I met this problem when making diazonium salt from p-aminobenzoic acid.
0.02 mol of p-aminobenzoic acid (PABA) was added to the HCl solution ( 5 ml of concentrated HCl in 35 ml of water). The HCl solution was heated up to 60oC before adding PABA. At that time, PABA was totally dissolved and I had a clear solution.
But after I put the mixture in the icebath to cool it down, PABA returned to solid state.
Could anyone give me a solution for this problem, please?
Thank you in advance.
2HN-C6H4-COOH +HCl
When hydrochloric acid is added, a salt is formed over the amine functional group. It is not the PABА that precipitates, but its salt.
Yuri Mirgorod Apologies for reactivating an old thread, but why is this the case? Looking at the structure of pABA, it seems like the nucleophilic Cl ions in the acidic HCl solution should react with the electrophilic site on the carboxyl group (the central carbon), rather than the amino group. What am I missing?
Om Kar
PABA is an amphoteric compound. Inside the molecule, the proton of the carboxyl group passes to the amine group + H3N-C6H4- COO- internal salt is formed. When dissolved in HCl, the salt dissociates and the amine group interacts with the stronger acid
-Cl + H3N-C6H4- COOH
Yuri Mirgorod Thanks for getting back so quickly! I understand now, but this severely complicates matters, doesn't it? How would one perform traditional substitution reactions on pABA in this case? I'm concerned that acid chloride formation with thionyl chloride would be limited by the interference of the internal salt formation. (That is my current goal, synthesizing 4-Aminobenzoyl chloride). Do you have any suggestions for performing this synthesis/reaction?
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