further reacts with alkyl halide to form secondary and tertiary amines, and finally forms quaternary ammonium salt.

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

Ammonolysis has the disadvantage of yielding a mixture of primary, secondary and tertiary amines and also a quaternary ammonium salt. However, primary amine is obtained as a major product by taking large excess of ammonia.
The order of reactivity of halides with amines is RI > RBr >RCl.

Example 13.1: Write chemical equations for the following reactions:

(i) Reaction of ethanolic $NH_3$ with $C_2H_5Cl$.

(ii) Ammonolysis of benzyl chloride and reaction of amine so formed with two moles of $CH_3Cl$.

Solution:

3. Reduction of nitriles

Concept box:
Hydrogenation refers to treatment with hydrogen. It can be defined as the chemical reaction between molecular hydrogen ($H_2$) and another compound or element, in the presence of a catalyst such as nickel, palladium or platinum.

Nitriles, on reduction with lithium aluminium hydride ($LiAlH_4$) or by catalytic hydrogenation produce primary amines.
This reaction is used for ascent of amine series. That is, for preparation of amines containing one carbon atom more than the starting amine.

4. Reduction of amides

Amides on reduction with lithium aluminium hydride yield amines.

5. Gabriel phthalimide synthesis

Gabriel synthesis is used for the preparation of primary amines.

Phthalimide on treatment with ethanolic potassium hydroxide forms: potassium salt of phthalimide.
The salt, on heating with alkyl halide forms N- Alkylphthalamide.
N- Alkylphthalamide is subjected to alkaline hydrolysis to produce the corresponding primary amine.

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

6. Hoffmann bromamide degradation reaction

Hoffmann developed a method for preparation of primary amines.
In this method, an amide is treated with bromine in an aqueous or ethanolic solution of sodium hydroxide.
By degradation reaction, migration of an alkyl or aryl group takes place from carbonyl carbon of the amide to the nitrogen atom to form an amine.

· The amine so formed contains one carbon less than that present in the amide.

Example 13.2: Write chemical equations for the following conversions:

(i) $ \ce { CH3–CH2–Cl into CH3–CH2–CH2–NH2}$

(ii) $ \ce { C6H5–CH2–Cl into C6H5–CH2–CH2–NH2}$

Solution:

Example 13.3: Write structures and IUPAC names of

(i) the amide which gives propanamine by Hoffmann bromamide reaction.

(ii) the amine produced by the Hoffmann degradation of benzamide.

Solution:

(i) Propanamine contains three carbons. Hence, the amide molecule must contain four carbon atoms. Structure and IUPAC name of the starting amide with four carbon atoms are given below:

(ii) Benzamide is an aromatic amide containing seven carbon atoms.Hence, the amine formed from benzamide is aromatic primary amine containing six carbon atoms.

Questions from section 13.4:

1. Explain the preparation of amine by reduction of nitro compounds with examples.

2. Explain the preparation of amine by Ammonolysis of alkyl halides with an example.

3. Explain the preparation of amine by reduction of nitriles.

4. Explain the preparation of amine by reduction of amides.

5. Explain the processes:

a) Gabriel phthalimide synthesis.

b) Hoffmann bromamide degradation reaction.

13.5 Physical Properties

The lower aliphatic amines are gases with fishy odour.
Primary amines with three or more carbon atoms are liquid and with even more carbon atoms are solid.
Aniline and other arylamines are usually colourless but get coloured on storage due to atmospheric oxidation.
Lower aliphatic amines are soluble in water because they can form hydrogen bonds with water molecules. However, solubility decreases with increase in molar mass of amines due to increase in size of the hydrophobic alkyl part. Higher amines are essentially insoluble in water.
Out of butan-1-ol and butan-1-amine, amines are more soluble in butan-1-ol. This is because amines are soluble in organic solvents like alcohol, ether and benzene. This can be attributed to the fact that, alcohols are more polar than amines and form stronger intermolecular hydrogen bonds than amines.
Primary and secondary amines exhibit intermolecular association due to hydrogen bonding between nitrogen of one and hydrogen of another molecule. This intermolecular association is more in primary amines than in secondary amines as there are two hydrogen atoms available for hydrogen bond formation in it. Tertiary amines do not have intermolecular association due to the absence of hydrogen atom available for hydrogen bond formation.
Therefore, the order of boiling points of isomeric amines is as follows:

$\quad$ $\quad$ Primary > Secondary > Tertiary

Intermolecular hydrogen bonding in primary amines is shown in Fig. 13.2.

Fig. 13.2 Intermolecular hydrogen bonding in primary amines

Boiling points of amines, alcohols and alkanes of almost the same molar mass are shown in Table 13.2.

