The Most Important Question To Ask When Learning a New Reaction

by James

in Chemical Bonds, Drawing Reaction Mechanisms, Organic Chem Study Tips, Organic Chemistry 1

About a month or so ago I had a big revelation as an instructor. Something that I’d been taking for granted, that was right under my nose.

I was spending so much time focusing on the why and how of chemical reactions, that I had neglected to make sure that students understood the what  in detail. 

I was assuming that the vast majority of students could look at a reaction and quickly see which bonds were being formed and which were broken. 

I was assuming this because I’d always ask “do you have any questions about this?” and they’d say “no”. So I assumed that they had possession of this most basic and important skill.

No no no no no.

Because when I thought to ask, “what specific bonds are formed, and what specific bonds are broken”, the variance in how quickly (and correctly) students could answer would be HUGE.

Take this reaction:

1-breakform
Now ask the question. What’s different? What bonds formed, and what bonds broke?

Someone professionally trained in organic chemistry could answer this question as quickly as they can speak.

An A-caliber student would take just a little bit longer.

But for someone struggling with organic chemistry, on the threshold between success and failure, answering this question could take minutes. I’ve seen it happen.

2-breakform

I think because I’ve spent the better part of the past 10 years talking with other Ph.D. caliber organic chemists, I couldn’t imagine someone not being able to see this. It’s like trying to imagine not being able to read English. But now things are much clearer.

I think that asking “what bonds were formed, and what bonds were broken” is the most important question you can ask about a chemical reaction. After all, knowing the answer to “what happens” is a prerequisite to being able to answer “how it happens”, or “why it happens”.

 And it’s a completely learnable skill. It’s a skill that someone could employ coming right out of gen chem. In fact, gen chem students do it all the time. Remember Hess’ law? Figuring out the enthalpy of a reaction by seeing which bonds break and form?

The other thing is, it’s a question you can answer without knowing how the reaction works. The only thing you need is to be able to read chemical structures – to be able to figure out which atoms are bound to what.  (That’s not trivial – a fair amount of effort in the first week or two of Org 1 is devoted to just that topic. In this case you’ll need to know about all the atoms that are hidden in these line drawings.)

It’s also a question you can ask about every chemical reaction you will ever encounter. 

From over 1000 hours of 1-on-1 tutoring, I don’t know that I’ve found anything that has a bigger educational impact than making sure students get this question right. It’s even more important than arrow pushing, in my opinion.

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{ 5 comments… read them below or add one }

csw

I am an undergrad who recently completed a three quarter o-chem series. Even after completing the course I don’t claim to have mastered o-chem, but I’m still hoping it will “click” for me one day. Can you expand a bit on how identifying bonds broken and bonds formed will fit into a complete understanding of a reaction? Without knowing how the arrow pushing works it seems impossible to definitively identify which bonds are broken. In the example above where did the other oxygen go (the one from NaOH)? Without arrow pushing how would I know whether the OH in the final molecule comes from the NaOH or is one of the original O in the reactants that gained a hydrogen?

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james

Good point. I should probably have illustrated this with a simpler example, as OH(-) is a catalyst in this process. That causes a little bit of confusion. I’m going to edit this when I get a moment.

I’m not saying that arrow pushing isn’t important – it is – I’m just saying that in order to draw an arrow pushing mechanism you need to be able to recognize the sites where bonds form and break first. If you think of understanding a reaction like a pyramid, just being able to recognize which bonds are broken and formed to form the product(s) would be at the base. From there, I’d say that recognizing the partial charges in the reactants comes next, followed by drawing the arrow pushing mechanism that connects those charges. At the top of the pyramid would probably be seeing how the individual reaction is related to other reactions.

Thanks for the comment.

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dave blackburn

After reading this, I added a couple of questions to an in-class worksheet about acid-base reactions. This is 3 1/2 weeks into O-Chem 1. Starting with skeleton structures of acetic acid and ethoxide ion going to ethanol and acetate, I asked:
1. On the product side circle the new bond(s) formed in this reaction.
2. On the reactant side, draw a slash through the bond(s) that break.
3. What happened to the third lone pair on the ethoxide O?
4. What happened to the electron pair that connected the H to the O in acetic acid?
5. Draw electron-pushing arrows showing what happens in the reaction.

It was worth asking such basic questions – a good third of the class didn’t really get it. I had people moving the ethoxide’s lone pair over to the acetic acid, all kinds of oddities.

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james

Thanks for sharing. You’ve described my experiences exactly. I’m not going to take this for granted anymore.

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aditya

good line for students:After all, knowing the answer to “what happens” is a prerequisite to being able to answer “how it happens”, or “why it happens”.

u should write and discuss often. thanks for u suggestions.

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