Preparation properties and Uses of Amines

Preparation of amines

Amines can be prepared through various chemical reactions. Below are the key methods used in their synthesis:

1. Alkylation of ammonia with haloalkanes: Primary amines can be obtained by the reaction of alkyl halides with excess ammonia. The use of excess ammonia is crucial to prevent the formation of secondary or tertiary amines. This method, however, is not ideal for the preparation of amines, as the substitution of hydrogen atoms by alkyl groups does not stop at the first stage, leading to the formation of more substituted amines.

   

2. Reduction of nitroalkanes (R–NO2): Primary amines can be synthesized by reducing nitroalkanes. Common reducing agents include Lithium aluminum hydride (LiAlH4), Tin and Hydrochloric acid (Sn/HCl), or Iron and Hydrochloric acid (Fe/HCl).

   

3. Reaction of amides with LiAlH4: Amides can be reduced to primary amines using Lithium aluminum hydride (LiAlH4) as the reducing agent.

   

4. Ammoniation of carbonyl compounds followed by reduction with hydrogen in the presence of nickel: In this method, carbonyl compounds undergo ammoniation to form intermediate compounds, which are then reduced to form amines using hydrogen and a nickel catalyst.

   

5. Reduction of nitriles: Nitriles (R-CN) can be reduced to primary amines using reducing agents like LiAlH4 or catalytic hydrogenation.

   

6. Reaction of alcohols with ammonia: Catalytic ammoniation of alcohols leads to a mixture of amines.

   

7. Hoffmann's degradation of amides: This is an important reaction used to convert amides into amines, reducing the number of carbon atoms by one in the process.

   

Properties of amines

a. Physical properties of amines

Amines have distinctive physical properties due to their ability to form hydrogen bonds. However, they differ significantly from alcohols and carboxylic acids in terms of boiling points and solubility.

1. Boiling point and melting point: Amines have higher boiling and melting points than hydrocarbons due to their hydrogen bonding. However, their boiling points are lower than those of alcohols (R-OH) because alcohols have two lone pairs of electrons on the oxygen atom, making hydrogen bonding more efficient. Amines also have lower boiling points than carboxylic acids. For example, primary amines have higher boiling points than secondary amines, and secondary amines have higher boiling points than tertiary amines, which cannot form hydrogen bonds.
2. Solubility: Lower aliphatic amines are soluble in water due to their ability to form hydrogen bonds. As the molecular size increases, solubility decreases due to the increasing hydrophobic nature of the alkyl group. Amines containing six or more carbon atoms are generally insoluble in water. For example, aniline is insoluble in water due to its large benzene ring.

b. Chemical properties of amines

Amines exhibit a variety of chemical properties that are mainly due to the nitrogen atom's lone pair of electrons, making them basic in nature. Below are some of their key chemical reactions:

1. Basic character: Amines are the most important organic bases. Aliphatic amines are slightly stronger Lewis bases than ammonia because the lone pair of electrons on nitrogen is more readily available in amines than in ammonia. The basicity of amines decreases from primary to tertiary amines due to the inductive and hydration effects. For example, primary amines are more basic than tertiary amines.

   

2. Effect of substituent groups on basicity: In aromatic compounds like aniline, the presence of electron-withdrawing groups (such as nitro groups) decreases basicity, while electron-donating groups (such as alkyl groups) increase basicity. In aniline, the lone pair of electrons on nitrogen is less available for protonation due to resonance effects with the benzene ring, making aniline less basic than alkylamines.

3. Alkylation (reaction with haloalkanes): Alkylation of amines with haloalkanes results in the formation of substituted amines, and further alkylation can lead to the formation of quaternary ammonium salts.

   

4. Reaction with ketones and aldehydes: Amines react with aldehydes and ketones to form Schiff's bases, which are compounds that contain a C=N double bond.

   

   Schiff's base

   E.g:

   
   • Lewis acid is a substance that accepts a lone pair of electrons
   

5. Carbylamine reaction (reaction with chloroform): Primary amines react with chloroform and alcoholic potassium hydroxide (KOH) to form isocyanides (carbylamines), which have a foul odor. This reaction is used as a test for primary amines.

   

6. Acylation: Amines undergo acylation with acyl chlorides to form amides. This reaction is an important method for modifying amines.

   

7. Reaction with nitrous acid: Aliphatic amines react with nitrous acid (HONO) to form alcohols and nitrogen gas, while aromatic amines react with nitrous acid at low temperatures to form diazonium salts, which are key intermediates in the synthesis of various aromatic compounds.

   

   Amine

   · Aliphatic amine reacts with HONO to form alcohol and nitrogen gas (you will see bubbles)

   

   Eg:

   

   – For aromatic amine react with HNO2 in cold (below 5°C) to form diazonium salt.

   

   Amine

   · amines react with nitrous acid to give N – nitrosoamine which separate out as a yellow oily liquid.

   

   N- nitrosoamine (Yellow oil liquid)

   It is a distinguishing test since we see yellow oil liquid

   Eg:

   

   Amine

   Amine reacts with nitrous acid to form a soluble trialkylammonium salt which is colourless.

   It is not a distinguishing test since there is no observable change.

   

   Aromatic 3 amines react with HONO to give green colour p – nitrosoamine

   

   This is a distinguishing Test since there is a green coloured

8. Reaction of amines with metal ions Ag+ & Cu2+

   Reaction of aromatic amine due to benzene ring

   1. reaction with bromine water

   · Aniline undergoes halogenations even in the absence of catalyst. With bromine it gives

   2, 4, 6 – tribromoaniline (white solid)

   

   Here benzene ring is highly activated due to the presence of amine

   To obtain mono substituted derivative, aniline should be first acylated then halogenation. After halogenations, the acyl group is removed by hydrolysis to obtain mono substituted aniline.

   
   2. Nitration

   Direct Nitration is not possible because aniline gets oxidized. Amine group should be protected by acylation, then nitration and finally hydrolysis.

   
   3. Sulphonation
   

   We have different products due to H – bonding. At 1800C, H – bonding will not be effective hence SO3H will be at para position.

   4. Diazonium salt

   · It is used to form many other products.

   · Reaction is nucleophilic substitution .

   

Uses of amines

Amines are important in both industrial and biological processes. Some of their key uses include:

1. Pharmaceuticals: Amines are widely used in the synthesis of drugs, such as antihistamines, antidepressants, and local anesthetics. For example, methamphetamine and morphine derivatives are amine-based compounds.
2. Agrochemicals: Amines are used in the production of herbicides, insecticides, and fungicides, which play an essential role in agriculture.
3. Polymers: Amines are used in the production of synthetic polymers like nylon and polyurethane.
4. Textile industry: Amines are used as intermediates in the production of dyes, particularly for textiles.
5. Water treatment: Amines are used in water treatment plants to remove impurities and as corrosion inhibitors in boilers and cooling systems.