leaving group strength question in TBR

[Oh_Gee]

so in TBR Ochem1 ch3 passage 9 question 59, it asks

9. Which of the following is the MOST stable leaving group?
A. HCN pka 9.1
B. CN-
C. H3CCH2S-
D. H3CCH2SH pka 10.5

in the passage it says
This implies that a good leaving group is a weak base. The weaker the compound is as a base, the stronger its conjugate acid is. As acid strength increases, the pKa of the acid decreases. This ultimately means that the lower the pKa of the conjugate acid of the leaving group, the more reactive the electrophile.

the answer is D but I picked B because I thought that the best leaving group is a weak base which means that the electrophile should be a stronger acid (lower pka)

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[Czarcasm]

so in TBR Ochem1 ch3 passage 9 question 59, it asks

9. Which of the following is the MOST stable leaving group?
A. HCN pka 9.1
B. CN-
C. H3CCH2S-
D. H3CCH2SH pka 10.5

in the passage it says
This implies that a good leaving group is a weak base. The weaker the compound is as a base, the stronger its conjugate acid is. As acid strength increases, the pKa of the acid decreases. This ultimately means that the lower the pKa of the conjugate acid of the leaving group, the more reactive the electrophile.

the answer is D but I picked C because I thought that the best leaving group is usually charged when it leaves. is this not true?

I agree with you. Given the pKa's, it's unusual that the answer is D. Considering the pKa of HCN is lower than the pKa for ethanethiol in choice D, would imply that HCN is the stronger, more reactive acid. As the stronger, more reactive acid, this would imply that it's conjugate base is more stable by comparison (and therefore a better leaving group). I would have picked B, based on that reasoning.

 

[Oh_Gee]

so you think it's a mistake on TBR's part?

 

[Concrete Jungle]

It is D. Why could a charged leaving group be more stable than an uncharged one??? Choice A is just a F#CK YOU answer, it cant be a leaving group.

Edit: In case you don't see it add a -H to each answer (expect A) to find the conjugate acid...

 

[Czarcasm]

It is D. Why could a charged leaving group be more stable than an uncharged one??? Choice A is just a F#CK YOU answer, it cant be a leaving group.

Edit: In case you don't see it add a -H to each answer (expect A) to find the conjugate acid...

We realize choice B and C are conjugate bases of A and D. Where the confusion was, was relative stability of the leaving groups in choice A and D after they leave. Sure, they'd have to go from a positive to neutral charge (atleast in Choice D) and that's more favorable than say a negative charge. But if they were say, in a protic solvent, they would still be relative reactive as weak acids and dissociate to the more favorable conjugate base form. I guess the more appropriate reason why choice D is the better answer is because (as weak acids) the predominate form is the neutral form and so, I suppose having the higher pKa (more neutral molecule, less dissociaton) is favored over a solution entirely of conjugate base.

This is just one of those scenarios where over-thinking it does more harm than good. Probably best to just keep it simple and focus on the charges.

 

[Concrete Jungle]

All you have to do, like I said, is look at the conjugate acid of EACH choice. Choice A is non-realistic, you can't have 5 bonds to carbon. Choice D is CH3CH2SH2+ while choice B and C are neutral. Even in a protic solvent, CH3CH2SH is not readily going to donate its lone pair to reform its conjugate acid while the two would relatively do so.