Amines Class 12 Chemistry Chapter 9 Notes

Amines

Definition: Amines are organic compounds derived from ammonia (NH3) by replacing one or more of its hydrogen atoms with alkyl or aryl groups. They are characterized by the presence of a nitrogen atom bonded to carbon atoms.

Occurrence in Nature:

Proteins: Amines are integral components of proteins, where they play a crucial role in the structure and function of these biomolecules.
Vitamins: Several vitamins, such as niacin and pyridoxine, contain amine groups in their structures.
Alkaloids: A large group of naturally occurring compounds with diverse pharmacological effects, including morphine, caffeine, and nicotine, are alkaloids, which often contain amine groups.
Hormones: Some hormones, like adrenaline and serotonin, contain amine functional groups.

Synthetic Applications:

Polymers: Amines are used in the synthesis of various polymers, including nylon and polyurethane.
Dye Stuffs: Amines are crucial intermediates in the production of dyes, which are widely used in textiles and other industries.
Pharmaceuticals: Many drugs and medications, such as Novocain (a local anesthetic) and antihistamines like Benadryl, contain amine functional groups.
Surfactants: Quaternary ammonium salts, a type of ammonium compound with four organic groups attached to a central nitrogen atom, are used as surfactants in detergents and cleaning products.
Diazonium Salts: Diazonium salts are intermediates in the synthesis of various aromatic compounds, including dyes, pigments, and pharmaceuticals.

Biological and Medicinal Uses:

Adrenaline and Ephedrine: These compounds, both containing secondary amine groups, are used to increase blood pressure and have various medical applications.
Anaesthetics: Novocain, a synthetic amine compound, is used as a local anesthetic in dentistry.
Antihistamines: Benadryl is a well-known antihistamine drug used to relieve allergy symptoms. It contains a tertiary amine group.

Industrial Applications: Amines find application in various industrial processes, including the production of chemicals, pharmaceuticals, and polymers.

Diazonium Salts: These are essential intermediates in the preparation of a variety of aromatic compounds, making them valuable in the dye and pigment industries.

Structure of Amines

Hybridization: The nitrogen atom in amines is sp3 hybridized. This means that one s orbital and three p orbitals from nitrogen combine to form four equivalent sp3 hybrid orbitals. These hybrid orbitals then overlap with orbitals of hydrogen or carbon.

Geometry: Amines exhibit a pyramidal geometry around the nitrogen atom. The reason for this pyramidal shape is the presence of a lone pair of electrons on nitrogen. The nitrogen atom has three sigma bonds (two to hydrogens and one to a carbon or another nitrogen), and the lone pair of electrons occupies the fourth, non-bonding orbital. This arrangement results in a trigonal pyramidal geometry, where the three sigma bonds and the lone pair of electrons are arranged tetrahedrally around the nitrogen atom.

Bond Angle: Due to the presence of the lone pair of electrons on nitrogen, the bond angle in amines is less than the ideal tetrahedral angle of 109.5 degrees. This lone pair of electrons repels the bonding pairs, causing the bond angles to be slightly less than 109.5 degrees. For example, in the case of trimethylamine (CH3)3N, the bond angle around the nitrogen atom is approximately 108 degrees.

Classification of Amines

The classification of amines based on the number of hydrogen atoms replaced by alkyl or aryl groups in ammonia molecules is as follows:

Primary Amines (1o): In primary amines, one hydrogen atom of ammonia (NH3) is replaced by an alkyl (R) or aryl (Ar) group. The general formula for primary amines is RNH2 or ArNH2. Primary amines have one alkyl or aryl group and two hydrogen atoms bonded to the nitrogen atom.
Secondary Amines (2o): In secondary amines, two hydrogen atoms of ammonia or one hydrogen atom of a primary amine (RNH2 or ArNH2) are replaced by alkyl (R) or aryl (Ar) groups. The general formula for secondary amines is R2NH or ArNH. Secondary amines have two alkyl or aryl groups and one hydrogen atom bonded to the nitrogen atom.
Tertiary Amines (3o): In tertiary amines, all three hydrogen atoms of ammonia or two hydrogen atoms of a primary amine are replaced by alkyl (R) or aryl (Ar) groups. The general formula for tertiary amines is R3N or ArN. Tertiary amines have three alkyl or aryl groups bonded to the nitrogen atom and no hydrogen atoms.

