Physical and Chemical Properties of Amines - SS3 Chemistry Lesson Note

Amines are organic compounds that contain a nitrogen atom bonded to one or more carbon atoms. They exhibit a wide range of physical and chemical properties, which are determined by the presence of the nitrogen atom and the nature of the substituents attached to it. Let's explore the physical and chemical properties of amines in detail:

Physical Properties of Amines:

1.    State of Matter: Amines with smaller carbon chains are typically gases or liquids at room temperature, while those with larger carbon chains can be liquids or solids.

2.    Odour: Amines often have characteristic odours, especially the smaller ones. For example, methylamine is often described as having a fishy odour, and ethylamine has an ammonia-like smell.

3.    Solubility: Amines are generally soluble in water due to their ability to form hydrogen bonds with water molecules. However, the solubility decreases with increasing carbon chain length. Amines with long hydrophobic carbon chains become less soluble in water and more soluble in organic solvents.

4.    Boiling Points: Amines have higher boiling points compared to hydrocarbons of similar molecular weight due to the presence of hydrogen bonding between amine molecules.

5.    Basicity: Amines are weak bases, meaning they can accept a proton (H+) from an acid to form a positively charged ammonium ion. The basicity of amines depends on the availability of lone pair electrons on the nitrogen atom. Primary and secondary amines are more basic than tertiary amines because they have more available lone pairs.

Chemical Properties of Amines:

1.    Acid-Base Reactions: Amines act as Lewis bases, meaning they can donate lone pair electrons to form a coordinate covalent bond with Lewis acids. They react with acids to form ammonium salts through the protonation of the nitrogen atom.

2.    Nucleophilic Substitution: Amines can undergo nucleophilic substitution reactions with electrophiles. For example, they can react with alkyl halides to form alkylated amines, also known as substituted amines.

3.    Schiff Base Formation: Amines can react with aldehydes or ketones to form Schiff bases, which are imine derivatives. Schiff bases are important intermediates in organic synthesis and are involved in the formation of various natural products.

4.    Reductive Amination: Amines can be synthesised through reductive amination, where an aldehyde or ketone reacts with ammonia or a primary amine in the presence of a reducing agent (e.g., sodium borohydride) to form the corresponding amine.

5.    Hofmann Rearrangement: The Hoffmann rearrangement involves the conversion of a primary amide to a primary amine by treatment with bromine and a strong base. This rearrangement is useful in the synthesis of primary amines.

6.    Gabriel Synthesis: The Gabriel synthesis is a method for preparing primary amines from phthalimide and alkyl halides in the presence of a strong base.

Overall, the physical and chemical properties of amines make them versatile functional groups in organic chemistry. They participate in various reactions and can be synthesised using different methods, making them essential building blocks for the synthesis of numerous organic compounds, pharmaceuticals, and bioactive molecules.