Questions for section 13.5:

1. List the physical properties of amines.

2. Complete the following acid-base reactions and name the products:

(i) $CH_3CH_2CH_2NH_2$ + HCl →

(ii)$ (C_2H_5)_3N + HCl$ →

13.6 Chemical Reactions

Difference in electronegativity between nitrogen and hydrogen atoms and the presence of unshared pair of electrons over the nitrogen atom makes amines reactive.
The number of hydrogen atoms attached to nitrogen atom also decides the course of reaction of amines; that is why primary (–$NH_2$), secondary: and tertiary amines: differ in many reactions.
Amines behave as nucleophiles due to the presence of unshared electron pair.

Some of the reactions of amines are described below:

1. Basic character of amines

Concept box:
A compound or ionic species which can donate an electron pair to an acceptor compound is known as Lewis base.

Amines, being basic in nature, react with acids to form salts:

Amine salts on treatment with a base like NaOH then regenerate the parent amine:

Amine salts are soluble in water but insoluble in organic solvents like ether. Thus, amines get separated from the non-basic organic compounds which are insoluble in water.
The reaction of amines with mineral acids to form ammonium salts shows that these are basic in nature.
Amines have an unshared pair of electrons on nitrogen atom due to which they behave as Lewis base. Basic character of amines can be better understood in terms of their K_b and pK_b values as explained below:

Larger the value of $K_b$ or, smaller the value of $pK_b$, stronger is the base.
Aliphatic amines are stronger bases than ammonia due to +I effect (Inductive effect) of alkyl groups leading to high electron density on the nitrogen atom. Their $pK_b$, values lie in the range of 3 to 4.22.
On the other hand, aromatic amines are weaker bases than ammonia due to the electron withdrawing nature of the aryl group.

Structure-basicity relationship of amines

Basicity of amines is related to their structure. Basic character of an amine depends upon the ease of formation of cation by accepting a proton from the acid. The more stable the cation is relative to the amine, more basic is the amine.

(a) Alkanamines versus ammonia:

We shall compare the relative basic strengths of Ammonia and Methyl amine in this part of the section.

Ammonia and Amines are basic in nature because of the presence of lone pair of electrons on N- atom. Easier the donation of lone pair of electrons on nitrogen, stronger will be the base.
Let us consider the reaction of an alkanamine and ammonia with a proton to compare their basicity.

Due to the electron releasing nature of alkyl group, R pushes electrons towards nitrogen and thus makes the unshared electron pair more available for sharing with the proton of the acid.
Moreover, the substituted ammonium ion formed from the amine gets stabilised due to dispersal of the positive charge by the +I effect of the alkyl group. Hence, alkylamines are stronger bases than ammonia. Thus, the basic nature of aliphatic amines will increase with increase in the number of alkyl groups. This trend is followed in the gaseous phase. Thus, the order of basicity of amines in the gaseous phase follows the expected order:
$\quad \quad$ tertiary amine > secondary amine > primary amine > $NH_3$
The trend is not regular in the aqueous state as evident by their pK_b values given in Table 13.3. In the aqueous phase, the substituted ammonium cations get stabilised not only by electron releasing effect of the alkyl group (+I) but also by solvation with water molecules. The greater the size of the ion, lesser will be the solvation and the less stabilised is the ion. The order of stability of ions are as follows:

Decreasing order of extent of H-bonding in water and order of stability of ions by solvation

Greater the stability of the substituted ammonium cation, stronger will be the corresponding amine as a base.
Thus, the order of basicity of aliphatic amines should be:
$\quad \quad$ primary > secondary > tertiary,

The order would be opposite to the inductive effect based order.

Secondly, when the alkyl group is small, like –$CH__3$ group, there is no steric hindrance to H-bonding. In case the alkyl group is bigger than $CH__3$group, there will be steric hindrance to H-bonding. Therefore, the change of nature of the alkyl group, from –$CH_3$ to –$C_2H_5$ results in change of the order of basic strength.
Thus, interactions of the inductive effect, solvation effect (hydration effect) and steric hinderance of the alkyl group decide the basic strength of alkyl amines in the aqueous state. The table below shows the extent of these effects:

Factors favouring the increase in basic character	Basic strength
Electron releasing inductive effect	$3^o > 2^o > 1^o $
Steric effect	$ 1^o > 2^o > 3^o$
Hydration effect	$ 1^o > 2^o > 3^o$

