TBR o-chem section 1 passage XIII Question 86

[badmintondr]

It says that at a pH of 9.0 , which compound would be the most reactive nucleophile?

the answer states that as ph increases, the species is longer cationic

Should I be assuming that the de-acylated ester hydroxide group becomes protonated at a pH of 9 thus making the molecule neutral and not cationic..?
Above statement may be totally wrong so please correct me, thanks.

Thanks again

---

Members don't see this ad.

 

[BerkReviewTeach]

It says that at a pH of 9.0 , which compound would be the most reactive nucleophile?

the answer states that as ph increases, the species is longer cationic

Should I be assuming that the de-acylated ester hydroxide group becomes protonated at a pH of 9 thus making the molecule neutral and not cationic..?
Above statement may be totally wrong so please correct me, thanks.

Thanks again

This is one of those those wtf questions that you scratch your head on and wonder how the heck it was solved. You need to look at the three graphs and note that the greatest rate is found with the amine at a pH above 9, so the amine above pH = 9.0 must be the best nucleophile of the three choices. It can be reasoned that the pKa of a protonated amine is around 9, so when the pH is raised above 9.0, then the amine is deprotonated and therefore neutral. Amines are better nuclophiles than alkoxides and according to the graphs alkyl sulfides as well. This question requires that you use the graphs in the passage.