General Introduction of Amines: Amines are one of the most important classes of organic compounds that can be synthesised by substituting an alkyl group for one or more hydrogen atoms in ammonia molecules. As surfactants, quaternary ammonium salts are used. Diazonium salts are used to create useful aromatic compounds like dyes. To learn more about the general introduction of amines, read the article below.

Amines

Amines may be considered as derivatives of ammonia \(\left( {{\rm{N}}{{\rm{H}}_{\rm{3}}}} \right)\) in which one or more \({\text{H}}\) atoms have been replaced by equivalent numbers of alkyl or aryl groups. The \({\text{N}}\) atom of each amino group contains a lone pair of electrons. For example,

\({\text{N}}{{\text{H}}_3}\xrightarrow[{ + {\text{R}}}]{{ – {\text{H}}}}{\text{RN}}{{\text{H}}_2}\xrightarrow[{ + {\text{R}}}]{{ – {\text{H}}}}{{\text{R}}_2}{\text{NH}}\xrightarrow[{ + {\text{R}}}]{{ – {\text{H}}}}{{\text{R}}_3}{\text{N}}\)

Where \({\text{R}}\) may be an alkyl or aryl group.

Examples of amines are as follows;

Structure of Amine

Amines have a pyramidal structure with the bond angle at \({\rm{N}}\)-atom approximately \({108^{\rm{o}}}\) due to greater repulsion of \({\rm{Ar}}\) or \({\rm{R}}\) groups than \({\rm{H}}\)-atom. Amines are not optically active about nitrogen atoms because of very fast flip rotation.

Classification of Amines

Amines may be classified as follows:

1. Primary, secondary and tertiary amines
2. Simple and mixed amines
3. Aliphatic and aromatic amines

l. Primary, secondary and tertiary amines:
Depending upon the number of \({\rm{H}}\) atoms of ammonia replaced by alkyl groups, the amines are classified as primary, secondary, and tertiary.

1. Primary Amine: When one \({\rm{H}}\) atom of ammonia is replaced by one alkyl or aryl group, then the obtained amine is called a primary amine.

2. Secondary Amine: When two \({\rm{H}}\) atoms of ammonia are replaced by two alkyl or aryl groups, then the obtained amine is known as secondary amine.

3. Tertiary Amine: When all the three atoms of ammonia are replaced by three alkyl or aryl groups, then the obtained ammonia is known as a tertiary amine.

ll. Simple and mixed amines
Depending upon the types of alkyl groups attached to \({\rm{N}}\)-atom, secondary and tertiary amines are further classified as simple amines or mixed amines.

1. Simple Amines: In secondary and tertiary amines, if all the alkyl or aryl groups are identical, then the amine is known as simple amines. For example,

2. Mixed Amines: In secondary and tertiary amines, if the alkyl or aryl groups are different, then the amines are known as mixed amines. For example,

lll. Aliphatic and aromatic amines
Aliphatic Amines: Amines in which the nitrogen atom is directly attached to one or more alkyl groups are called aliphatic amines.

For example,

1. Aromatic Amines: Aromatic amines are further classified into two groups: They are
1. Arylamines: Amines in which the nitrogen atom is directly bonded to one or more aryl groups are called arylamines. Arylamines may be primary, secondary, or tertiary amines.
For example,

2. Alkyl amines: The amines in which nitrogen atoms are bonded to the side chain of the aromatic ring are called aryl alkyl amines. These amines are also called aryl-substituted alkylamines.
For example,

Preparation of Amines

Mixtures of primary, secondary, and tertiary aliphatic amines are prepared by the following methods. This method is used for the industrial preparation of aliphatic amines.

1. From Haloalkanes (Hofmann’s method)

In this process, haloalkanes are converted into amines by treatment with an alcoholic solution of ammonia.

\(\underset{{{\text{Ammonia}}}}{\mathop {{\text{N}}{{\text{H}}_3}}} \xrightarrow{{{\text{RX}}}}\underset{{{\text{primary}}\,{\text{amine}}}}{\mathop {{\text{R-N}}{{\text{H}}_2}}} \xrightarrow{{{\text{RX}}}}\underset{{{\text{secondary}}\,{\text{amine}}}}{\mathop {{{\text{R}}_2}{\text{N}}{{\text{H}}_2}}} \xrightarrow{{{\text{RX}}}}\underset{{{\text{Tertiary}}\,{\text{amine}}}}{\mathop {{{\text{R}}_3}{\text{N}}}} \xrightarrow{{{\text{RX}}}}\underset{{{\text{Quaternary}}\,{\text{ammonium}}\,{\text{salt}}}}{\mathop {{{\text{R}}_4}{\text{NX}}}} \)

Mechanism of ammonolysis of Haloalkane

1. Ammonia behaves as a nucleophile.
2. When haloalkane is treated with alcoholic ammonia, a nucleophilic substitution reaction takes place. In the reaction, the halogen atom is replaced by an amino group (\({\rm{ – N}}{{\rm{H}}_2}\) group).