The resultant of all the above factors gives the order of basic strength as: 2⁰>1⁰>3⁰.
The order of basic strength in case of methyl substituted amines and ethyl substituted amines in aqueous solution is as follows:
$$ \ce { (C_2H_5)_2NH > (C_2H_5)_3N > C_2H_5NH_2 > NH_3}$$
$$\ce {(CH_3)_2 NH > CH_3NH_2 > (CH_3)_3N > NH_3}$$

(b) Arylamines versus ammonia

$pK__b$ value of aniline is quite high because, in aniline (or other arylamines), the -$NH__2$ group is attached directly to the benzene ring. It results in the unshared electron pair on nitrogen atom to be in conjugation with the benzene ring; thus making it less available for protonation (addition of proton). Lesser the proton acceptability, lesser is the basic strength.
Aniline is a resonance hybrid of the following five structures:

On the other hand, anilinium ion—obtained by accepting a proton can have only two resonating structures (kekule’s structures):

We know that, greater the number of resonating structures, greater is the stability. Thus, we can infer that, aniline (five resonating structures) is more stable than anilinium ion. Hence, the proton acceptability or the basic nature of aniline or other aromatic amines would be less than that of ammonia.
In case of substituted aniline, it is observed that, electron releasing groups like –$OCH_3, –CH_3$ increase basic strength whereas electron withdrawing groups like –$NO_2, –SO_3$, –COOH, –X decrease the basic strength.

Example 13.4: Arrange the following in decreasing order of their basic strength:

$$ \ce { C6H5NH2, C2H5 NH2, (C2H5)2 NH, NH3 }$$

Solution: The decreasing order of basic strength of the above amines and ammonia follows the following order:
$$ \ce { (C2H5)2NH > C2H5NH2 > NH3 > C6H5NH2 }$$

Reactions of amines with various substances:

1. Alkylation

Amines undergo alkylation on reaction with alkyl halides.

2. Acylation

Aliphatic and aromatic primary and secondary amines react with acid chlorides, anhydrides and esters by nucleophilic substitution reaction. This reaction is known as acylation. We can also consider this reaction as the replacement of hydrogen atom of –$NH_2$ or >N–H group by the acyl group.
The products obtained by acylation reaction are known as amides. The reaction is carried out in the presence of a base stronger than the amine, like pyridine, which removes HCl so formed and shifts the equilibrium to the right hand side.

3. Benzoylation:

Amines also react with benzoyl chloride ($C_6H_5COCl$). This reaction is known as benzoylation as shown below:

$$ \ce {CH_3NH_2 + C_6H_5COCl -> CH_3NHCOC_6H_5 + HCl }$$

Methanamine Benzoyl chloride N-Methylbenzamide

4. At room temperature, Amines react with carboxylic acids to form salts with amines.

5. Carbylamine reaction

Aliphatic and aromatic primary amines on heating with chloroform and ethanolic potassium hydroxide form isocyanides or carbylamines. These products are foul smelling substances.
Secondary and tertiary amines do not show this reaction.
This reaction is known as carbylamines reaction or isocyanide test and is used as a test for primary amines.
$$ \ce { R-NH_2 + CHCl_3 + 3KOH ->[{Heat}] R-NC + 3KCl + 3H_2O}$$

6. Reaction with nitrous acid

The three classes of amines react differently with nitrous acid. The nitrous acid is prepared in situ from a mineral acid and sodium nitrite as described:

(a) Primary aliphatic amines react with nitrous acid to form aliphatic diazonium salts which being unstable, liberates nitrogen gas quantitatively along with alcohols.

(Quantitative evolution of nitrogen is used for estimation of amino acids and proteins)

(b) Aromatic amines react with nitrous acid at low temperatures (273-278 K) to form diazonium salts. These salts are used for synthesis of a variety of aromatic compounds

8. Benzenesulphonyl chloride ($C_6H_5SO_2Cl$), which is also known as Hinsberg’s reagent, reacts with primary and secondary amines to form sulphonamides as described:

(a) The reaction of benzenesulphonyl chloride with primary amine yields N-ethylbenzenesulphonyl amide.

The hydrogen attached to nitrogen in sulphonamide is strongly acidic due to the presence of strong electron withdrawing sulphonyl group. Hence, it is soluble in alkali.

(b) Benzenesulphonyl chloride reacts with secondary amine to form N,N-diethylbenzenesulphonamide.

Since N, N-diethylbenzene sulphonamide does not contain any hydrogen atom attached to nitrogen atom, it is not acidic and hence insoluble in alkali.

This property of amines reacting with benzenesulphonyl chloride in a different manner is used for the distinction of primary, secondary and tertiary amines and also for the separation of a mixture of amines.

Note box:
These days, benzenesulphonyl chloride is replaced by p-toluenesulphonyl chloride for the distinction test.

