Chemical Reactions of Amines: Definition, Chemical Properties, Examples

Chemical Reactions of Amines: Amines are organic compounds that have the functional group \( – {\rm{N}}{{\rm{H}}_2}\). Amines are derived from ammonia. Substitution of alkyl or aryl group for a hydrogen atom in ammonia leads to the formation of alkyl amines and aryl amines, respectively. In aryl amines, the amino group is attached to the benzene ring directly.

Amines are classified as primary \(\left( {{1^ \circ }} \right)\), secondary \(\left( {{2^ \circ }} \right)\), and tertiary \(\left( {{3^ \circ }} \right)\) amines, based on the number of alkyl or aryl substitutions in ammonia. If single hydrogen in ammonia is substituted with an alkyl group, it forms primary amine, and with two hydrogens substituted by the alkyl group makes up for secondary amines. When all the three hydrogen atoms are substituted by alkyl groups, then tertiary amine is formed.

What are Amines: Definition, Examples

Amines are organic compounds that have the functional group \( – {\rm{N}}{{\rm{H}}_2}\). Amines are named alkanamines, according to the IUPAC system. Examples are as follows:

Physical Properties of Amines

a. Amines are colourless in pure form and develop colour on storage due to oxidation.
b. Lower members like methyl amine and ethyl amine smell like ammonia and are gases.
c. Higher alkyl amines have a fishy odour.
d. As the number of carbon increases from lower amines, the physical state varies from gas, liquid and then solids.
e. They are basic in nature.
f. Lower members of the series are soluble in water since they have the ability to form hydrogen bonds with water.
g. The increase in the hydrophobic alkyl part with the increase in molecular weight of the amines reduces their solubility in water. Hence, higher amines are practically insoluble in water.
h. Aromatic amines are colourless when pure and generally gets air oxidised and are discoloured.
i. Aromatic amines are toxic in nature.

Chemical Properties of Amines

Amines are very reactive, because of:

1. The difference in electronegativity between hydrogen and nitrogen atoms in amines.
2. The unshared electrons present on the Nitrogen atom.

Given below are some significant reactions of amines:

1. Basic Nature of Amines

Amines are basic in nature. Amines have a lone pair of electrons on the nitrogen atom, thereby making them behave as Lewis bases. This is confirmed by the reaction with acids to form salts.
\({\rm{R}} – {\rm{N}}{{\rm{H}}_2} + {\rm{HCl}} \to {\rm{R}} – {{\rm{N}}^ + }{{\rm{H}}_3}{\rm{C}}{{\rm{l}}^ – }\)

Aliphatic amines are stronger than ammonia due to the +I effect or electron-donating power of the alkyl groups present in them. Aromatic amines are weaker than ammonia due to the electron-withdrawing aryl group. 

The order of basic strength varies as follows

Aliphatic amines > Ammonia > Aromatic amines

The basic nature arises due to the formation of a cation. More stable is the cation formed than the amine; more basic is the amine. Hence, the basic nature of amines increases with the addition of more alkyl groups in the molecule.

The order of basicity for amines in the gaseous phase is:

Tertiary amines > Secondary amines > Primary amines > Ammonia

In an aqueous solution, the basicity largely depends upon the stability of the ammonium cation, formed by accepting a proton from water. Its stability (ammonium cation) depends upon factors such as:

1. \({\rm{ + l}}\) effect (attached alkyl group
2. Steric effects (alkyl groups)
3. Solvation effect

The basicity of amines is well explained by their \({{\rm{K}}_{\rm{b}}}\) and \({{\rm{pK}}_{\rm{b}}}\) values.   Also, for amines, the larger the \({{\rm{K}}_{\rm{b}}}\) value or smaller the \({{\rm{pK}}_{\rm{b}}}\) value, the stronger is the basicity of the amines. According to this, aliphatic amines are stronger bases than ammonia because of the \({\rm{ + l}}\) effect or electron-donating power of the alkyl groups attached. Aromatic amines, on the other hand, are weaker bases because of the electron-withdrawing aryl group.

Reaction with NaOH

As such, amines do not react with \({\rm{NaOH}}\). The salt formed on reaction with \({\rm{HCI}}\) is reversed on the addition of \({\rm{NaOH}}\).

\({\rm{R}} – {{\rm{N}}^ + }{{\rm{H}}_3}{\rm{C}}{{\rm{l}}^ – } + {\rm{NaOH}} \to {\rm{R}} – {\rm{N}}{{\rm{H}}_2} + {\rm{NaCl}} + {{\rm{H}}_2}{\rm{O}}\)

2. Alkylation Reaction

Amines react with alkyl halides in an alkylation reaction. The reaction is as follows:

\({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{N}}{{\rm{H}}_2} + {\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{Br}}\quad \to \quad {\left( {{\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}} \right)_2}{\rm{NH}} + {\rm{HBr}}\)
Ethylamine      Ethyl Bromide             Diethylamine

\(\left( {{\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}} \right){\rm{NH}} + {\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{Br}} \to {\left( {{\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}} \right)_3}\;{\rm{N}} + {\rm{HBr}}\)
Diethylamine              Ethyl Bromine           Triethylamine

\({\left( {{\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}} \right)_3}\;{\rm{N}} + {\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{Br}} \to {\left( {{\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}} \right)_4}\;{{\rm{N}}^ + }{\rm{B}}{{\rm{r}}^ – }\)
Triethylamine        Ethyl Bromine     Tetraethylammonium bromide

The process by which a \({1^ \circ }\) amine is converted into quaternary ammonium salt is known as Hofmann exhaustive alkylation.

