Ch22 : Amines

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.


ammonia	ethyl amine	dimethyl amine	trimethyl amine	tetramethyl ammonium
	1^o	2^o	3^o	4^o

Question
Check the designations by counting the number of C atoms attached to the N atom in each of the examples shown above.

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

Structure:

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).

Reactivity:

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 pK_a ~ 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: