Another Exercise In Mind Reading

by James

in Organic Chemistry 1, Teaching

Here’s another great example of how one reagent used in a certain context performs one thing, and in a different context performs a completely different task. These are the types of inconsistencies which lead students to find organic chemistry difficult, and in many cases to mistakenly believe that it is arbitrary. It is not.

First example. In Org 1 we learn that alkenes can be converted to alcohols when treated with aqueous acid (i.e. H3O+) :

1-mindread

Although these technical details are never mentioned, the laboratory procedure for this reaction  involves treating the alkene with concentrated acid, often at high temperatures [edit: see link in comments for a specific example, done with cooling, not heat. Thanks to Mary Beth for the suggestion] Somewhat forcing conditions, in other words.

All’s well and good. However, skip ahead a semester, to the addition of Grignard reagents to ketones. After the Grignard reagent adds to the ketone, we add H3O+ in a “workup” step to protonate the negatively charged oxygen (alkoxide) to give the alcohol.

Although often not mentioned, in practice, this is done with dilute acid, at room temperature, and briefly (often in a separatory funnel). Mild conditions, in other words.

I recently met a student who was confused by the following problem:

2-mindread

See the issue? We are doing a Grignard in the presence of an alkene. How is the student supposed to know that the second step is merely a workup step and not a hydration of the alkene?

One answer is, “experience”. If you do enough Grignard problems, you see that H3O+ step at the end and you come to associate it with working up a Grignard.

A second answer is “context”. If one is doing problems from a chapter on Grignard reactions, it’s unlikely (although still possible) that alkene addition reactions will be thrown in there alongside.

Regardless, there is an aspect of ambiguity in the use of H3O+ in these two contexts. It’s not unreasonable to suggest that adding the word “workup”  or “dilute” in the case of the second reaction would in some cases  prevent the type of error shown here.

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{ 8 comments… read them below or add one }

Mary Beth

I agree that as instructors, we are often less clear than we could/should be about reaction conditions, which absolutely impact the outcome of a reaction, and that this ambiguity can lead to student confusion. However, I would argue that in your top example, using high temperatures and very concentrated acid could favor the alkene side of the equilibrium. (We teach students that heating alcohols in concentrated (sulfuric, usually) acid leads to dehydration, in this case via an E1 mechanism.) The identify of the acid is important too – what’s the counterion for the hydronium? If it’s something nucleophilic (bromide, chloride, etc.), then it can also act as a nucleophile, if the acid is concentrated enough.

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james

Good point. I’ve been looking for a good preparation of alcohols through this method; my copy of March doesn’t lead me to a good example, however it does give this example of addition of hydrogen peroxide to an alkene using H2SO4 at 5-10 deg C.
http://www.orgsyn.org/orgsyn/pdfs/CV5P0818.pdf
They report combining (with cooling) 30% hydrogen peroxide w/ 95% H2SO4, and then slowly adding diisobutylene to obtain the adduct.

So I was incorrect in stating that heat was required. I’ll correct that. Thanks for the comment!

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Mary Beth

You’re welcome…I haven’t encountered a good example of that, either. The peroxide example is an interesting one, though!

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mamid

OMG, “seperatory funnel”?!!! Not from you! :-(

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james

Fixed (thank you) I blame it on sleep deprivation.

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Paul

That last problem is pure evil to give to undergrads. Aside from the choice of hydration, dehydration, or just protonation, you’ve got the issue that Grignards typically give a mix of 1,2 and 1,4 addition to a,b-unsaturated compounds. Evil.

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azmanam

I try to avoid this by explaining that, in so many words, H3O+ doesn’t exist. You don’t go to the shelf to find the bottle labeled ‘H3O+.’ You don’t flip open the Sigma Aldrich catalog and say ‘boy howdy, look at the prices on the 20L drum of H3O+ these days!’

H3O+ is just a placeholder. It’s what you make when you add real acids to water (strong acids, at least). It’s just a lazy shortcut for chemists when we don’t really care what the acid is, just generic acid will do.

I also try to avoid this by not using H3O+ as my quenching acid. In lab, I’d really reach for my sat’d aq NH4Cl, so I make a concerted effort to use NH4Cl on my Grignard (and LAH, and aldol, and Claisen) reactions instead of H3O+. In fact, in the aldol, generic H3O+ will likely lead to dehydration of the aldol adduct!

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Mia

what exactly is a workup step?

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