An organic conversion
After this, concentrated HCl was added, to react with any excess NaOH, as well as to replace the Na + ions with H + ions to form solid phthalic acid crystals.
The next step was to convert the phthalic acid into phthalic anhydride. The crystals were poured into a beaker, which was first heated to evaporate off any water vapour left in the crystals, and then the beaker was sealed with a flask of cold water, and continually heated. This process was driving off more water and converting the acid into the anhydride, by the loss of a H 2 O from each molecule. The anhydride vapour then sublimed back into fluffy crystals inside the beaker and on the flask, which could be collected. This was repeated until no more anhydride crystals were being formed, and the reaction was complete.
Phthalic anhydride
The next step was to convert this phthalic anhydride into phthalimide, an imide with a nitrogen in place of an oxygen. The fluffy anhydride was first compressed before being added to urea in a round- bottomed flask, and was then heated in an oil bath. After melting, the urea and phthalic anhydride were reacting to form phthalimide and kicking off CO 2 gas. The lone pair from one of the nitrogen atoms in the urea molecule first attacks one of the electropositive carbons which is bonded to 2 oxygen atoms (with the nitrogen becoming positively charged) and so the pair of electrons in the single bond to the oxygen atom are donated to it, becoming an oxide ion. The lone pair from that oxide ion then attacks the electropositive carbon to restore the pi bond, causing its bond to the other oxygen atom to break heterolytically, and it becomes an oxide ion. Next, that ion attacks the delta-positive carbon, restoring the bond and causing the bond with the positively charged nitrogen ion to break, with both electrons going to the nitrogen as a lone pair to neutralize the charge. This lone pair then attacks the other electropositive carbon, in order to close the cyclic structure; the bonded pair of electrons on this carbon are donated to restore the C=O double bond. Finally, the remaining NH 2 O 2 molecule deprotonates the nitrogen to form phthalimide, and splits into ammonia and carbon dioxide.
The solid reaction mixture was then poured into a beaker. To purify the imide, boiling water was added. This was done because unreacted urea would dissolve into the water, and any unreacted phthalic anhydride would be rehydrated and also dissolve into the water, however, phthalimide is practically insoluble in water, and could then easily be filtered off. The phthalimide was then dried in a warm oven.
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