In addition to this classification, amines can also be categorized as “simple” or “mixed” based on the nature of the alkyl or aryl groups attached to the nitrogen atom:

Simple Amines: Simple amines have all the alkyl or aryl groups on the nitrogen atom identical. For example, if all three R groups in a tertiary amine are the same (e.g., RRRN), it is a simple tertiary amine.
Mixed Amines: Mixed amines have different alkyl or aryl groups attached to the nitrogen atom. For example, if in a tertiary amine, two or more of the R groups are different (e.g., R1R2R3N), it is a mixed tertiary amine.

Nomenclature of Amines

The nomenclature of amines can vary depending on whether you are using the common system or the IUPAC (International Union of Pure and Applied Chemistry) system. Here’s a summary of the nomenclature of amines in both systems:

Common System:

Aliphatic Amines: Aliphatic amines are named by prefixing the name of the alkyl group to “amine” as one word. For example, CH3NH2 is named as “methylamine.”
Secondary and Tertiary Amines: In secondary and tertiary amines, when two or more alkyl groups are the same, you can use the prefixes “di” or “tri” before the name of the alkyl group. For example, (CH3)2NH can be named as “dimethylamine.”

IUPAC System:

Primary Amines: Primary amines in the IUPAC system are named as “alkanamines.” To name them, replace the ‘e’ of the corresponding alkane with “amine.” For example, CH3NH2 is named as “methanamine.”
Multiple Amino Groups: When there is more than one amino group (-NH2) in the parent chain, you need to specify their positions by assigning numbers to the carbon atoms bearing these groups. Use suitable prefixes like “di” or “tri” before the amine name. The ‘e’ at the end of the hydrocarbon part’s suffix is retained. For example, H2N-CH2-CH2-NH2 is named as “ethane-1,2-diamine.”
Secondary and Tertiary Amines: In the IUPAC system, for secondary and tertiary amines, use the locant “N” to designate the substituents attached to the nitrogen atom. For example, CH3NHCH2CH3 is named as “N-methylethanamine,” and (CH3CH2)3N is named as “N,N-diethylethanamine.”

Arylamines: In arylamines, where the -NH2 group is directly attached to a benzene ring (C6H5), you can use both the common and IUPAC names.

Common System: The common name is used, such as “aniline.”
IUPAC System: In the IUPAC system, replace the ‘e’ of the arene name with “amine.” For example, C6H5-NH2 is named as “benzenamine.”

Preparation of Amines

The preparation of amines involves several methods, each suitable for different starting materials and desired amine products. Here’s a summary of the methods for preparing amines:

1. Reduction of Nitro Compounds:

Nitro compounds (R-NO2) are reduced to amines (R-NH2) by passing hydrogen gas in the presence of catalysts like finely divided nickel, palladium, or platinum.
They can also be reduced with metals in acidic medium.
Nitroalkanes can be reduced to the corresponding alkanamines.

2. Ammonolysis of Alkyl Halides:

Alkyl or benzyl halides (R-X) can undergo ammonolysis, a nucleophilic substitution reaction, in which the halogen atom is replaced by an amino group (NH2).
The reaction is carried out in a sealed tube with an ethanolic solution of ammonia at an elevated temperature.
This method can yield a mixture of primary, secondary, and tertiary amines, along with quaternary ammonium salts.
Primary amines can be isolated by using excess ammonia.

3. Reduction of Nitriles:

Nitriles (RC≡N) can be reduced to primary amines (R-NH2) by using reducing agents like lithium aluminum hydride (LiAlH4) or by catalytic hydrogenation.
This method is useful for increasing the number of carbon atoms in the amine structure.

4. Reduction of Amides:

Amides (RCO-NH2) can be reduced to amines (R-NH2) using lithium aluminum hydride (LiAlH4) as a reducing agent.

5. Gabriel Phthalimide Synthesis:

This method is used for the preparation of primary amines.
Phthalimide is treated with ethanolic potassium hydroxide to form the potassium salt of phthalimide.
This salt, when heated with an alkyl halide, followed by alkaline hydrolysis, produces the corresponding primary amine.
Aromatic primary amines cannot be prepared by this method.

