24.2 Structure and Properties of Amines

Amines, organic compounds with a nitrogen atom, play a crucial role in chemistry. Their unique structure, featuring a lone pair of electrons on nitrogen, gives them special properties. These properties influence their behavior in reactions and their physical characteristics.

Understanding amines is key to grasping many organic reactions. From their basicity to their ability to undergo pyramidal inversion, amines showcase fascinating chemical behavior. Their solubility and boiling points also set them apart from other organic compounds, highlighting their importance in various applications.

Structure and Properties of Amines

Bonding and geometry of alkylamines

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• Alkylamines contain nitrogen atom bonded to carbon atoms
  • Nitrogen is sp³ hybridized resulting in tetrahedral geometry around nitrogen atom (ammonia, methylamine)
  • Lone pair on nitrogen occupies one of four sp³ hybrid orbitals
• Bond angles in alkylamines slightly less than ideal 109.5° due to lone pair-bond pair repulsion
  • Typical bond angles range from 106° to 108° (trimethylamine, triethylamine)
• Nitrogen-carbon (N-C) bond lengths in alkylamines approximately 1.47 Å
  • Slightly shorter than carbon-carbon (C-C) single bond length of 1.54 Å due to higher electronegativity of nitrogen compared to carbon

Pyramidal inversion in amines

• Pyramidal inversion process where nitrogen atom in amine passes through plane formed by three substituents attached to it
  • Lone pair on nitrogen changes orientation during this process
  • Energy barrier for pyramidal inversion relatively low, typically around 24-26 kJ/mol (6-7 kcal/mol)
• Rapid pyramidal inversion in amines at room temperature has consequences for chirality
  • If amine has three different substituents attached to nitrogen atom, it can potentially be chiral
  • However, due to fast inversion, enantiomers rapidly interconvert making it impossible to isolate individual enantiomers at room temperature (methylethylamine, methylisopropylamine)
• Pyramidal inversion can be slowed down or prevented by:
  • Increasing size of substituents on nitrogen atom, which raises energy barrier for inversion (tert-butylamine)
  • Lowering temperature, which reduces kinetic energy available for inversion
  • Protonating amine, which eliminates lone pair and prevents inversion (ammonium salts)

Physical properties of amines vs organics

• Solubility:
  • Amines more soluble in water compared to hydrocarbons of similar molecular weight due to ability to form hydrogen bonds with water (ethylamine vs ethane)
  • Solubility of amines in water decreases as size of alkyl group increases, as hydrophobic nature of alkyl group becomes more dominant (octylamine vs ethylamine)
  • Amines less soluble in water than alcohols of similar molecular weight because amines have one less hydrogen bond donor site (N-H) compared to alcohols (O-H) (ethylamine vs ethanol)
• Boiling points:
  • Amines have higher boiling points than hydrocarbons of similar molecular weight due to presence of intermolecular hydrogen bonding between amine molecules (propylamine vs propane)
  • Boiling points of amines increase with increasing molecular weight and surface area, as strength of intermolecular forces increases (ethylamine vs butylamine)
  • Primary amines have higher boiling points than secondary amines, which in turn have higher boiling points than tertiary amines (ethylamine vs diethylamine vs triethylamine)
    • Trend due to decreasing number of N-H hydrogen bond donor sites from primary to tertiary amines
  • Amines have lower boiling points than alcohols of similar molecular weight because N-H···N hydrogen bond is weaker than O-H···O hydrogen bond in alcohols (propylamine vs propanol)

Chemical properties of amines

• Basicity: Amines are generally basic due to the lone pair of electrons on the nitrogen atom, which can accept a proton
• Nucleophilicity: The lone pair also makes amines good nucleophiles, allowing them to participate in various reactions
• Hofmann elimination: A reaction where quaternary ammonium salts undergo elimination to form alkenes
• Hinsberg test: A method used to distinguish between primary, secondary, and tertiary amines based on their reactivity with benzenesulfonyl chloride