What are Amines?

Amines are organic compounds with a nitrogen atom having a lone pair of electrons on it. They are derived from ammonia (NH₃) wherein hydrogen atoms are substituted by an aryl or alkyl group.

Structure of Amines

In amines, the nitrogen atom is trivalent with a lone pair of electrons. Thus, the nitrogen atom of amines is sp³ hybridized. Each of these three sp³ hybrid orbital overlaps with the hydrogen atom or carbon atom depending on the substituent present on the amine. Also, due to the presence of a lone pair of electrons, it acquires pyramidal geometry with bond angle between C-N-H to be 108^o rather than the usual tetrahedral geometry with the bond angle of 109^o.

Types of amines

Based on nature

Amines are classified as aliphatic or aromatic amines based on their nature.

• Aliphatic amines: They have hydrogen and alkyl groups. For example, methane, CH₄.
• Aromatic amines: They have an aromatic ring that is connected to the nitrogen atom. For example, aniline.

Based on connection between carbon and nitrogen atoms

Depending on how the carbon atoms are connected to the nitrogen, the amines are classified as primary amines (1° amines), secondary amines (2° amines) and tertiary amines (3° amines):

• Primary amines: In primary amines, one of the hydrogens of ammonia is substituted by an alkyl or aryl substituent. For example, ethyl amine.
• Secondary amines: In secondary amine, two hydrogens of ammonia are substituted by an alkyl or aryl substituent. For example, diethyl amine.
• Tertiary amines: In tertiary amines all of the hydrogens of ammonia are substituted by an alkyl or aryl substituent. For example, triethyl amine.

In ammonia, if all the four substituents, including the unshared pair of electrons of nitrogen are substituted, then it is termed as quaternary ammonium cations. It is represented as:

Here, R, R', R", R"' groups can be alkyl groups or aryl groups.

Naming of Amines

While naming the amines using common naming system, the group attached to nitrogen is identified and then the word “amine” is added at the ending. Use the prefix such as di or tri before the substituent when two or more identical groups are connected to the nitrogen atom. For example, the common name for CH₃NH₂ is methyl amine.

While naming the amines using IUPAC naming system, they are named as alkanamines as they are derived from the alkanes and the ending letter "e" is replaced by the word amine. For example, the IUPAC name for CH₃NH₂ is named as methanamine.

Basicity of Amines

Due to the presence of an unshared pair of electrons on the nitrogen atom, amines are basic in nature that also results in its electron density. With increase in electron density, the basic nature of a compound also increases. Therefore, the basicity increases in the presence of an electron donating group while it decreases in the presence of the electron withdrawing group.

Preparation of Amines

Amines are prepared by the following methods:

Reduction of nitro compounds

In this method, the nitro compounds are treated with the hydrogen gas and catalysts like nickel, palladium or platinum to form amines. Nitrobenzene on treatment with H₂/Pd in presence of ethanol forms aniline.

Ammon lysis of alkyl halides 

During this process, the alkyl halide is heated with ammonia in presence of ethanol where it undergoes nucleophilic substitution reaction by breaking the halogen bond and forming a substituted ammonium salt.  This ammonium salt is further treated with a strong base, NaOH, to form a free amine.

Reduction of nitriles 

Nitriles on treatment with lithium aluminum hydride (LiAlH₄ ) undergo reduction reaction to form amines.

Reduction of amides 

Amines on treatment with lithium aluminum hydride undergo reduction reaction to form amines.

Gabriel phthalimide synthesis 

During this synthesis process, phthalimide is treated with ethanolic potassium hydroxide and forms a potassium salt which is further heated with alkyl halide followed by alkaline hydrolysis producing the corresponding primary amine.

Physical Properties of Amines

• Primary amines are liquids at room temperature when they have three or four carbon atoms whereas the amines with carbon atoms higher than these are solids at room temperature.
• The lower aliphatic amines which are gases at room temperature have fishy odour.
• Organic compounds like aniline, other aryl amines are colourless, but when exposed to the atmosphere, they get oxidized.
• Organic solvents such as benzene dissolve the amines.
• Due to the presence of hydrogen atom, the intermolecular association occurs in primary and secondary amines. Since there are no hydrogen atoms in tertiary amines, the intermolecular association is not present in it. Therefore, based on the intermolecular association, the order of boiling point of amines is: primary amines > secondary amines > tertiary amines.

Chemical Properties of Amines

• The electronegativity difference between the hydrogen and nitrogen atoms and also the presence of a lone pair of electrons on the nitrogen atom make the amines more reactive.
• Since the amines have an unshared pair of electrons, it behaves as a nucleophile.
• Based on the number of hydrogen atoms attached to the central nitrogen atom of amine, the primary, secondary and tertiary amines react differently during the chemical reaction.
• Based on these different chemical properties, amines undergo different chemical reactions. These reactions are as follows:
  • Amines are basic in nature; they react with acids to form salts which on treatment with a strong base regenerate the parent amines.

• Amines add the acyl group to the compound. In this acylation reaction, amines react with acid chlorides to form amides.

• Amines undergo alkylation reaction when treated with alkyl halides. This method is used to make higher amines. 

• When amines are heated with chloroform and potassium hydroxide, it forms foul smelling isocyanides or carbylamines. This type of reaction is known as carbylamines reaction and is applicable only for primary amines as secondary and tertiary amines do not show undergo this type of reaction.

Uses of Amines

• Amines are used in pharmaceutical industry to make pain relieving medicines such as morphine and demerol.
• They act as a starting material for the preparation of azo-dyes.
• They are used widely in pest control industries.
• Amines are used in the leather industry for tanning.
• Many molecules that are essential to life such as amino acids, neurotransmitters, hormones are based on the amines.

Common Mistakes

• The primary, secondary and tertiary amine structures are flipped.
• When attempting to write the reaction, the reagents are forgotten.
• Amine is confused with amide.

Context and Applications 

Amines concept is significant in the professional exams for both undergraduate and graduate courses; this chemistry topic is especially for Bachelors and Masters in Chemistry and Biochemistry.