3. Acylation (Nucleophilic Substitution)

Aromatic and aliphatic amines (primary and secondary) react with acid chlorides, anhydrides and esters, form amides. This reaction occurs in the presence of a strong base like pyridine. (To remove \({\rm{HCI}}\) formed, and shift the reaction to the right side). The reaction is a nucleophilic substitution reaction and is called an acylation reaction.

\({\rm{C}}{{\rm{H}}_3}{\rm{N}}{{\rm{H}}_2} + {\rm{C}}{{\rm{H}}_3}{\rm{COCl}} \to {\rm{C}}{{\rm{H}}_3}{\rm{NHCOC}}{{\rm{H}}_3} + {\rm{HCl}}\)

The process of introducing a \({{\rm{C}}_6}{{\rm{H}}_5} – {\rm{C}} = {\rm{O}} – \) group in an amine molecule by treating it with benzoyl chloride is called a benzoylation reaction. With Benzoyl chloride, amines form \({\rm{ N – }}\)methyl benzamide.

\({\rm{C}}{{\rm{H}}_3}{\rm{N}}{{\rm{H}}_2} + {{\rm{C}}_6}{{\rm{H}}_5}{\rm{COCl}} \to {\rm{C}}{{\rm{H}}_3}{\rm{NHCO}}{{\rm{C}}_6}{{\rm{H}}_5} + {\rm{HCl}}\)

4. Carbylamine Reaction

Primary amines, on heating with ethanolic potassium hydroxide and chloroform, gives isocyanides or carbylamines, which are foul-smelling in nature. The reaction is called the carbylamine reaction. It is also referred to as the test for isocyanides. It is a test for primary amines because secondary and tertiary amines do not give this test.

\({\rm{R}} – {\rm{N}}{{\rm{H}}_2} + {\rm{CHC}}{{\rm{l}}_3} + 3{\rm{KOH}} \to {\rm{R}} – {\rm{NC}} + 3{\rm{KCl}} + 3{{\rm{H}}_2}{\rm{O}}\)

\({\rm{C}}{{\rm{H}}_3} – {\rm{N}}{{\rm{H}}_2} + {\rm{CHC}}{{\rm{l}}_3} + 3{\rm{KOH}} \to {\rm{C}}{{\rm{H}}_3} – {\rm{NC}} + 3{\rm{KCl}} + 3{{\rm{H}}_2}{\rm{O}}\)
Methylamine                                Methyl Isocyanide

5. Reaction with \({\rm{HN}}{{\rm{O}}_2}\)

Aryl amines on reaction with \({\rm{HN}}{{\rm{O}}_2}\left( {{\rm{NaN}}{{\rm{O}}_2} + {\rm{HCl}}} \right)\) under cold conditions give diazonium salts that are used as dyes. Primary, secondary and tertiary amines react differently with nitrous acid.

Primary Amines
Both aliphatic and aromatic primary amines react with nitrous acid to form diazonium salts, which are unstable in aliphatic amines, and therefore they hydrolyze to form alcohols. Aromatic amines, though, form diazonium salts.

\({\rm{C}}{{\rm{H}}_3}{\rm{N}}{{\rm{H}}_2} + {\rm{HN}}{{\rm{O}}_2} \to \left[ {{\rm{C}}{{\rm{H}}_3} – {\rm{N}}_2^ + {\rm{C}}{{\rm{l}}^ – }} \right] \to {\rm{C}}{{\rm{H}}_3} – {\rm{OH}} + {{\rm{N}}_2} + {\rm{HCl}}\)

\({{\rm{C}}_6}{{\rm{H}}_5}{\rm{N}}{{\rm{H}}_2} + {\rm{NaN}}{{\rm{O}}_2} + 2{\rm{HCl}}\quad \to \quad {{\rm{C}}_6}{{\rm{H}}_5} – {\rm{N}}_2^ + {\rm{C}}{{\rm{l}}^ – } + {\rm{NaCl}} + 2{{\rm{H}}_2}{\rm{O (at 273 – 278K) }}\)
Aniline                                           Benzene diazonium chloride

Secondary Amines
With nitrous acid, secondary amines form \({\rm{N}} – \) nitroso amines as a yellow-green oily layer. This reaction is also called the ‘Libermann’s nitrosoamine’ test.

