In the case of a series mechanism it may help to select the reaction time (low conversion). You can also try to change the molar ratio of substrates. However, this is not the case with the parallel mechanism. Look for a solution in selecting a selective catalyst. Previously, however, try to get information on the esterification rate of the particular carboxyl groups.

Regards,

Esterification of 1,1,2,2-tetrayl tetrabenzoic acid use excess of alcohol (25 ml)and 3 ml of con. H2SO4 for 10 g. of aryl acid & reflux for 8 hrs. , cool and neutralize same with sodium carnonate when thick layer of ester separates out , separate using separatory funnel. If ester separates as a solid , separate using Buckner funnel.

As far as I know the esterification process, the temperature is important outside of the time. And in the case of an excess of esterifying agent (I do not know what the abbreviation NHS is) in moles, it is more likely to esterify the remaining carboxylic groups. - unless one of the groups is privileged or/and the esterification process is very slow in this case ?

Regards,