6. Hoffmann Bromamide Degradation Reaction:

This reaction is employed to prepare primary amines.
An amide is treated with bromine in an aqueous or ethanolic solution of sodium hydroxide.
In this degradation reaction, an alkyl or aryl group migrates from the carbonyl carbon of the amide to the nitrogen atom.
The resulting amine contains one carbon less than the original amide.

The choice of method depends on the starting material and the desired amine product, whether it is primary, secondary, or tertiary amine, and whether an increase or decrease in the number of carbon atoms is required in the amine structure.

Physical Properties of Amines

The physical properties of amines are influenced by their molecular structure, particularly the presence of amino groups and the size of alkyl or aryl substituents. Here are some key points regarding the physical properties of amines:

1. State and Odor:

Lower aliphatic amines (those with fewer carbon atoms) are typically gases at room temperature and have a fishy odor.
Primary amines with three or more carbon atoms are liquids, and higher primary amines are solid at room temperature.
Aniline and other arylamines are usually colorless but may develop color upon exposure to air due to atmospheric oxidation.

2. Solubility:

Lower aliphatic amines are soluble in water because they can form hydrogen bonds with water molecules.
However, the solubility of amines decreases with an increase in molar mass due to the hydrophobic nature of the alkyl part.
Higher amines (those with longer alkyl chains) are essentially insoluble in water.

3. Comparison with Alcohols:

Amines and alcohols have different polarities. The electronegativity of nitrogen in amines is lower (3.0) than the electronegativity of oxygen in alcohols (3.5).
Because of this, alcohols form stronger intermolecular hydrogen bonds with water than amines.
In a comparison between butan-1-ol (an alcohol) and butan-1-amine (a primary amine), butan-1-ol is more soluble in water due to its stronger hydrogen bonding capabilities.

4. Hydrogen Bonding and Boiling Points:

Primary and secondary amines can engage in intermolecular association due to hydrogen bonding between the nitrogen of one molecule and the hydrogen of another.
Primary amines have two hydrogen atoms available for hydrogen bond formation, making their intermolecular association stronger than secondary amines, which have one such hydrogen.
Tertiary amines lack a hydrogen atom available for hydrogen bond formation and do not exhibit significant intermolecular association.
As a result, the order of boiling points of isomeric amines is as follows: Primary > Secondary > Tertiary. Primary amines have the highest boiling points due to the strongest hydrogen bonding.

Chemical Reactions of Amines

1. Basic character of amines

The basicity of amines is a fundamental property that determines their ability to accept protons (H+) and form salts. Amines are considered basic due to the presence of an unshared pair of electrons on the nitrogen atom, which can readily interact with proton-donating species, such as acids. Here’s an overview of the basic character of amines and how it is understood through Kb (basicity constant) and pKb (negative logarithm of Kb) values:

1. Reaction with Acids: Amines react with acids to form salts. This is a characteristic reaction of basic compounds, where the nitrogen atom of the amine accepts a proton from the acid, resulting in the formation of an ammonium salt. For example:

R-NH2 + HCl → R-NH3+ Cl-

This reaction demonstrates the basic nature of amines.

2. Formation of Ammonium Salts: Amine salts can be treated with a base, such as NaOH, to regenerate the parent amine. This highlights the reversible nature of the protonation and deprotonation of amines:

R-NH3+ Cl- + NaOH → R-NH2 + NaCl + H2O

3. Solubility: Amine salts are soluble in water but insoluble in organic solvents like ether. This solubility behavior is utilized for the separation of amines from non-basic organic compounds that are insoluble in water.

4. Basicity Constant (Kb) and pKb: Basic character can be quantitatively assessed using the basicity constant (Kb) or its negative logarithm (pKb). A higher value of Kb or a lower value of pKb indicates a stronger base. Kb can be expressed in terms of the concentration of the base and its conjugate acid:

Kb = [BH+][OH-] / [B]

where B represents the amine, and BH+ represents the ammonium ion.

Basicity Trends:

Aliphatic amines are generally stronger bases than ammonia (NH3) due to the electron-donating nature of alkyl groups (+I effect). This leads to higher electron density on the nitrogen atom, making it more available for protonation. The pKb values of aliphatic amines typically fall in the range of 3 to 4.22.
Aromatic amines, such as aniline, are weaker bases than ammonia due to the electron-withdrawing nature of the aryl group. The presence of the benzene ring in arylamines can lead to resonance effects, which further reduce their basicity. For example, aniline is more stable than its ammonium ion due to resonance stabilization.

