Hello sujata

Chemistry is a subject which is tough and also it is very easy.

My teacher told me that don't learn or cram reaction ,go with the mechanism , nature and the properties of the substance and you will easily get to know that what will be the product and what is going on in the reaction. I think yes you should learn all the mechanism of reactions because it is not definitely sure that the question will come from this topic not from that and if you need to get a good rank you should go with all and try your best but if you will be not able to do so than don't worry and just relax and don't take any tension because tension can ruin your other preparation and there are a lots of other topics in which you can score and still get a nice rank.

So good luck

Dear,

For JEE advanced it is important to know the mechanisms for reactions, as u may be asked to follow the flow of reaction with a different reagent. However for jee mains mechanisms are not so important, try knowing mechanisms of some standard reactions and the job is done.

Hope this helps!