9. Electrophilic substitution

As we now know, aniline is a resonance hybrid of five structures. Ortho and Para positions to the –$NH_2$ group become centres of high electron density. Thus, –$NH_2$ group is ortho and para directing and a powerful activating group.

(a) Bromination: Aniline reacts with bromine water at room temperature to give a white precipitate of 2,4,6-tribromoaniline.

Test to distinguish between 1⁰, 2⁰ and 3⁰ amines:

Test	$1^o$ amine	$2^o$ amine	$3^o$ amine
Methylation test Amine + methyl iodide, heated	React with 3 moles of $CH_3$I/mole of amine to form qua ammino salt	React with 2 moles of $CH_3$I/mole of amine to form qua ammno salt	React with 1 mole of $CH_3$I/mole of amine to form qua ammo salt
Reaction with $HNO_2$ Amine + $HNO_2$ N – 273 – 278 k	$N_2$ gas evolved alcohol formed	Yellow oily layer is obtained	Clear solutions of trialkylammo salts are obtained
Hinsberg’s regeant Amine + benzene sulphonyl chloride + cold aqueous NaOH acidified with HCl	Homogenous solution Insoluble material	Ppt is formed Insoluble material	Remains insoluble Homogenous solution

The lone pair of electrons on nitrogen of acetanilide interacts with oxygen atom due to resonance as shown below:

Hence, the lone pair of electrons on nitrogen is less available for donation to benzene ring by resonance. Therefore, activating effect of –$NHCOCH_3$ group is less than that of amino group.

(b) Nitration: Nitration of aniline yields tarry oxidation products in addition to the nitro derivatives. Moreover, in the strongly acidic medium, aniline is protonated to form the anilinium ion which is meta directing. That is why, besides the ortho and para derivatives, significant amount of meta derivative is also formed.

However, by protecting the –$NH_2$ group by acetylation reaction with acetic anhydride, the nitration reaction can be controlled and the p-nitro derivative can be obtained as the major product.

(c) Sulphonation: Aniline reacts with concentrated sulphuric acid to form anilinium hydrogensulphate which on heating with sulphuric acid at 453-473K produces p-aminobenzene sulphonic acid, commonly known as sulphanilic acid, as the major product.

Note box:
Aniline does not undergo Friedel Craft’s reaction (alkylation and acetylation) due to salt formation with aluminium chloride (which acts as Lewis acid) used as a catalyst. Due to this, nitrogen of aniline acquires positive charge and hence acts as a strong deactivating group for further reaction.

Questions for section 13.6:

1. What makes amines reactive compounds?

2. Why do amines behave as nucleophiles?

3. Explain the basic nature of amines with specific examples.

4. Explain the reaction of alkamines with ammonia.

5. Explain the effect of steric hindrance in amines.

6. List the three effects that affect the basic strength of amines.

7. State the order of basicity of aliphatic amines.

8. Draw the resonance structures of aniline.

9. Draw the resonance structures of anilinium ion or Kekule’s structures.

10. How does the number of resonating structures for a compound affect its stability?

11. How do electron releasing and withdrawing groups affect the basic strength of amines?

12. Explain the following reactions:

a) Alkylation

b) Acylation

c) Reaction of amines with carboxylic acids

d) Carbylamine reaction

e) Reaction of amines with nitrous acid

f) Reaction of amines with nitrous acid with Benzenesulphonylchloride

g) Electrophilic substitution (with all sub-types)

13. Write reactions of the final alkylation product of aniline with excess of methyl iodide in the presence of sodium carbonate solution.

13.7 Diazonium Salts:

The diazonium salts have the general formula: –R₂X (page 16 ncert); where “R” stands for an aryl group and “X” ion may be $Cl^–, Br^–, HSO_4^− $and $BF_4^−$.

Nomenclature:

They are named by adding the suffix: “diazonium” to the name of the parent hydrocarbon (from which the salt is formed), followed by the name of anion such as chloride, hydrogensulphate and so on.
The $N_2^+$ group is called diazonium group.
Forexample: $C_6H_5N_2^+Cl^-$ is named benzenediazonium chloride and $C_6H_5N_2^+HSO_4^–$ is known as benzenediazonium hydrogensulphate.

Stability of Diazonium salts:

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).
The stability of arenediazonium ion is explained on the basis of resonance as depicted:

Method of Preparation of Diazoniun Salts:

Benzenediazonium chloride is prepared by the reaction of aniline with nitrous acid at 273-278K.
The Nitrous acid in the reaction is produced by reaction of sodium nitrite with hydrochloric acid.
$$ \ce { C_6H_5NH_2 + NaNO_2 + 2 HCl ->[{273-278 K}] C_6H_5N_2Cl + NaCl + 2H_2O}$$
This reaction in which primary aromatic amines is converted into diazonium salts is known as diazotisation.

