AAMC 5 OChem #120

[BenZq]

---

Members do not see this ad.

To anyone who has access and can help: Question 120 about the first step in the reaction of acetic anhydride with R-NH2, I orginally narrowed this down to B and C, but put B, that the carbonyl oxygen is protonated first. After drawing out the mechanism, I realize that this is wrong, and that it is obviously the attack by nitrogen on the carbonyl carbon first. Are there any glaring clues that could help me solve this problem without having to draw out/ memorize the mechanism?

 

[loveoforganic]

I don't have access - what are the full reaction conditions given? Solvent/acidic conditions?

 

[michaelfranklin]

This question is actually pretty tricky, and I understand why you would choose choice B, but I agree with loveoforganic. The solvent (whether acidic or basic) can sometimes give you a hint. The solvent (R-NH2) is somewhat basic, so you wouldn't expect free H+ ions to be around to protonate the carbonyl oxygen.

Additionally, you know that anhydrides are relatively reactive (only second to acyl chlorides) and that R-NH2 is a good nucleophile (enhanced somewhat by having a methyl group attached).

Nonetheless, I think the biggest give away would be the solvent. Choice B would possible if the solvent was acidic.