ACIDITY OF ACIDS

               Acidity of Aromatic Acids

             Acidity of carboxylic acids depends on the stability of acid anion  a conjugate base thus.

            Electron withdrawing group stabilises anion, hence increases acidity

                                                  

            Similarly , electron donating group destabilises anion, hence decreases acidity.

                                                      

            The First member of aromatic acid is a benzoic acid which dissociates as follows

                                                

            Thus  acidity  of  benzoic acid  will  depends on the stability of benzoate anion .  .Greater the stability of carboxylate ion ,it is a weaker conjugate base and tendency to react with H+ decreases  resulting in the increase of acidic nature .

            Acidity of substituted Acids: Ortho substituted benzoic acid is always a strong acid than m- and    p-derivative due to the ortho effect.

            Case I :           When group is –M and –I group.

               at  ORTHO  POSITION           

            Thus , anion is stabilised by  – M and  – I  effect and  – I  power is maximum

             AT META POSITION                  

            Anion is stabilised only by –I effect of NO2 group .

                        AT PARA POSITION           

Anion is stabilised by –M and –I effect of NO2 group

                                    NO SUBSTITUENT    

                                   

            Thus decreasing order of the stability of these anions is follows:

                                    I > III > II > IV

            Ortho derivative  is  the  most acidic  due to ortho effect therefore decreasing  order of acidity of these acids is as follows :

  

Case II :          When group has +I effect.

                                                  Ortho derivative will be most acidic due to ortho effect. The anion is destabilised due to +I effect.As distance between + I group and carboxylate increases +I influence decreases .Thus methyl at para is less donating compared to methyl at meta .At ortho position eventhough the distance between CH3 group and carboxylate is very less DUE TO ORTHO EFFECT IT IS MOST ACIDIC.

              

Case III :   When group has +M and –I effect and - I dominates +M

                                              

 Case IV :   When group has +M > –I effect 

   
 ultimately

 

PSK CHAKRAVARTHY