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Topic: aldehyde + tert-butoxide,exercise  (Read 476 times)

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Offline xshadow

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aldehyde + tert-butoxide,exercise
« on: April 06, 2019, 02:04:38 PM »
HI, I  have to write the products of these two reactions:





Are these  written products right?
In the first  "question", I think that no  reaction can occour....because I haven't any  C-H in alpha position; tert-butoxide con acts only as a strong base..I think it can't act as nucleohile (C=O attack)  ---> so I suppose no reaction .

Is it right?

Offline rolnor

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Re: aldehyde + tert-butoxide,exercise
« Reply #1 on: April 06, 2019, 02:45:22 PM »

Offline spirochete

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Re: aldehyde + tert-butoxide,exercise
« Reply #2 on: April 06, 2019, 10:50:14 PM »
If this is basic sophomore organic, the original poster may have been taught it is simply "no reaction" under any circumstances for an aldehyde with no alpha hydrogens + base. Professors sometimes don't think very hard about questions like this, and in that case they also don't expect the student to.

If they are indeed responsible for something like benzaldehyde + alkoxide or hydroxide, OP should also read up on "Cannizaro reaction" to understand the result: https://en.wikipedia.org/wiki/Cannizzaro_reaction . The metal also seems to be important as a Lewis acid, but you could imagine a simplified mechanism that does not involve the metal.

For the second aldol, that looks correct. You might also consider aldol condensation, but your professor may have taught you that this requires "heat."
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Offline xshadow

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Re: aldehyde + tert-butoxide,exercise
« Reply #3 on: April 07, 2019, 04:46:01 AM »
Thanks ... but We haven't done the Tishchenko reaction...only Cannizzaro one.

Is benzaldehyde +  potassium tert-butoxide  campatible with Cannizzaro reaction?

Because usually we need NaOH....here we have tert -butoxide: a very weak nucleophilic (steric hindrance)  but strong base.. but in the first step of Cannizzaro reaction we have the OH-  nucleophilic attack towards the C of   C=O : tert-butoxide is a weak nucleophilic

Offline spirochete

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Re: aldehyde + tert-butoxide,exercise
« Reply #4 on: April 07, 2019, 05:11:19 PM »
If you have studied cannizarro, then it would be reasonable to think Tischenko might be fair game. Draw a cannizarro mechanism first, and then below it try to draw a similar mechanism for Tischenko.

Disclaimer: I don't specifically know if it works with t-butoxide. But t-butoxide is occasionally a nucleophile, mainly in situations where it has no possible way for it to act like base. For example, you can do Williamson ether synthesis (Sn2) with t-butoxide to make tert-butyl methyl ether. In this case the electrophile is a methyl halide.
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Offline kriggy

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Re: aldehyde + tert-butoxide,exercise
« Reply #5 on: April 08, 2019, 01:20:21 AM »
I dont think canizzaro works with t-butoxide.T-butoxide is not a great nucleophile and tbutanol is more acidic than hydrogen so it should be a better leaving group compared to hydride

Offline spirochete

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Re: aldehyde + tert-butoxide,exercise
« Reply #6 on: April 08, 2019, 03:39:21 PM »
t-butoxide is also more basic than an average alkoxide, which could increase equilibrium constant for formation of the reactive intermediate. This could even make reaction faster if hydride transfer is still the rate limiting step. If the basicity of hydride was a critical factor, than no canizzarro or tischenko reaction would ever work in the first place. I guess hydride never forms as a leaving group in these reactions.
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Offline xshadow

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Re: aldehyde + tert-butoxide,exercise
« Reply #7 on: April 10, 2019, 07:03:19 AM »
 I suppose that t-butoxide doesn't give Cannizzaro reaction (weak nucleophilc,strong steric hindrance ).

So can i say that there wasn't any reaction?(haven't studied Tischenko reaction)
Thanks
« Last Edit: April 10, 2019, 07:45:01 AM by xshadow »

Offline rolnor

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Re: aldehyde + tert-butoxide,exercise
« Reply #8 on: April 10, 2019, 09:50:11 AM »
I think you could say that Cannizarro is imposible but Tischenko is possible.

Offline xshadow

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Re: aldehyde + tert-butoxide,exercise
« Reply #9 on: April 10, 2019, 11:59:47 AM »
Is potassium terz- butoxide a good alcoxide for Tischenko reaction...or has too much steric hindrance?

I never see terz -butoxide in this reaction...

Offline rolnor

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Re: aldehyde + tert-butoxide,exercise
« Reply #10 on: April 10, 2019, 01:36:48 PM »
I dont think is good but not impossible.

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