Effect of Substituents: The basicity of amines can also be influenced by substituents. Electron-releasing groups (e.g., -OCH3, -CH3) increase basicity, while electron-withdrawing groups (e.g., -NO2, -SO3H, -COOH, -X) decrease it.

2. Alkylation

Amines can indeed undergo alkylation reactions, which involve the introduction of alkyl groups into the amine molecule. This reaction is a type of nucleophilic substitution reaction where the nitrogen atom in the amine acts as a nucleophile, attacking the electrophilic carbon atom of an alkyl halide (RX), leading to the formation of a new carbon-nitrogen (C-N) bond.

The general reaction for alkylation of amines with alkyl halides can be represented as follows:

R-X + H-NH2 → R-NH2 + H-X

In this reaction:

R-X represents an alkyl halide, where X is typically a halogen atom like chlorine (Cl), bromine (Br), or iodine (I).
H-NH2 represents the amine, usually ammonia (NH3) or a primary amine (R-NH2).
R-NH2 is the product, which is an alkylamine with the alkyl group (R) attached to the nitrogen atom.
H-X represents the halide ion (X-) that is released during the reaction.

The reaction can be carried out using various conditions, such as heating in the presence of a solvent or a base to facilitate the reaction. The choice of amine and alkyl halide determines the specific product formed, and this reaction is widely used in organic synthesis to introduce alkyl groups into amine molecules, leading to the synthesis of a wide range of organic compounds, including secondary and tertiary amines.

3. Acylation

Acylation is a reaction where aliphatic and aromatic primary and secondary amines react with acid chlorides, anhydrides, and esters by nucleophilic substitution reactions. In this reaction, the hydrogen atom of the -NH2 or >N-H group in the amine is replaced by an acyl group, resulting in the formation of amides. This reaction can be catalyzed by a base, typically a stronger base than the amine itself, such as pyridine, to facilitate the removal of HCl that forms as a byproduct and to shift the reaction equilibrium to the right.

For example, when methanamine (CH3NH2) reacts with benzoyl chloride (C6H5COCl), it undergoes benzoylation to form N-methylbenzamide:

CH3NH2 + C6H5COCl → CH3C6H5CONH2

In this reaction, the acyl group (-C6H5CO-) from benzoyl chloride replaces one of the hydrogen atoms in the amine, resulting in the formation of the amide N-methylbenzamide.

When amines react with carboxylic acids, they can form salts. This reaction typically occurs at room temperature, and the amine and carboxylic acid combine to produce an ammonium salt. For example, when ethylamine (C2H5NH2) reacts with acetic acid (CH3COOH), the following salt is formed:

C2H5NH2 + CH3COOH → C2H5NH3+CH3COO-

The salt formed is ethylammonium acetate, which is soluble in water. This reaction is similar to the neutralization reaction between acids and bases, where the amine acts as a base and the carboxylic acid acts as an acid.

4. Carbylamine reaction

The carbylamine reaction, also known as the isocyanide test, is a chemical test used to detect the presence of primary amines in a given compound. This reaction is specific to primary amines and does not occur with secondary or tertiary amines.

In the carbylamine reaction, an aliphatic or aromatic primary amine is heated with chloroform (CHCl3) and ethanolic (ethyl alcohol) potassium hydroxide (KOH). The reaction results in the formation of isocyanides or carbylamines, which are characterized by their foul-smelling nature. The reaction can be represented as follows:

Primary Amine + Chloroform + Ethanolic KOH → Carbylamine (Isocyanide) + Water + Potassium Chloride

The carbylamine produced in this reaction is often recognized by its pungent and unpleasant odor. This reaction is useful in distinguishing primary amines from secondary and tertiary amines, as the latter two do not give the carbylamine product when subjected to this test.

The carbylamine test serves as a qualitative test for the presence of primary amines in a mixture or compound and is often used in chemical analysis and identification of organic compounds.

5. Reaction with nitrous acid

The reaction of amines with nitrous acid (HNO2) is a significant chemical transformation that leads to the formation of diazonium salts. The nature of this reaction depends on the type of amine involved:

(a) Primary Aliphatic Amines: When primary aliphatic amines react with nitrous acid, they form aliphatic diazonium salts. These diazonium salts are generally unstable and decompose readily, liberating nitrogen gas (N2) and an alcohol as the products. This reaction is utilized for the quantitative estimation of amino acids and proteins, as the release of nitrogen gas can be measured to determine the amine content.

