See 12th is so much connected with 11th topics. There are many such topics in 12th that will require the basics that have been covered in 11th without which you'll be unable to master the topics. Okk don't worry, I'm not telling you to sit and study complete 11th std right now, what's gone is already gone, let's focus on what's left now. Whenever you study topics like Halogen Derivatives in 12th chemistry, you'll require some basics covered in Basic principles and techniques in chemistry and aromatic compounds of 11th, that time you can just go and read the related concepts only once. For some other important topics of 11th that you need to study, I'll recommend you to finish Ray Optics, Refraction of Light, Motion, Magnetic Effects of electric current, Current electricity of Physics, States of Matter, Redox Reaction, Periodic Table, Nature of Chemical Bonds in chemistry, Probability, Permutations and Combinations in Mathematics.

Best wishes. Thankyoh.