Note box:
Due to instability of diazonium salt, it is not generally stored and is used immediately after its preparation.

13.8 Physical Properties

Benzenediazonium chloride is a colourless crystalline solid.
It is readily soluble in water.
It is stable in cold water but reacts with warm water.
It decomposes easily in the dry state.
Benzenediazonium fluoroborate is water insoluble and stable at room temperature.

Questions from sections 13.7 and 13.8:

1. State the general formula to represent diazonium salts. Explain its nomenclature.

2. Write a note on stability of diazonium salts.

3. Explain diazotisation reaction.

4. List the physical properties of diazonium salts.

5. Write chemical reaction of aniline with benzoyl chloride and write the name of the product obtained.

6. Write structures of different isomers corresponding to the molecular formula, $C_3H_9N$ Write IUPAC names of the isomers which will liberate nitrogen gas on treatment with nitrous acid.

13.9 Chemical Reactions

The reactions of diazonium salts can be broadly divided into two categories:

(A) Reactions involving displacement of nitrogen

(B) Reactions involving retention of diazo group.

Reactions involving displacement of nitrogen:

Diazonium group being a very good leaving group gets substituted by other groups such as $Cl^–, Br^–, I^–, CN^– $ and $OH^–$ which displace nitrogen from the aromatic ring. The nitrogen formed escapes from the reaction mixture as a gas.

We shall now look at some of such substitutions:

1. Replacement by halide or cyanide ion:

The $Cl^–, Br^–$ and $CN^–$ nucleophiles can easily be introduced in the benzene ring in the presence of Cu(I) ion. This reaction is called Sandmeyer reaction.

Alternatively, chlorine or bromine can also be introduced in the benzene ring by treating the diazonium salt solution with corresponding halogen acid in the presence of copper powder. This is referred as Gatterman reaction.

Note box:
The yield in Sandmeyer reaction is found to be better than in Gattermann reaction.

2. Replacement by iodide ion:

Iodine cannot be easily introduced into the benzene ring. However, when the diazonium salt solution is treated with potassium iodide, iodobenzene is formed.

3. Replacement by fluoride ion:

When arenediazonium chloride is treated with fluoroboric acid, arene diazonium fluoroborate is precipitated which on heating decomposes to yield aryl fluoride.

4. Replacement by H:

Mild reducing agents like, hypophosphorous acid (phosphinic acid) or ethanol reduce diazonium salts to arenes.
The phosphinic acid itself gets oxidised to phosphorous acid and ethanol gets oxidized to ethanal during the course of the reaction.

5. Replacement by hydroxyl group:

If the temperature of the diazonium salt solution is allowed to rise upto 283 K, the salt gets hydrolysed to phenol.

6. Replacement by –$NO_2$ group:

N₂Cl ring reacts with fluroboric acid to produce diazonium fluoroborate. When this diazonium fluoroborate is heated with aqueous sodium nitrite solution, in the presence of copper, the diazonium group is replaced by –$NO_2$ group.

Reactions involving retention of diazo group

Coupling reactions:

Definition box:
Azo coupling: An azo coupling is a reaction between a diazonium compound and another aromatic compound that produces an azo compound. In most cases, the diazonium compound produced is also aromatic.

The azo products obtained, form a conjugate system having both the aromatic rings joined through the –N=N– bond.
These compounds are often coloured and are used as dyes.

Benzene diazonium chloride reacts with phenol—in which, the phenol molecule at its para position is coupled with the diazonium salt to form p-hydroxyazobenzene. This type of reaction is an example for coupling reaction.

Similarly, the reaction of diazonium salt with aniline yields p-aminoazobenzene. This is an example of electrophilic substitution reaction.

13.10 Importance of Diazonium Salts in Synthesis of Aromatic Compounds

From the above reactions, it is clear that, the diazonium salts are very good intermediates for the introduction of –F, –Cl, –Br, –I, –CN, –OH, –$NO_2$ groups into the aromatic ring.
Aryl fluorides and iodides cannot be prepared by direct halogenation. Also, the cyano group cannot be introduced by nucleophilic substitution of chlorine in chlorobenzene. However, cyanobenzene can be easily obtained from diazonium salt.
Thus, the replacement of diazo group by other groups is helpful in preparing those substituted aromatic compounds which cannot be prepared by direct substitution in benzene.

Example 13.5: How will you convert 4-nitrotoluene to 2 bromobenzoic acid?