The reaction can be summarized as follows:

Primary Aliphatic Amine + Nitrous Acid → Aliphatic Diazonium Salt → Nitrogen Gas + Alcohol

(b) Aromatic Amines: Aromatic amines react with nitrous acid under specific conditions, typically at low temperatures (273-278 K), to form aromatic diazonium salts. Aromatic diazonium salts are important intermediates in organic synthesis and are used for various transformations to prepare a wide range of aromatic compounds. This reaction is essential in the synthesis of azo dyes, among other applications.

(c) Secondary and Tertiary Amines: Secondary and tertiary amines react with nitrous acid differently from primary amines. Nitrous acid is not very effective in diazotizing secondary and tertiary amines, and this reaction is not commonly employed for these types of amines. Instead, other reactions, such as N-nitrosation, are often observed with secondary and tertiary amines in the presence of nitrous acid.

6. Reaction with arylsulphonyl chloride

The reaction of amines with benzenesulphonyl chloride (C6H5SO2Cl), often referred to as Hinsberg’s reagent, is a useful method for distinguishing between primary, secondary, and tertiary amines, as well as for the separation of a mixture of amines. The specific products formed in these reactions depend on the type of amine involved:

(a) Primary Amine Reaction: When benzenesulphonyl chloride reacts with a primary amine, it forms an N-alkylbenzenesulfonyl amide. For example, if the primary amine is ethylamine, the product will be N-ethylbenzenesulfonyl amide. In this reaction, the sulfonyl chloride group (-SO2Cl) replaces one of the hydrogen atoms attached to the amino group in the amine.

(b) Secondary Amine Reaction: With secondary amines, benzenesulphonyl chloride forms N,N-dialkylbenzenesulfonyl amides. For instance, if the secondary amine is diethylamine, the product will be N,N-diethylbenzenesulfonyl amide. In this case, both hydrogen atoms attached to the amino group in the secondary amine are replaced by sulfonyl chloride groups.

(c) Tertiary Amine Reaction: Tertiary amines do not react with benzenesulphonyl chloride under these conditions. This lack of reaction is due to the absence of a hydrogen atom directly attached to the nitrogen atom in tertiary amines. Therefore, tertiary amines do not form sulfonyl amides in this reaction.

One notable feature of sulfonyl amides is that they exhibit different solubility properties in alkali. Sulfonyl amides containing a hydrogen atom attached to the nitrogen atom (as in primary and secondary amides) are acidic and soluble in alkali. In contrast, sulfonyl amides derived from secondary amines, where no hydrogen is attached to the nitrogen atom, are not acidic and, therefore, insoluble in alkali.

This reaction with benzenesulphonyl chloride is commonly used in organic chemistry for the differentiation and separation of primary, secondary, and tertiary amines due to the distinct products and solubility characteristics. In modern practice, p-toluenesulphonyl chloride is often used as a more effective reagent to achieve similar results.

7. Electrophilic substitution

Electrophilic substitution reactions of aromatic amines, such as aniline, are influenced by the electron-donating nature of the amino group (-NH2) and its resonance effects. Let’s discuss some key electrophilic substitution reactions:

(a) Bromination: Aniline readily reacts with bromine water at room temperature. The amino group (-NH2) is an ortho- and para-directing group, meaning it directs the incoming electrophile to the ortho and para positions. As a result, 2,4,6-tribromoaniline is formed. The electron density is highest at the ortho and para positions due to the resonance effects of the amino group.

One challenge with electrophilic substitution reactions of aromatic amines is their high reactivity, leading to multiple substitutions. To control the reactivity and achieve monosubstitution, the amino group can be protected by acetylation with acetic anhydride. After protection, the desired substitution can be performed, followed by hydrolysis of the substituted amide to regenerate the substituted amine. The acetyl group (-NHCOCH3) is less activating than the amino group (-NH2) due to resonance, making it easier to control the reaction.

