Comments on: Another Exercise In Mind Reading

By: James Ashenhurst

James Ashenhurst — Sat, 29 Jun 2019 03:03:39 +0000

In reply to Rahul. Try this; DOI:10.15227/orgsyn.040.0076

By: Rahul

Rahul — Tue, 05 Dec 2017 00:44:32 +0000

In reply to james. This link no longer works. Any chance you could reupload it?

By: Mia

Mia — Mon, 16 Jun 2014 06:13:55 +0000

what exactly is a workup step?

By: Mary Beth

Mary Beth — Fri, 26 Apr 2013 02:21:04 +0000

In reply to james. You're welcome...I haven't encountered a good example of that, either. The peroxide example is an interesting one, though!

By: azmanam

azmanam — Mon, 22 Apr 2013 20:13:18 +0000

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!

By: Paul

Paul — Mon, 22 Apr 2013 19:54:57 +0000

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.

By: james

james — Thu, 18 Apr 2013 13:24:27 +0000

In reply to mamid. Fixed (thank you) I blame it on sleep deprivation.

By: mamid

mamid — Thu, 18 Apr 2013 12:44:26 +0000

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

By: james

james — Wed, 17 Apr 2013 19:41:40 +0000

In reply to Mary Beth.

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!

By: Mary Beth

Mary Beth — Wed, 17 Apr 2013 16:00:33 +0000

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.