Question

How can one understand and memorize the mechanism of the preparation and reactions of alkene?

Answer 1

We should be familiar with the basic idea of alkene preparation and its reactions. The steps can be learnt like a story. Think about why reactions would take place in an alkene, and what makes the bond break, and familiarize what the end products will be. Understand how alkene is synthesized and also remember which atoms can be removed from the carbon atoms.

Complete step by step answer:
I would not suggest memorizing concepts in Chemistry; we need to approach the subject with understanding. If we want to excel in a sport we need to practice for it, just like that, if we keep reading the concepts and practicing the mechanisms we can easily do this topic well.
To understand the mechanisms and reactions, we need to understand why the reactions take place. Alkenes in simple words are hydrocarbons with a double bond between two carbons. They are unsaturated compounds. The double bond of an alkene automatically tells us that they have a weak $\pi$ bond. A weak bond means that it can be broken easily, so whichever agent broke that bond, would then form a new bond with the atoms which were there in the double bond.
Now we need to know what a $\pi$ bond is and what an electrophile is. A pi bond consists of loose electrons so it is electron rich, and an electrophile is something that craves electrons. So naturally something that wants electrons that is electrophile would want to catch the electrons from the pi bond, so pi bond will donate its electrons to the electrophile. This is the process of breaking the double bond.
But remember that the reaction cannot take place as easily as explained; it will be easy only if the atoms attached to the carbons are good leaving groups. A leaving group is one that is able to exist stably on its own. In case they are not easily leaving groups, we must help them by adding something into that reaction for making them into an easy leaving group.

Note: The two types of hydrocarbons are: saturated and unsaturated. When only a single bond exists between carbons, it is an alkane and they are saturated compounds. When a double or triple bond exists between carbons, then they are alkenes or alkynes and they are unsaturated compounds. Unsaturated hydrocarbons are called so because they have a scope of getting more hydrogen atoms in order to become a single bond that is saturated hydrocarbons.