(b) Nitration: Direct nitration of aniline can lead to the formation of tarry oxidation products, and in strongly acidic conditions, aniline is protonated to form the anilinium ion, which is meta-directing. This leads to the formation of ortho, para, and meta derivatives. To obtain primarily the para-nitro derivative, the amino group can be protected by acetylation with acetic anhydride before nitration.

(c) Sulphonation: Aniline reacts with concentrated sulfuric acid to form anilinium hydrogensulfate. Upon heating with sulfuric acid at elevated temperatures, p-aminobenzenesulfonic acid, commonly known as sulphanilic acid, is the major product. In this reaction, the electron-donating nature of the amino group contributes to the reactivity.

It’s worth noting that aniline does not undergo Friedel-Crafts reactions (alkylation and acetylation) because it forms a salt with aluminum chloride, a Lewis acid commonly used as a catalyst. This salt formation leads to a positive charge on the nitrogen atom of aniline, making it a strong deactivating group for further reactions. This phenomenon helps to control the reactivity of aniline in Friedel-Crafts reactions.

Diazonium salts

Diazonium salts, often referred to as arenediazonium salts in the context of aromatic amines, have a general formula of R-N2^+X^-. Here’s what each component represents:

R: This stands for an aryl group, typically derived from an aromatic hydrocarbon.
N2^+: This is the diazonium group itself, which consists of a nitrogen atom (N) bonded to two other nitrogen atoms (N2) with a positive charge (+).
X^-: This represents the anion associated with the diazonium salt. It can be chloride (Cl^-), bromide (Br^-), hydrogen sulfate (HSO4^-), tetrafluoroborate (BF4^-), and so on.

The naming of diazonium salts follows a systematic pattern. The salt is named by suffixing “diazonium” to the name of the parent hydrocarbon from which it is derived, followed by the name of the associated anion. For example:

C6H5N2^+Cl^- is named as benzenediazonium chloride.
C6H5N2^+HSO4^- is known as benzenediazonium hydrogensulfate.

It’s important to note that primary aliphatic amines form highly unstable alkyldiazonium salts. In contrast, primary aromatic amines can form arenediazonium salts, which are relatively stable for a short time when maintained in a solution at low temperatures (typically between 273-278 K or 0-5°C).

The stability of arenediazonium ions can be explained by resonance effects, which help disperse the positive charge associated with the diazonium group and make it less reactive compared to alkyldiazonium salts. This increased stability allows for certain reactions and transformations involving arenediazonium salts, which are important in organic synthesis.

Method of Preparation of Diazoniun Salts

The method of preparing benzenediazonium chloride (or other diazonium salts) involves the reaction of aniline with nitrous acid at a low temperature range of 273-278K. Here’s a step-by-step description of the process:

Preparation of Nitrous Acid (HNO2): Nitrous acid is generated in situ within the reaction mixture. This is achieved by combining sodium nitrite (NaNO2) with hydrochloric acid (HCl). The reaction can be represented as follows:NaNO2 + 2HCl → HNO2 + NaCl + H2OThis reaction produces nitrous acid (HNO2) along with sodium chloride (NaCl) and water (H2O).
Diazotization Reaction: Aniline (C6H5NH2) is then added to the reaction mixture containing the generated nitrous acid (HNO2). The diazotization reaction takes place between aniline and nitrous acid. This reaction converts aniline into benzenediazonium chloride (C6H5N2^+Cl^-). The specific reaction can be represented as:C6H5NH2 + HNO2 → C6H5N2^+Cl^- + H2OThe diazonium salt, benzenediazonium chloride, is formed as a result of this reaction.

It’s important to maintain a low temperature range (273-278K) during the diazotization process because diazonium salts are relatively unstable and can decompose at higher temperatures. Due to their instability, diazonium salts are typically used immediately after their preparation and are not stored for extended periods.

Diazotization reactions are essential in various organic syntheses and are a key step in the preparation of various aromatic compounds and dyes.

Physical Properties of Diazoniun Salts

Benzenediazonium chloride and benzenediazonium fluoroborate are examples of diazonium salts with different properties:

Benzenediazonium Chloride:
- Physical State: Benzenediazonium chloride is a colorless crystalline solid at room temperature.
- Solubility: It is readily soluble in water, which allows it to be easily dissolved in aqueous solutions.
- Stability: Benzenediazonium chloride is stable in the cold (at lower temperatures) but reacts with water when warmed. It decomposes easily in the dry state, making it less suitable for long-term storage.
Benzenediazonium Fluoroborate:
- Physical State: Benzenediazonium fluoroborate is another diazonium salt.
- Solubility: It is water-insoluble, which means it does not readily dissolve in water.
- Stability: Benzenediazonium fluoroborate is stable at room temperature, making it more suitable for storage under typical conditions.

