Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: VittoVG on March 25, 2020, 04:23:00 PM
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As practice on organic reaction mechanisms we've been given several examples. I've written the mechanism down which you can see in the attachement. While I get the end result I feel like I've made certain mistakes on how it should actually go. I've tried finding examples but the results I get aren't 100% what I want or they don't show the mechanism at all. Could perhaps someone take a look at this and maybe point out mistakes and correct if needed.
Thanks in advance.
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You drew a hydrogen being kicked off as a hydride in step 4, that can't happen. Something has to abstract it as a proton.
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This is the hydrogen in step 4 correct? And yes this is one my steps I was uncertain about and can't seem to find how exactly I can create a double bond like shown in the end result. I sadly can't seem to find examples where DMAP is used together with Acetic anhydride to create a double bond. Or is the double bond created by the 2nd reaction with t-BuOK?
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Either base is strong enough to cause elimination. potassium tertbutoxide is especially excellent at it. Since equivalencies aren't specified, you can assume these reagents are in excess.
For the ester hydrolysis step, I don't think there is any reason to form the tert butyl ester first, its not a very favorable reaction. You can just hydrolyze the ethyl ester with KOH.
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Thank you for your help so far. I've made changes to the mechanism which you again can see in the attachement. For my hydrolysis of the ethyl ester to a carboxylic acid with KOH I would require an addition of a H+ proton. Would my acetic acid that is being formed as a side product be able to do this?
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I don't understand where you need a proton to do the hydrolysis step. Its correct as written. potassium ethoxide is the byproduct. FYI its not typical to show an ionic bond between K and OH. Usually its K+ OH- just written next to one another, or merely plain KOH.
A small quibble, which has a higher pKa acetic acid or DMAP? That should tell you which one holds that proton from the first step.
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Thanks again for the feedback! I added the proton for my hydrolysis cause I've been learned and when I looked it up I thought you needed to have an acidic environmont to be able to get a carboxylic acid cause if you didn't wouldn't you get an ester with K+ having an ionic bond with O- .
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I take slight issue with the first step. compare the pKa of an alcohol to DMAP, do you think DMAP will deprotonate an alcohol. look up the roll of DMAP in acylation reactions to find out what DMAP does.
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I take slight issue with the first step. compare the pKa of an alcohol to DMAP, do you think DMAP will deprotonate an alcohol. look up the roll of DMAP in acylation reactions to find out what DMAP does.
Thank you again for your feedback! I've looked up DMAP in acylation reactions and have made some adjustments according to the info I've found which you can see in my attachment. The issue now was to create an ester group I require a base as seen in the following link: https://pt.slideshare.net/KianBagheri/fusubtilisin-presentation-2?ref=&smtNoRedir=1
I then found this mechanism ( https://commons.wikimedia.org/wiki/File:DMAP_esterification_mechanism.png )which gives me the same end result but uses the acetic anhydride. I find the mechanism a bit confusing and seems an unlikely reaction to me or is the mechanism that most likely will take place?
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I'm fine with your latest mechanism.
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I'm glad to hear that. Thanks a ton for all the feedback.
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Why wouldn't DMAP just deprotonate the alpha proton in the starting compound, since it's more acidic and eliminate the hydroxyl to form the alkene? Then have KOH come in and hydrolyze the ester to form the acid?
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Why wouldn't DMAP just deprotonate the alpha proton in the starting compound, since it's more acidic and eliminate the hydroxyl to form the alkene? Then have KOH come in and hydrolyze the ester to form the acid?
probably not quite a strong enough base.