Which is more reactive as an electrophile: esters or carboxylic acids?

[FeralisExtremum]

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Comparing carboxylic acids and esters acting as electrophiles (e.g. in nucleophilic acyl substitution): intuitively it seems that esters would be more electron donating than carboxylic acids, and therefore less reactive. But some sources I've read say that esters are more reactive, why is this?

Similarly I often see that OH is listed as more activating than OR in EAS reactions, implying that it is more electron donating. How could this be the case? Isn't -OR a stronger nucleophile than -OH? I would expect the alkyl chain to cause more donation via hyperconjugation.

 

[BYU4you]

Not as sure for the first what you are asking but I'm assuming it is because -OR is a better leaving group than -OH unless I misunderstand the question.

As for the second, the Lone pairs on OR are being shared between the O and Cs on both sides where as OH the lone pairs are more free to be donated in.

 

[FeralisExtremum]

Not as sure for the first what you are asking but I'm assuming it is because -OR is a better leaving group than -OH unless I misunderstand the question.

As for the second, the Lone pairs on OR are being shared between the O and Cs on both sides where as OH the lone pairs are more free to be donated in.

That pretty much answers both, thank you!

 

[DAT Destroyer]

Comparing carboxylic acids and esters acting as electrophiles (e.g. in nucleophilic acyl substitution): intuitively it seems that esters would be more electron donating than carboxylic acids, and therefore less reactive. But some sources I've read say that esters are more reactive, why is this?

Similarly I often see that OH is listed as more activating than OR in EAS reactions, implying that it is more electron donating. How could this be the case? Isn't -OR a stronger nucleophile than -OH? I would expect the alkyl chain to cause more donation via hyperconjugation.

An ester can be an activating or a deactivating group when on a benzene ring, depending on how it is connected. If the O is directly connected, it is activating.....but the carbonyl group adjacent to it removes some electron density. This will of course DECREASE its activating ability. I hope that is clear to you. IF THE CARBONYL is directly bonded to the benzene ring, it will be deactivating. An OR group such as methoxy will donate electron density and NOT have a carbonyl group interfering with the electron-donating ability. Now.....for the tougher question. An OH would be more activating than an OR group This has been confirmed by many studies. . The exact answer here is still debated by the best organic chemists. It turns out that the transition state for the OH pathway is a bit less than an OR group.....perhaps a better orbital interaction occurs whereby a less energetic intermediate is formed. For this exam.....make sure you know that NH2, OH, RO, R, and halogens would be the general order of reactivity. Halogens directing o/p but slightly deactivating.

Hope this helps

Dr. Romano

 

[BYU4you]

Would NRR' be more activating than NH2 or would it be less activating like the OH OR debate?