Short answer: reproduce them from a blank page, repeatedly, and stop reading worked examples. Mechanisms are the part of CHEM191 that rewards deliberate practice the most and responds to cramming the least, so this is where method matters most.
Why reading mechanisms fails
Following a curly-arrow mechanism on a slide is easy. Every step looks inevitable once someone has drawn it. That fluency convinces you that you know it, right up until you have to generate the arrows yourself with nothing in front of you.
This is the recognition versus recall gap. Testing yourself produces markedly better retention than restudying the same material (Roediger & Karpicke, 2006), and mechanisms are the clearest case of it in the whole paper.
The drill
- Take a reaction you have covered. Write only the starting material and reagents.
- Close everything. Draw the full mechanism from memory: every arrow, every intermediate, every charge.
- Check against your notes. Mark only the steps you got wrong.
- Redraw just those steps. Then redraw the whole thing again the next day.
The day gap matters. Coming back after a little forgetting is what makes it stick, rather than repeating it four times in one sitting.
Arrows mean something, so say what
The most common failure is drawing arrows that look plausible without knowing what they represent. Every arrow shows movement of electrons, from a source of electron density to somewhere that wants it.
For each arrow, force yourself to say: what is the nucleophile, what is the electrophile, why here. If you cannot say it, you are pattern-matching a picture rather than understanding a reaction, and the exam will reword it until that breaks.
Learn the families, not the individual reactions
CHEM191 mechanisms group into a handful of behaviours: nucleophilic substitution, elimination, addition, and the reactions of carboxylic acid derivatives. Within each family the logic repeats.
Once you know why substitution and elimination compete, and what pushes a reaction toward one or the other, you can reason about a substrate you have never seen. That is the transfer the harder questions are testing, and it is far more efficient than memorising each example separately.
Predict before you check
Take a substrate and predict the product before looking. Getting the prediction wrong is useful information; reading the answer first destroys the exercise. Same rule as everywhere else: attempt, then check.
Where this sits in the paper
Both CHEM191 exams are multi-choice, so you will not be hand-drawing mechanisms for marks. But the questions test whether you understand what a mechanism does, and the students who can draw them cold are the ones who answer those questions quickly and correctly.
The awkward part is that CHEM191 gives you very little practice material of its own. That gap is exactly what CHEM191 practice questions on Cutline exist to fill, worked lecture by lecture and free to start.
References
- Roediger, H. L., & Karpicke, J. D. (2006). Test-enhanced learning. Psychological Science, 17(3), 249–255.
