Amines: Structure, Nomenclature, Properties, and Reactions

Chapter 16: Amines

16.1 Classifying Amines

Amines are organic compounds derived from ammonia (NH3) in which one or more hydrogen atoms are replaced by organic groups (alkyl or aryl groups). The classification of amines depends on the number of organic groups attached to the nitrogen atom.

• Primary (1°) amine: Nitrogen is bonded to one organic group and two hydrogens. General formula: RNH2

• Secondary (2°) amine: Nitrogen is bonded to two organic groups and one hydrogen. General formula: R2NH

• Tertiary (3°) amine: Nitrogen is bonded to three organic groups and no hydrogens. General formula: R3N

• Quaternary ammonium ion: Nitrogen is bonded to four organic groups and carries a positive charge. General formula: R4N+

Example: Pyrrolidine is a cyclic secondary amine. Quaternary ammonium ions are important in biochemistry and medicine.

16.2 Naming and Drawing Amines

Amines are named according to the number and type of organic groups attached to the nitrogen atom. Both common and IUPAC naming conventions are used.

• Simple amines: Name the alkyl group(s) followed by "amine."

  • CH3NH2: methylamine

  • CH3CH2NH2: ethylamine

  • Cyclohexylamine: cyclohexane ring with an amino group

• Secondary and tertiary amines: List the alkyl groups in alphabetical order, followed by "amine." If the same group appears more than once, use prefixes (di-, tri-).

  • CH3NHCH3: dimethylamine

  • CH3N(CH3)2: trimethylamine

  • Methyl ethyl amine: one methyl and one ethyl group attached to nitrogen

• Substituents on the nitrogen: Use "N-" to indicate substituents attached to the nitrogen atom.

  • N,N-dimethylpropylamine: two methyl groups and one propyl group attached to nitrogen

• Amino as a substituent: When the amino group is a substituent, use the prefix "amino-" and indicate its position.

  • 4-amino-2-pentene: amino group on carbon 4 of a pentene chain

• Aromatic amines: The simplest is aniline (aminobenzene). Substituents are named as prefixes.

  • p-bromoaniline: para-bromo substituent on aniline

  • N-methylaniline: methyl group attached to the nitrogen of aniline

Example: N,N-dimethylpropylamine has two methyl groups and one propyl group attached to the nitrogen atom.

16.3 Properties of Amines

Amines exhibit characteristic physical and chemical properties due to the presence of the nitrogen atom with a lone pair of electrons.

• Basicity: Amines act as weak bases (Lewis bases) because the nitrogen atom can accept a proton (H+).

• Hydrogen bonding: Primary and secondary amines can form hydrogen bonds with each other, leading to higher boiling points than alkanes but lower than alcohols.

• Physical state and odor: Simple amines are often liquids with unpleasant, fishy odors.

• Solubility: Amines with up to 6 carbon atoms are generally water-soluble due to hydrogen bonding with water.

• Physiological activity: Many amines are biologically active; some are toxic.

Example: Methylamine is a water-soluble, volatile liquid with a strong odor.

16.4 Heterocyclic Nitrogen Compounds

Heterocyclic compounds are ring structures that contain atoms other than carbon, such as nitrogen. These compounds are common in nature and pharmaceuticals.

• Pyridine: A six-membered aromatic ring with one nitrogen atom.

• Pyrrolidine: A five-membered saturated ring with one nitrogen atom.

Example: Pyridine is used as a solvent and in the synthesis of drugs.

16.5 Basicity of Amines

Amines act as bases by accepting protons to form ammonium ions. The basicity depends on the structure of the amine and its environment.

• Ammonium ion formation: Amines react with acids to form ammonium ions.

• Acid-base reactions: In water, amines accept protons from acids, forming their conjugate acid (ammonium ion) and the conjugate base of the acid.

• Relative basicity: Nonaromatic amines (aliphatic) are generally stronger bases than aromatic amines (such as aniline).

Equations:

• General base reaction: $\mathrm{RNH_2 + H^+ \rightarrow RNH_3^+}$

• Example with methylamine: $\mathrm{CH_3NH_2 + H^+ \rightarrow CH_3NH_3^+}$

• Example with dimethylamine: $\mathrm{(CH_3)_2NH + H^+ \rightarrow (CH_3)_2NH_2^+}$

Comparison of basicity:

Type	Example	Relative Basicity
Nonaromatic amine	CH3NH2	Strongest
Ammonia	NH3	Intermediate
Aromatic amine	Aniline (C6H5NH2)	Weakest

16.6 Amine Salts

Amine salts are ionic compounds formed when amines react with strong acids. They are important in pharmaceuticals and biochemistry.

• Formation: Amines react with acids to form ammonium salts, which are water-soluble, crystalline solids.

• Regeneration: The free amine can be regenerated by treating the salt with a base.

• Quaternary ammonium ions: These have four organic groups attached to nitrogen and no lone pair; they are always positively charged.

Equations:

• Formation of amine salt: $\mathrm{RNH_2 + HCl \rightarrow RNH_3^+Cl^-}$

• Regeneration of amine: $\mathrm{RNH_3^+Cl^- + NaOH \rightarrow RNH_2 + NaCl + H_2O}$

Example: Methylamine reacts with hydrochloric acid to form methylammonium chloride, a water-soluble salt.

16.7 Alkaloids

Alkaloids are naturally occurring amines found in plants. They often have pronounced physiological effects and can be toxic or medicinal.

• Properties: Bitter taste, often poisonous, many are used as drugs or stimulants.

• Examples: Morphine, quinine, nicotine, and caffeine are all alkaloids.

Example: Quinine, an alkaloid from cinchona bark, is used to treat malaria.