In organic chemistry there are five general types of reactions.
1. Substitution – break a sigma bond with one group and form a sigma bond with a new group
2. Elimination – break two sigma bonds on neighboring atoms to form a pi bond
3. Addition – break a pi bond to form two sigma bonds on neighboring atoms
4. Radical – break and form bonds by moving only one electron at a time
5. Electrocyclic – break and form sigma and pi bonds to rearrange the connectivity of atoms
An organic chemistry textbook has A LOT more than five reactions though. That is because how each reaction occurs (think reagents needed and mechanistic pathway) depends on the properties of the starting material. Traditionally, textbooks organize reactions by functional group. Rather than memorizing the list of reactions each functional group undergoes, it is much more efficient to understand the fundamentals of each of these reaction types.
In Mechanisms you will find puzzles covering substitution, elimination, and addition reaction patterns. Review examples of each reaction type by watching the videos for the puzzles listed below. Then practice pushing the electrons yourself to achieve the reaction indicated.