I will be honest with you. You need to learn and memorize those reactions. Find a way which suits you the best. But you *need* to find a way to memorize them.
But the trick in learning anything in physics derivations or chemistry reactions is not bluntly memorizing them, but trying to understand them first.
Spend some time understanding the very basics (which we ignore most of the times) and then go for the actual reactions. This will help you not only for IIT-JEE but also in your engineering college where first 3 semesters are common and you study most of the stuff you learn while preparing for IITJEE.
1. Try revisiting these things often. You are still a human being, you will forget things if you don't revisit them. 2. Make a strict schedule and follow it!!