\(\underset{\begin{subarray}{l} {\text{Ammonia}} \\ {\text{Nucleophile}} \end{subarray} }{\mathop {{\text{N}}{{\text{H}}_3}}} + \underset{{{\text{Haloalkane}}}}{\mathop {{\text{R-X}}}} \to \underset{{{\text{p}} – \,{\text{Amine}}}}{\mathop {{\text{R-N}}{{\text{H}}_2}}} + {\text{HX}}\)

3. The process of cleavage of the \({\rm{C—X}}\) bond of haloalkane by ammonia is called ammonolysis.
4. The primary amine thus formed also behaves as a nucleophile. It reacts further with haloalkanes to form secondary and tertiary amines.

2. From alcohol

When vapours of a mixture of alcohol and ammonia are passed over the heated aluminium ​or copper chromite at \({350^{\rm{o}}}{\rm{C}}\), a mixture of amines is obtained. This process is used for the industrial production of lower aliphatic amines. The amines obtained may be separated from the mixture by fractional distillation.

\(\underset{{{\text{alcohol}}}}{\mathop {{\text{R-OH}}}} \xrightarrow[{{\text{-}}{{\text{H}}_2}{\text{O}}}]{{{\text{N}}{{\text{H}}_3}}}\underset{{{\text{primary}}\,{\text{amine}}}}{\mathop {{{\text{R}}_2}{\text{NH}}}} \xrightarrow[{{\text{-}}{{\text{H}}_2}{\text{O}}}]{{{\text{ROH}}}}\underset{{{\text{secondary}}\,{\text{amine}}}}{\mathop {{{\text{R}}_2}{\text{NH}}}} \xrightarrow[{{\text{-}}{{\text{H}}_2}{\text{O}}}]{{{\text{ROH}}}}\underset{{{\text{tertiary}}\,{\text{amine}}}}{\mathop {{{\text{R}}_3}{\text{N}}}} \)

3. Gabriel-phthalimide reaction

This procedure is used to make aliphatic / arylalkyl primary amines only. Potassium phthalimide is reacted with an alkyl halide or an alkyl aryl halide to obtain \({\rm{N}}\)-alkyl phthalimide. \({\rm{N}}\)-alkyl phthalimide gives the corresponding primary amine by hydrolysis with dilute \({\rm{HCl}}\) under pressure or with \({\rm{NaOH}}\). Potassium phthalimide is obtained by reacting phthalimide with alcoholic \({\rm{KOH}}\). The phthalic acid may be converted to phthalimide and used again.

1. By reducing an alkyl isocyanide: Alkyl isocyanide when reduced using \({{\rm{H}}_2}\) and \({\rm{Ni}}\) catalyst gives secondary amines.

Properties of Amines

Physical Properties of Amines

1. Lower aliphatic amines smell like ammonia, while higher have a fishy smell.
2. Aromatic amines generally turn into oxo derivatives and turn brown while remaining in the air. Pure amines are generally colourless.
3. The boiling points of isomeric amines are in the order of magnitude from \({1^{\rm{o}}}\; > {\rm{ }}{2^{\rm{o}}}\; > {\rm{ }}{3^{\rm{o}}}\) as the degree of intermolecular hydrogen bonding decreases.
4. Lower amines are soluble in water due to the formation of hydrogen bonds with \({{\rm{H}}_2}{\rm{O}}\) molecules.

Chemical Properties of Amines

Following chemical properties are shown by all amines.

1. Basic Character: All amines contain a nitrogen atom that has only one pair of electrons. The tendency of nitrogen to share these electrons is responsible for the basic nature of amines. All amines are more basic than water and protonate with water. Aqueous amine solutions are basic in nature due to the release of \({\rm{O}}{{\rm{H}}^ – }\) ions in the solutions. The tendency of such reactions in the forward direction, i.e. the basic strength of an amine, is described by the base ionization constant \({\rm{(}}{{\rm{K}}_{\rm{b}}}{\rm{)}}\) as described below.

