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Strength Of Bases

Basicity in Amines. The tendency to accept a proton determines the strength of a base measured in terms of pkb values. Larger the pkb value, weaker is the base. For example ammonia: NH3 is a weak base in aqueous medium but nitrogen trifluoride is not at all basic and CH3NH2 is stronger base than ammonia
                                          NH3  > NH2-NH2   > NH2OH   >  NF3
                                           ( CH3)2NH   >   CH3NH2  >  NH3
This may be attributed to the inductive effects of substations on nitrogen atom. The replacement of H in NH3 by electron withdrawing groups such as –NH2, -OH, -F etc. decreases the basicity as these groups withdraw the electron density form the nitrogen atom make in it slightly positive and hence reduces basicity.

On the other hand, replacement of H in NH3 by electron releasing alkyl group increases basicity because of increased electron density of nitrogen making the acceptance of proton easier. In general basic character of amines decreases in the order R2NH>RNH2>NH3.

However, this increase in basicity does not go beyond secondary amines and it has been observed that tertiary amines R3N are weaker bases with basicity of the order of primary amines or less. The reason for this anomaly is the increased satiric effect of three alkyl groups on a small nitrogen atom and lower energy of salvation tertiary amines as compared to others.

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