Success in the second semester relies heavily on what you got out of the first term. Organic 2 covers mainly new reactions, mechanisms, and applications and requires a solid understanding of core topics such as stereochemistry, reactivity, resonance, and arrow pushing. The best way to prepare for Organic 2 is to review and understand the foundational mechanisms of substitution, elimination, and addition, linked through the video playlists below as they form the bulk of the events in bigger second term pathways. Knowing when a carbocation may or may not be formed (yes in neutral and acidic conditions, no in base) will help, as will being able to apply delocalization patterns to structures to explain reactivity or stability.
There could be up to 100 mechanisms covered in Organic 2 so it is essential that you stay organized and try to see similarities in related pathways. Many of the required structures will be delocalization patterns, which should not need to be memorized if you genuinely understand resonance. Most of the mechanisms, other than radical or some concerted processes, will feature the same bond-forming and bond-breaking events of proton transfer, nucleophilic attack, loss of leaving group and, rarely, alkyl or hydride migrations in rearrangement steps. Try to see the helicopter view to make your studying more efficient.

Organic 1 Concepts

Substitution Reactions

Elimination Reactions

Addition Reactions

This one may seem obvious but you aren’t going to be able to draw a mechanism successfully if you don’t know where you are going. This means knowing the expected products that form under certain conditions. In reality, chemists figure out mechanistic pathways after they know what a reaction produces using structural and stereochemical clues. Whenever you encounter a new reaction you should take a step back and look at the reactants, the conditions (acidic, basic, or neutral?), and the products. Assess which bonds are formed and broken, and then make sure that your mechanism includes a logical description of each of those events remembering that only so many things can happen in polar pathways.
For example, in the SN1 pathway for a tertiary alkyl bromide in alcohol solution, the C-Br bond has to break and a C-O bond has to form. This will be either concerted or stepwise (as are all reactions) and your study will tell you that the pathway is stepwise with formation of a carbocation. Loss of leaving group followed by nucleophilic attack gives the observed ether product. In the second semester, many of our students have that “aha!” moment in which mechanism clicks and they are able to predict products in new reactions because they recognize the limits of what can happen in different environments.

If the product of a reaction has a certain stereochemical or regiochemical identity, this is very helpful in working out how that product was formed. In the SN2 reaction the only product’s stereochemistry has inverted (stereospecific), which is only possible through a concerted attack of nucleophile at the back of the substrate (where the antibond space is located) and loss of leaving group at the same time. The only other option, stepwise progress through a carbocation, would produce a racemic mixture of products as the flat, prochiral intermediate could be attacked from two directions.
In the E2 reaction of an alkyl halide with KOt-Bu the less-substituted Hofmann regioisomer is the major product. This tells us that the reaction is irreversible since equilibration would give us the more stable Zaitsev outcome. The kinetically-controlled pathway then must be governed by steric considerations in transition states in which the very large base attacks the most accessible proton, which inevitably will be attached to the less crowded carbon. Working backwards from these clues helps us understand the factors governing mechanism and make its study much more rewarding than trying to memorize everything.