Hello Yashwant, In chemistry it is widely recommended to learn all reactions as it helps you to get concept more clearly, but you mentioned ' Important Organic Reaction' there is huge number of important reactions in organic chemistry, but from the Exam's point of view these are the some important organic reactions ---

• Sandmeyer Reaction
• Wurtz Reaction
• Aldol Condensation
• Nucleophilic Substitution
• Electrophilic Aromatic Substitution
• Oxidation of Alcohols

To see important reactions --- Click Here

To see all reactions --- Click Here