Chapter 22: Amines

Amines

Nomenclature:
Functional group suffix = -amine (review)
Functional group prefix = amino
Primary, secondary, tertiary or quaternary ? Amines are described as primary (1^o), secondary (2^o), tertiary (3^o) or quaternary (4^o) depending on how many alkyl substituents are attached to the N atom (note the difference compared with alcohols).  Quaternary amines are also known as ammonium cations.
 

Physical Properties:

• The polar nature of the N-H bond (due to the electronegativity difference of  the two atoms) results in the formation of hydrogen bonds with other amine molecules, see below, or other H-bonding systems (e.g. water). The implications of this are:
• high melting and boiling points compared to analogous alkanes
• high solubility in aqueous media
 

intermolecular H-bonding in amines

• The amine functional group consists of an N atom bonded either to C or H atoms via σ bonds.
• Both the C-N and the N-H bonds are polar due to the electronegativity of the N atom.
• The trigonal pyramidal arrangement of bonds around nitrogen is shallower in aryl amines vs alkyl amines.
• This is a result of resonance delocalisation of the lone pair into the aromatic π system

(such delocalisation is also responsible for the decreased basicity of aryl vs alkyl amines).

The image shows the electrostatic potential for methylamine, CH3NH2. The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density.  The amine N atom is a region of high electron density (red) due to the lone pair. Amine N atoms are Lewis bases, (alkyl ammonium pKa ~ 10, aryl ammonium pKa ~ 5) Amines can react as either bases or nucleophiles at the nitrogen. There is low electron density (blue) on H atom of the -NH group. Removal of the proton generates the amide ion (care : not to be confused with the carboxylic acid derivative RCONH2)  The -NH group is a very poor leaving group and needs to be converted to a better leaving group before substitution can occur.

The image shows the electrostatic potential for methylammonium, CH3NH3+. The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density.  The ammonium N atom is a region of low electron density (blue) due to the positive charge. Ammonium ions are not nucleophilic. There is low electron density (blue) on H atom of the -NH3 group. Removal of the proton regenerates the amine. The -NH3 group is a potential leaving group.

Basicity:

• Amines are more basic than analogous alcohols (R-NH₃⁺  pK_a~ 10, R-OH₂⁺  pK_a~ -3)
• Factors (e.g. resonance, electronegativity) that affect the availabilty of the lone pair will affect the basicity.
• N is less electronegative than O and therefore N is a better electron donor.
• alkyl and non-aromatic heterocyclic amines are slightly stronger bases than ammonia
• aryl amines are much weaker bases than ammonia, a result of the delocalisation of the lone pair into the π system of the ring.

 
• The anion derived by the deprotonation of an amine is the amide ion, NH₂^-
• Amide ions are important bases in organic chemistry (example)
• Amines react with Na (or K) to give the amide ion.
• The basicity of aryl amines is:

Increased by the presence of electron-donating substituents on the ring, by counter-acting the delocalisation of the lone pair into the π system of the ring. Decreased by the presence of electron-withdrawing substituents which enhance the delocalisation of the lone pair into the π system of the ring (especially those ortho or para to the amine functional group, see right).

 
• Inclusion of a heteroatom into an aromatic ring generally decreases basicity, unless protonation leads to an ion that can be stabilised by electron delocalisation:

© Dr. Ian Hunt, Department of Chemistry