2. Alkylation: Amines are alkylated with alkyl halide on heating. The primary amine is converted into secondary and tertiary amines and finally into a quaternary ammonium salt. The process is known as exhaustive alkylation.

\(\underset{{{\text{p}} – {\text{Amine}}}}{\mathop {{\text{RN}}{{\text{H}}_2}}} \xrightarrow[{{\text{-HX}}}]{{ + {\text{RX}}}}\underset{{{\text{s}} – {\text{Amine}}}}{\mathop {{{\text{R}}_2}{\text{NH}}}} \xrightarrow[{{\text{-HX}}}]{{ + {\text{RX}}}}\underset{{{\text{t}} – {\text{Amine}}}}{\mathop {{{\text{R}}_3}{\text{N}}}} \xrightarrow{{ + {\text{RX}}}}\underset{{{\text{Quaternary}}\,{\text{ammonium}}\,{\text{halide}}}}{\mathop {{{\text{R}}_4}{{\text{N}}^ + }{{\text{X}}^ – }}} \)

3. Acylation: The aliphatic amines form substituted amides when treated with an acid chloride or acid anhydride.

4. Carbylamine reaction: Aliphatic and aromatic primary amines, when heated with chloroform and ethanolic potassium hydroxide, form isocyanides or carbylamines, which are foul-smelling substances. Secondary and tertiary amines do not show this reaction. This reaction is known as the carbylamine reaction or isocyanide test and is used as a test for primary amines.

\({\text{R-N}}{{\text{H}}_2} + {\text{CHC}}{{\text{l}}_3} + 3{\text{KOH}}\xrightarrow{{{\text{Heat}}}}{\text{R}} – {\text{NC + 3KCl + 3}}{{\text{H}}_2}{\text{O}}\)

5. Reaction with nitrous acid: Three classes of amines react differently with nitrous acid, which is produced in situ from a mineral acid and sodium nitrite.

\({\text{NaN}}{{\text{O}}_2} + {\text{HCl}} \to {\text{NaCl + }}\mathop {{\text{HN}}{{\text{O}}_2}}\limits_{{\text{nitrous}}\,{\text{acid}}} \)

1. Primary aliphatic amines react with nitrous acid to form aliphatic diazonium salts, which, being unstable, quantitatively release nitrogen gas and give alcohols. The quantitative evolution of nitrogen is used in the estimation of amino acids and proteins.

\(\mathop {{\text{RN}}{{\text{H}}_2}}\limits_{{\text{primary}}\,{\text{amine}}} + \mathop {{\text{HN}}{{\text{O}}_2}}\limits_{{\text{nitrous}}\,{\text{acid}}} \xrightarrow{{ < {5^{\text{o}}}{\text{c}}}}\mathop {{\text{ROH}}}\limits_{{\text{alcohol}}} {\text{ + }}{{\text{N}}_2}({\text{g}}) + {{\text{H}}_2}{\text{O}}\)

2. Aromatic amines react with nitrous acid at low temperatures \(\left( {{\rm{273 – 278 K}}} \right)\) to form diazonium salts, a very important class of compounds used in the synthesis of a wide variety of aromatic compounds.

Basicity of Amines

1. Amines behave as Bronsted and Lewis bases in organic solvents and vapour phases.
2. Amines readily form salts with acids. \({{\rm{H}}_2}{\rm{PtC}}{{\rm{l}}_6}\) is specially used for determining molecular masses of amines.

\({\text{RN}}{{\text{H}}_2} + {\text{HCl}} \to {\text{R}}\overset{ + }{\mathop {\text{N}}} {{\text{H}}_3}{\text{C}}{{\text{l}}^ – }\)
\({\rm{2RN}}{{\rm{H}}_2} + {{\rm{H}}_2}{\rm{PtC}}{{\rm{l}}_6} \to {\left( {{\rm{R}}\mathop {\rm{N}}\limits^ + {{\rm{H}}_3}} \right)_2} + \left( {{\rm{Pt}}{{\mathop {{\rm{Cl}}}\limits^{2 – } }_6}} \right) \downarrow \)

3. Aromatic amines are less basic than ammonia due to the of lone pair of \({\text{N}}\)-atom towards benzene ring.
4. The aqueous solution of \({\rm{C}}{{\rm{H}}_3}{\rm{N}}{{\rm{H}}_2}{\rm{\;}}\left( {\rm{g}} \right)\) and \({{\rm{C}}_2}{{\rm{H}}_5}{\rm{N}}{{\rm{H}}_2}{\rm{\;}}\left( {\rm{g}} \right)\) function as hydroxides.
a.

