Basicity of amines, effect of substituents on basicity, and synthetic uses of aryl diazonium salts

Basicity of amines, effect of substituents on basicity, and synthetic uses of aryl diazonium salts

Session Objectives

By the end of this session, students will be able to:

Ø  Discuss the Basicity of amines.


Organic derivatives of ammonia, NH3.

Nitrogen atom have a lone pair of electrons, making the amine both basic and nucleophilic

Amines Nomenclature

alkylamines                                              arylamines

Amines are classified according to the degree of nitrogen

substitution: 1° (RNH2), 2° (R2NH), 3° (R3N) and 4° (R4N+)

primary (1°) amines                  secondary (2°) amines           tertiary (3°) amines       quarternary (4°) ammonium ion

Structure and bonding.  The nitrogen of alkylamines is sp3 hybridized and tetrahedral. 

The nitrogen of arylamines (aniline) is slightly flatten, reflecting resonance interactions with the aromatic ring.

Physical Properties

Basicity of Amines.  The basicity is reflective of and is expressed as the pKa of the conjugate acid.

The conjugate base of a weak acid is a strong base:

      Higher pKa = weaker acid = stronger conjugate base

The conjugate base of a strong acid is a weak base

      Lower pKa = stronger acid = weaker conjugate base

pKa values of ammonium ions

Alkyl ammonium ions, R3NH+ X-, have pKa values in the range of 10-11 (ammonium ion, H4N+ X-, has a pKa ~ 9.3)

The ammonium ions of aryl amines and heterocyclic aromatic amines are considerably more acidic than alkyl amines (pKa < 5).  The nitrogen lone pair is less basic if it is in an sp2 hybridized orbital (versus an sp3) 

NH4+                                    pKa=  9.3

(H3CH2C)NH3+             10.8

(H3CH2C)2NH2+             11.1

(H3CH2C)3NH+               10.8

Arylamines are much less basic than alkylamines.  The lone pair of electrons on the nitrogen of aniline are conjugated to the p-electrons of the aromatic ring and are therefore less available for acid-base chemistry.  Protonation disrupts the conjugation.

Substitutents can greatly influence the basicity of the aniline. The effect is dependent upon the nature and position of the substitutent.

Electron-donating substituents (-CH3, -OH, -OCH3) make the substituted aniline more basic than aniline itself (the pKa of the anilinium ion is higher than 4.6)

Electron-withdrawing substituents (-Cl, -NO2) make the substituted aniline less basic than aniline itself (the pKa of the anilinium ion is lower than 4.6)

Basicity-Aliphatic Amines

Diazonium Salts

       Diazonium: there are 2 nitrogen atoms joined together in the positive ion.

       In French, nitrogen is still called by its old name ‘azote’ which means unable to support life.

       Notice the triple bond between the nitrogen atoms

       The positive charge is on the nitrogen that is attached to the benzene ring

Why are they important? They look pretty weird!

       They are essential in the dye industry.

       A Diazonium salt is produced then reacted with a phenol. If the correct phenol is used, almost any colour can be produced.

Formation of the Diazonium salt.

       Formed by reacting phenylamine with sodium nitrite and hydrochloric acid.

       These reagents form in situ nitrous acid HONO.

       The Diazonium salt is unstable above 10°C, so the reaction is normally carried out in ice.

       An aliphatic Diazonium salt is very unstable, so only aromatics are used.

       The lone pairs present in the salt can participate in the benzene ring, making it more stable. More correctly this is due to overlap of p-orbitals in the diazo group with the p-system in the ring.

       So phenylamine would give benzenediazonium chloride.

       The conditions are 5°C and remember the HONO (nitrous acid) is prepared in situ by reacting sodium nitrite with hydrochloric acid.

       The diazonium salt can ten do one of two things depending on the temperature

Reactions of aromatic diazonium salts


       Triacylglycerols undergo stepwise enzymatic hydrolysis to finally liberate free fatty acids and glycerol

       Hydrolysis of triacylglycerols by alkali to produce glycerol and soaps is known as saponification

       Rancidity is the term used to represent the deterioration of fats and oils resulting in an unpleasant taste

       Oils which on exposure to air, change into hard solids e.g., linseed oil

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