Strength Of Bases

Basicity in Amines. The tendency to accept a proton determines the strength of a base measured in terms of pk_b values. Larger the pk_b 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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