\({\text{FeC}}{{\text{l}}_3}\left( {{\text{aq}}{\text{.}}} \right) + 3\left( {{\text{C}}{{\text{H}}_3}\mathop {\text{N}}\limits^ + {{\text{H}}_3}} \right){\text{O}}{{\text{H}}^ – } \to 3\left( {{\text{C}}{{\text{H}}_3}\mathop {\text{N}}\limits^ + {{\text{H}}_3}} \right){\text{C}}{{\text{l}}^ – } + {\text{Fe}}{\left( {{\text{OH}}} \right)_3} \downarrow \)

5. In organic solvents (nonpolar or polar aprotic) and in vapour phase, the order of basicity of aliphatic amines is \({3^{\rm{o}}}\; > {\rm{ }}{2^{\rm{o}}}\; > {\rm{ }}{1^{\rm{o}}}\; > {\rm{ N}}{{\rm{H}}_3}\).
6. Electron withdrawing groups, like \({\rm{N}}{{\rm{O}}_2},{\rm{ CN}}\), etc., make aniline weaker base and weaker when present at ortho-position.
7. Electron donating groups, like \({\text{C}}{{\text{H}}_3}, – {\text{OC}}{{\text{H}}_3},\) etc., make aniline, stronger base and still stronger when present at \({\rm{p}}\)-position but at ortho position; these are weaker than aniline.

Uses of Amines

Amines are very useful compounds. These are used as follows:

1. Lower members of the aliphatic amine series are used as solvents, intermediate members in the pharmaceutical industry, and in organic synthesis.
2. Detergents are made from quaternary salts derived from long-chain aliphatic amines. A common detergent is an n-hexadecyltrimethylammonium chloride.
3. Aromatic amines, such as aniline, are widely used in the manufacture of polymers, dyes, drugs, and intermediates used in the rubber industry. These are also used as synthetic reagents via diazonium salts.

Summary

Amines may be considered as derivatives of ammonia \(({\rm{N}}{{\rm{H}}_3})\) in which one or more \({\text{H}}\) atoms have been replaced by equivalent numbers of alkyl or aryl groups. Amines have a pyramidal structure. Amines may be classified as primary, secondary and tertiary amines, simple and mixed amines, and aliphatic and aromatic amines. Amines have many applications. Amines are an important class of compounds. The amino groups are part of natural substances such as proteins, polymers, alkaloids, and hormones.

FAQs on General Introduction of Amines

Q.1. What is the importance of amines?
Ans: The importance of amines are as follows:
I. Quaternary ammonium salts are used as surfactants.
II. Secondary amino groups are found in biological compounds such as adrenaline and ephedrine. These substances are used to increase blood pressure.
III. The tertiary amino group is present in Benadryl. It is an antihistamine medication.
IV. Novocain is an amino compound that has been synthesised. In dentistry, it is used as an anaesthetic.
V. Diazonium salts are used to make useful aromatic compounds such as dyes.

Q.2. What makes amines unique?
Ans: Amine is a basic compound due to the presence of lone pair of electrons. This makes amines unique.

Q.3. How many types of amines are there?
Ans: There are three types of amines. They are as follows;
1. Primary, secondary and tertiary amines.
2. Simple and mixed amines
3. Aliphatic and aromatic amines

Q.4. What are amines, and how are they classified?
Ans: Amines may be considered as derivatives of ammonia \(({\rm{N}}{{\rm{H}}_3})\) in which one or more \({\text{H}}\) atoms have been replaced by equivalent numbers of alkyl or aryl groups.
Amines are classified as follows:
a. Primary, secondary and tertiary amines
b. Simple and mixed amines
c. Aliphatic and aromatic amines

Q.5. What are the amines formulas?
Ans: The amine formulas are \({\rm{RN}}{{\rm{H}}_2},{\rm{ }}{{\rm{R}}_2}{\rm{NH}}\), and \({{\rm{R}}_3}{\rm{N}}\), where \({\rm{R}}\) is aryl or alkyl group.

Q.6. Are amines acidic or basic?
Ans: Amines are basic in nature. This is because of the tendency of nitrogen to share its lone pair electrons.

Study Structure of Amines Here

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