Reaction with Aryl Sulphonyl Chloride
Primary amines react with aryl sulphonyl chloride (benzene sulphonyl chloride, otherwise called Hinsberg’s reagent) to form \({\rm{N}} – \) ethylbenzene sulphonamide. Since the hydrogen attached to the nitrogen is acidic due to the presence of the sulphonyl group (electron-withdrawing), the sulphonamide formed is soluble in alkali.

With secondary amines, benzene sulphonyl chloride gives \({\rm{N, N – }}\) diethyl sulphonamide. Tertiary amines do not react with benzene sulphonyl chloride.

6. Electrophilic Substitution Reactions

In aromatic amines, such as aniline, the ortho and para positions are electron-rich due to the \({\rm{N}}{{\rm{H}}_2}\) group attached. So, substitution reactions with ortho and para substituents are formed.

a. Halogenation

Bromination of aniline, in the absence of a catalyst, and on reaction with bromine water gives \(2, 4, 6\) – tribromoaniline.

In order to get the mono-substituted product, one needs to deactivate the \({\rm{N}}{{\rm{H}}_2}\) group with acylation by reaction with acetic anhydride. The product formed in acylation is then halogenated to get a mono-substituted derivative, which is again hydrolyzed to remove the acetyl group.

b. Nitration

In an acidic medium, aniline forms a mixture of ortho and para-nitroaniline and also a significant amount of meta-nitroaniline. Acylation with acetic anhydride followed by nitration yields para-nitroaniline.

c. Sulphonation

With concentrated sulphuric acid, aniline forms anilinium hydrogen sulphate, which on heating with sulphuric acid at \(455 – 475\;{\rm{K}}\) gives \({\rm{p}} – \)aminobenzene sulphonic acid or sulphanilic acid.

Summary

Amines are organic compounds that are derived from ammonia and are basic in nature. Amines are present in amino acids, the building units of protein hence present in DNA, hormones and neurotransmitters. Amines are classified as primary, secondary and tertiary and can be identified through unique reactions such as reactions with nitrous acid and aryl sulfonyl chloride. The basicity of amines is proved by several reactions involving acids to form salts. Amines also undergo both nucleophilic and electrophilic substitution, and the \({\rm{N}}{{\rm{H}}_2}\) group present in amines makes it highly reactive. Ortho and para positions have high electron density due to the functional group of amines in aromatic amines.

FAQs on Chemical Reactions of Amines

Q.1. What chemicals are amines?
Ans: Amines are organic compounds obtained by substituting alkyl or aryl groups for hydrogen in ammonia. The general representation is \({\rm{R}} – {\rm{N}}{{\rm{H}}_2}\).

Q.2. Do amines react with water?
Ans: Primary and secondary amines dissolve in water, forming hydrogen bonds. They do not react with water.

Q.3. What are the chemical reactions characteristics of amines?
Ans: Reaction with acids forming salts
\({\rm{R}} – {\rm{N}}{{\rm{H}}_2} + {\rm{HCl}} \to {\rm{R}} – {{\rm{N}}^ + }{{\rm{H}}_3}{\rm{C}}{{\rm{l}}^ – }\)
Acylation
Amines on reaction with acid chlorides, anhydrides or esters give amides.
\({\rm{C}}{{\rm{H}}_3}{\rm{N}}{{\rm{H}}_2} + {\rm{C}}{{\rm{H}}_3}{\rm{COCl}} \to {\rm{C}}{{\rm{H}}_3}{\rm{NHCOC}}{{\rm{H}}_3} + {\rm{HCl}}\)
Formation of diazonium salts
Aryl amines on reaction with \({\rm{HN}}{{\rm{O}}_2}\left( {{\rm{NaN}}{{\rm{O}}_2} + {\rm{HCl}}} \right)\) under cold conditions give diazonium salts.
\({{\rm{C}}_6}{{\rm{H}}_5}{\rm{N}}{{\rm{H}}_2} + {\rm{NaN}}{{\rm{O}}_2} + {\rm{HCl}}\quad \to \quad {{\rm{C}}_6}{{\rm{H}}_5}\;{\rm{N}} = {\rm{N}}{{\rm{C}}_6}{{\rm{H}}_5}\)
Aniline Benzenediazoniumchloride

Q.4. How is the basic strength of amines compared to that of ammonia?
Ans: The basic strength varies in order:
Aliphatic amines > Ammonia > Aromatic amines

Q.5. How do you neutralize amines?
Ans: Since amines are basic in nature, acids like \({\rm{HCl}},{\rm{Hl}}\) or \({\rm{HBr}}\) can be used to neutralize amines. Salts are formed as the product.

Q.6. Write three reactions that amine will undergo.
Ans: Amines undergo several reactions, including nucleophilic and electrophilic reactions. In electrophilic reactions, amines undergo nitration, Bromination and sulfonation reactions to form various ortho and para products.

Study Reactions of Haloarenes Here