I’ve read that Grignard reagants react pretty quickly with alcohols and carboxylic acids to form their respective conjugate bases. I’ve also learned that Grignards can be used for the prepararion of alcohols and carboxylic acids too. How is it possible to perform these reactions if the reagant would also react with the end product?
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$\begingroup$ The alcohol is the last product after acidifing. There is no acidic hydrogen is present before acidifing. $\endgroup$– Mathew MahindaratneCommented Jan 28, 2023 at 20:27
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The addition of a Grignard to a carbonyl doesn't form an alcohol—not directly at least. It forms an alkoxide ion (i.e. the conjugate base of an alcohol).
Only when that reaction is complete, is water (or some other acid, ammonium chloride is used quite regularly) added to the solution. The water protonates the alkoxide ion to form the alcohol, and also destroys any remaining Grignard. This step is usually referred to as 'protic workup' or just 'workup'.
So the Grignard never sees any alcohol.
The same is true of the carboxylic acid formation (assume you're referring to Grignard addition to carbon dioxide). It doesn't form a carboxylic acid straight away, it forms a carboxylate ion.
answered Jan 28, 2023 at 0:02
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$\begingroup$ Ah, but then why is it that alcohols on the Grignard’s reagant themselves DON’T exist in this alkoxide form instead? I was told that Grignard’s reagant will, under normal conditions, react with an alcohol and this is the reason we use protection with TMS. $\endgroup$– AllieCommented Jan 28, 2023 at 0:12
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5$\begingroup$ I'm not entirely sure what you mean. You mean why you can't have a hydroxyl group on a Grignard reagent? You can, it would just be very wasteful, as you'd be using up half of your Grignard reagent to deprotonate the hydroxyl group on the other half first. $\endgroup$ Commented Jan 28, 2023 at 0:18