Color change when heating organic compound

I did just attend a lab class where I should synthesize 3,6-dibromo- N-propylcarbazole from N-propylcarbazole by bromination. Once, the bromination was over the color of the reaction content was yellowish and I then started recrystallising of what I hope was 3,6-dibromo- N-propylcarbazole.


After recrystallization, the solid product I achieved was white in its color (which it should also be as described in the manual). But, unfortunately I did not achieve a high enough yield, so I decided once more to make a recrystallization of the reaction mixture (solvent) obtained upon the first recrystallization because I could clearly see a lot of the product was still dissolved in the solvent.
Unfortunately, the amount of solvent (glacial acetic acid) was too high, so i needed to make a destillation to get rid of the excess glacial acetic acid. However, once heating/boiling the solvent mixture, it started to change color from yellowish (probably due to bromine) to green - consequently I did obtain green crystals instead of white ones. It did not happen during my first recrystallization, but why now?


I asked my tutors, the simply did not know or they came with arguments like: "do not trust change color in organic chemistry", "your product may have been broken down while heating it, "it may haven been oxidized". I made a TLC to confirm whether the green crystals were the same as the white ones and it seemed so. In order to clarify it more precisely I have now made a H-NMR but I am still waiting for the answer.


Do you have an idea why my reaction mixture changed color to green during the second recrystallization upon heating it? I simply do not believe it is due to the impurities.