The stability and solubility properties of diazonium salts can vary depending on the specific diazonium compound and the counterion (anion) present. These properties are important considerations in organic synthesis reactions that involve diazonium salts. Diazonium salts are commonly used in various reactions, including Sandmeyer reactions and azo dye synthesis, where their stability and reactivity are carefully controlled to achieve the desired outcomes.

Chemical Reactions of Diazoniun Salts

Chemical reactions of diazonium salts involve both displacement of nitrogen and retention of the diazo group, leading to various products and applications:

A. Reactions Involving Displacement of Nitrogen:

Replacement by Halide or Cyanide Ion (Sandmeyer Reaction): Diazonium salts can be converted into halogenated or cyanated derivatives using copper(I) ions as a catalyst. For example, benzenediazonium chloride can be reacted with Cu(I) to form chlorobenzene, bromobenzene, or benzonitrile.
Alternative Halogenation (Gatterman Reaction): Instead of using Cu(I) ions, diazonium salts can also react with corresponding halogen acids (HCl, HBr) in the presence of copper powder to introduce chlorine or bromine into the benzene ring.
Replacement by Iodide Ion: Potassium iodide can be used to replace the diazo group with iodine, yielding iodobenzene.
Replacement by Fluoride Ion: Treatment of arenediazonium chloride with fluoroboric acid results in the formation of arenediazonium fluoroborate, which can be heated to produce aryl fluoride compounds.
Replacement by Hydrogen (Hydrolysis): When diazonium salt solutions are heated to around 283 K, they undergo hydrolysis to form phenols. This reaction is a form of diazo group displacement by hydrogen.
Replacement by Nitro Group: Diazonium fluoroborate can be heated with aqueous sodium nitrite in the presence of copper to replace the diazo group with a nitro group (-NO2).

B. Reactions Involving Retention of Diazo Group (Coupling Reactions):

Diazonium salts can participate in coupling reactions, where the diazo group is retained, and the salt reacts with other aromatic compounds. This leads to the formation of azo compounds, which have extended conjugated systems and are often colored. For example:

Coupling with Phenol: Benzene diazonium chloride can react with phenol to form p-hydroxyazobenzene.
Coupling with Aniline: The reaction of diazonium salt with aniline yields p-aminoazobenzene.

These coupling reactions are examples of electrophilic substitution reactions and are used extensively in the synthesis of azo dyes and other organic compounds. The resulting azo compounds often have vivid colors, making them valuable in the dye industry.

Importance of Diazonium Salts in Synthesis of Aromatic Compounds

Diazonium salts play a crucial role in the synthesis of various aromatic compounds due to their ability to introduce a variety of functional groups into the aromatic ring. Their importance lies in the following aspects:

Introduction of Halogen Atoms: Diazonium salts allow for the introduction of halogen atoms (-F, -Cl, -Br, -I) into the aromatic ring. This is particularly important because certain aryl halides, such as aryl fluorides and iodides, cannot be easily prepared by direct halogenation of the corresponding aromatic compounds. Diazonium salts provide a convenient route to these halogenated aromatic compounds.
Cyanation: Diazonium salts can be used to introduce the cyano group (-CN) into the aromatic ring. This is valuable because nucleophilic substitution reactions to replace a chlorine atom with -CN in chlorobenzene are often challenging. Diazotization followed by coupling with a suitable nucleophile allows for the efficient synthesis of cyanobenzene.
Introduction of Other Functional Groups: Diazonium salts can also be used to introduce other functional groups such as hydroxyl (-OH) and nitro (-NO2) groups into the aromatic ring. These groups have specific chemical properties and are essential in the synthesis of various organic compounds.
Azo Dye Synthesis: Diazonium salts are essential intermediates in the synthesis of azo dyes. Azo dyes are a class of organic compounds known for their vivid and diverse colors. The coupling of diazonium salts with other aromatic compounds results in the formation of azo compounds, which are extensively used as dyes in various industries, including textiles and cosmetics.