November 29, 2020, 08:12:34 AM
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Topic: Reversing/Opening a Hemiacetal  (Read 303 times)

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Offline NickDubz

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Reversing/Opening a Hemiacetal
« on: February 14, 2020, 10:04:17 PM »
Hi,

I only just started learning about organic reactions and mechanisms, so I'm sorry if this is really obvious or if I say something that's completely wrong.

https://ibb.co/SvQxjrR

In the picture I linked, I first had to show the first two curved arrows involved in forming the hemiacetal. I didn't struggle much with this. However, in the second part I now have to work backwards show the first two arrows of the reaction involved in "opening" the hemiacetal. Everything I drew in black (other than the words I wrote) was all that was given to me, and the magenta-ish arrows are what I drew in for question 1.

I thought it would just be the reverse of the original, but that's a bit difficult without visualizing the compound catalyzing the reaction. I originally thought that I would draw an arrow showing the electrons moving from the bond next to the non-hydroxyl oxygen to said oxygen, but I don't think any other combination worked out when I tried :/

I don't necessarily need someone to outright give me the answer (I'd actually prefer to figure it out myself, but I'm running out of attempts...), but if anyone could give me some sort of hint, I would really appreciate it! Thanks

Offline pgk

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Re: Reversing/Opening a Hemiacetal
« Reply #1 on: February 17, 2020, 02:12:10 PM »
1). Does it help you, drawing the hemiacetal hydroxyl as oxygen-single bond-hydrogen (-O-H) and not just -OH?
2). Theoretically, there is no need of a catalyst for reversing a hemiacetal and/or opening a cyclic hemiacetal (however, it is sometimes different in lab practice).
3). But if you insist for catalysis, then:
a). Acid catalysis:
Draw an oxonium cation in the place of the ether oxygen in the cyclic hemicatal, as (>O(+)-H) and not just >O(+)H .
b). Basic catalysis:
Draw a hydroxyl anion HO(-) attack to the hydrogen of the hemiacetal hydroxyl (-O-H), with curved arrows.

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