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Author Topic: Acid-catalysed ring opening of parasorbic acid to sorbic acid  (Read 746 times)

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DeanC

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I have attached the question and my attempt, any help appreciated!
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wildfyr

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Re: Acid-catalysed ring opening of parasorbic acid to sorbic acid
« Reply #1 on: April 18, 2017, 11:43:04 AM »

Looks good to me. Only nitpick would be to show water plucking off the proton in the elimination reaction at the end.
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lb2

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Re: Acid-catalysed ring opening of parasorbic acid to sorbic acid
« Reply #2 on: April 19, 2017, 06:22:40 PM »

You've got some of the basic ideas correctly identified but several of the details need to be adjusted.

First, if your instructor is ok with you using H+, then use it. But since H+ does not exist in solution, most instructors would usually prefer the use of H3O+. Anyway, just use whatever your instructor wants you to use.

The first stage of the reaction is a simple acid-catalyzed hydrolysis of an ester. Which oxygen is more basic in an ester?

The second stage, is an elimination. First, look carefully at the arrows that you drew for the elimination: they do not result in the formation of the desired product and, for that matter, are simply incorrect and impossible. Once you've figured out better electron-movement arrows (so which hydrogen needs to be removed?), then you will want to adjust your mechanism to include the formation of a dienol before kicking out H2O.
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kriggy

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Re: Acid-catalysed ring opening of parasorbic acid to sorbic acid
« Reply #3 on: April 19, 2017, 10:39:15 PM »

You've got some of the basic ideas correctly identified but several of the details need to be adjusted.

First, if your instructor is ok with you using H+, then use it. But since H+ does not exist in solution, most instructors would usually prefer the use of H3O+. Anyway, just use whatever your instructor wants you to use.


That is IMO not entirely correct. If I use anhydrous pTSA (for example) in dry solvent then there is no H3O+. Also, in my opinion H+ means in general acid condition while H3O+ is specificaly for aquaeous acidic conditions.

lb2

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Re: Acid-catalysed ring opening of parasorbic acid to sorbic acid
« Reply #4 on: April 19, 2017, 11:32:10 PM »

Agreed about what you mention about H3O+, it does indeed only apply to aqueous solutions. And obviously, H+ can be taken to mean "acidic conditions" in a general sense. But in my opinion it's a pretty terrible shortcut to use when you are learning this chemistry because (1) H+ does not exist in solution and (2) it neglects the whole concept of acids of different strengths and of using different acids for different purposes.

But my statement is entirely correct. If you use anhydrous pTSA, the acid is not H3O+ but pTSA itself. H+ is still not present.
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DeanC

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Re: Acid-catalysed ring opening of parasorbic acid to sorbic acid
« Reply #5 on: April 20, 2017, 07:41:01 AM »

You've got some of the basic ideas correctly identified but several of the details need to be adjusted.

First, if your instructor is ok with you using H+, then use it. But since H+ does not exist in solution, most instructors would usually prefer the use of H3O+. Anyway, just use whatever your instructor wants you to use.

The first stage of the reaction is a simple acid-catalyzed hydrolysis of an ester. Which oxygen is more basic in an ester?

The second stage, is an elimination. First, look carefully at the arrows that you drew for the elimination: they do not result in the formation of the desired product and, for that matter, are simply incorrect and impossible. Once you've figured out better electron-movement arrows (so which hydrogen needs to be removed?), then you will want to adjust your mechanism to include the formation of a dienol before kicking out H2O.

I noticed that I removed the proton from the wrong carbon after posting this alright. It results in the other carbon of the alkene now having 5 bonds. Thank you for the input!
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lb2

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Re: Acid-catalysed ring opening of parasorbic acid to sorbic acid
« Reply #6 on: April 20, 2017, 07:55:48 AM »

I'm glad you caught it!

Have you also looked into how you can correct your first step? Also, looked into the dienol thing?
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DeanC

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Re: Acid-catalysed ring opening of parasorbic acid to sorbic acid
« Reply #7 on: April 25, 2017, 12:30:01 PM »

I was thinking protonation of the carbonyl oxygen, attack again with the H2O nucleophile and then a proton transfer perhaps?

Not sure about the dienol.
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