A Primer On Organic Reactions
By James Ashenhurst
Learning Reactions: A Checklist (PDF)
Last updated: May 21st, 2019
Let’s Get More Specific About Where Memorization Works And Where It Doesn’t
Recently, we tried to understand what instructors mean when they say, “Don’t Memorize!”, or its darker variant, “If You Try To Memorize Everything You Are Doomed To Fail“. You can read the whole post here, but it basically boiled down to this:
• Brute force memorization of a large number of reactions and mechanisms is not efficient. If there are a lot of reactions and mechanisms to learn, spend significant time analyzing the common patterns. Like mlukeman said: “A terrible way to become good at chess is to watch a bunch of pro games and try to memorize the pattern of moves in each game. You instead need to understand the general principles that led the players to make those moves in the first place.”
• Still, in some Org 1 courses, you might be able to get away with memorizing reactions and mechanisms without understanding them, especially if you’re not tested on a deep level. Where the “Doomed To Fail” narrative really kicks in if you try to apply this strategy to Org 2, which is largely wall to wall reactions (>100, easily). Hence the advice: Learn the concepts, don’t just memorize reactions.
• Although some instructors would be loath to admit it, if you spend some time memorizing things like functional groups, a few useful pKa numbers, electronegativities, and so on, you are not “Doomed To Fail”, because memorizing these items is not that time-intensive.
Still, this doesn’t seem specific enough. It’s hard to visualize for a beginner. If I were taking organic chemistry over again, a detailed roadmap of what to expect would be useful.
So I thought it would be helpful to take a common problem students face – learning a new reaction – and break it all down into some very specific things to learn. You can think of this as a checklist, if you will.
What makes organic chemistry very different from most science courses is that there are many different categories of material to learn.
- Facts, such as bond strengths, pKa’s, the names of reagents, what bonds form/break in a given reaction, and so on. Generally, facts are answers to the question, “What…?” You can even include the reaction mechanism in this list of facts, although you’ll eventually find it more efficient to understand the key concepts involved. Bottom line: facts are things that can be memorized.
On the PDF, I’ve included a checklist of typical “facts” that one commonly wants to learn about a given reaction.
- Skills, like drawing curved arrows, calculating formal charge, drawing a cyclohexane chair flip. Skills require practice, which requires doing practice problems. Just like you can’t prepare for a piano recital exclusively by reading the sheet music, studying for organic chemistry forces you to get out your paper and pen.
The PDF includes a list of important skills one should practice in order to confidently be able to understand a reaction, such as calculating formal charge, interconverting resonance forms, recognizing implicit hydrogens and lone pairs, and many others.
- Concepts. For our purposes, concepts are useful generalizations based on facts. For instance, electronegativity is a useful concept for understanding the measured trends of electron affinities. We can use concepts to help our understanding of other concepts – for instance, electronegativity is helpful in understanding some acidity trends (such as H2O being more acidic than NH3 , for instance) as well as understanding concepts like polar vs. covalent bonding, inductive effects, and others. In organic chemistry, concepts require some level understanding beyond just memorizing facts. It can be useful to memorize trends based on concepts – such as that “electronegativity increases up and to the right on the periodic table” – but if the “why” behind the trends is understood then we can apply this to a huge range other situations.
I’ve tried to include some of the key concepts for learning reactions on the sheet. The good news is that there are a finite number of important concepts that will help you carry most of the freight, just like there are a finite number of important concepts that mechanics use for understanding engines, or chess players use for understanding positions on the board.
The bad news – for memorizers, anyway – is that if your instructor tests you on your ability to apply concepts to various reactions, there is a near-infinite number of questions they could ask, and it’s impossible to prepare for a test like this by memorization. The only way is to do a lot of advanced problem solving and gain practice by making a lot of mistakes.
To help things along in this regard, I’ve included a suggestive list of “deep” conceptual questions that you could try asking yourself about a new reaction, once you’ve understood it at a “factual” level.
Much as someone who understands how a lawn mower engine operates can apply some of the same concepts to, say, understanding how tractor and automobile engines work, if you keep asking these types of deep questions, you’ll eventually find yourself with a excellent mental “working model” of how organic chemistry reactions work that you can apply to other new reactions that you encounter.
The sheet also includes some “deeper’ questions to ask yourself about mechanisms, and finally, a few bonus exercises that will be useful for synthesis.
Thanks to Hubert Muchalski (@muchalh) and r/chemistry for helpful discussions. I’d appreciate any feedback to make this better.