Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: quantumnumber on September 02, 2014, 12:44:57 PM
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Hi everyone,
I was reading a part of Clayden Organic Chemistry about how to make ketones from esters and I realized there was a non-sense.
It states that, after the addition of the first organometallic molecule to the ester, the product ketone is more reactive than the starting ester, and so, the organometallic would continue reacting until the alcoxide. To get round this problem we can make the starting material more reactive. Then it gives the example I attach... The question is obvious: after the reaction with the acyl chloride, why the Grignard reagent doesn't go on reacting with the ketone formed this time?
It doesn't make any sense for me. Is it a mistake?
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You're right, if you used an excess of the Grignard reagent and threw everything in the flask at once, it would likely continue to attack the ketone. However, in the example you attached, the reaction was probably performed with equimolar amounts of Grignard and starting material, along with careful drop-wise addition of the Grignard reagent to prevent byproduct formation. This way there is little or no leftover Grignard to react.
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Yeah, I think so. With equimolar amounts of Grignard and starting material it must work. Just a lack of accuracy then. Thanks! ;)
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Besides controlling the reaction addition of equivalents as stated above, doing the reaction with very reactive reagents makes things more kinetically favorable. Pushing the starting materials to higher ΔG° makes the reaction more favorable under cold conditions (ΔGforward≠/ΔGreverse≠ ratio gets better). Thus the faster reaction affords less side products because it can be run colder. That is what I think the point of changing the reagents reactivity is here.