I love your app and I think it haves a lot of potential. But I also think there are things you could improve.
Firstly, the possibility of retry a puzzle, because if I want to learn of what I failed I can't retry it in the random mode (and it could be without modifying the user score).
Second, to add a mayor variety of mecanisms and reactions, other examples of synthesis, with a more variety of functional grups and more advanced reactions.
Also it could be so great to have agrupated the puzzles into groups like: just for practicing alkenes reactions, just for practicing ketones reactions, just for practicing elimination reactions, just for practicing oxidation reactions, etc.
And just another thing. To add information of that reactions to the user. Like comments, links to wikipedia, or whatever. With that your app will be more relevant and useful.
If you can I will suggesest it to the people of my university because it is a very good app idea.