Strength of nucleophile

[deleted647690]

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Why is HS- a stronger nucleophile than H3CS-? I would think the H3C would act as an EDG and therefore push electron density onto the S, therefore making it more able to give up electrons as a nucleophile

 

[beOncogene]

Why is HS- a stronger nucleophile than H3CS-? I would think the H3C would act as an EDG and therefore push electron density onto the S, therefore making it more able to give up electrons as a nucleophile

The strength of a nucleophile determines how fast it would react. So the stronger the nucleophile, the faster it would react in a reaction. The reason why HS- is a stronger nucleophile is because it experiences less of a steric hindrance than H3CS- (react faster).

 

[deleted647690]

The strength of a nucleophile determines how fast it would react. So the stronger the nucleophile, the faster it would react in a reaction. The reason why HS- is a stronger nucleophile is because it experiences less of a steric hindrance than H3CS- (react faster).

What is incorrect about seeing the methyl group as an EDG that would increase available electrons for S donate?

 

[aldol16]

What is incorrect about seeing the methyl group as an EDG that would increase available electrons for S donate?

Nothing particularly wrong but it's a weak effect. To see that, you have to understand how the methyl group pushes electron density, aka hyperconjugation. So the methyl carbon usually donates electron density via an MO pushing towards an empty p orbital of the adjacent atom. With carbon and sulfur, the sulfur p orbital is one energy level higher than the carbon orbitals and thus the overlap is not too good. So you wouldn't expect much delocalization via hyperconjugation here.

 

[deleted647690]

Nothing particularly wrong but it's a weak effect. To see that, you have to understand how the methyl group pushes electron density, aka hyperconjugation. So the methyl carbon usually donates electron density via an MO pushing towards an empty p orbital of the adjacent atom. With carbon and sulfur, the sulfur p orbital is one energy level higher than the carbon orbitals and thus the overlap is not too good. So you wouldn't expect much delocalization via hyperconjugation here.

Thank you, that seems pretty detailed. So what allows you to determine that the steric hindrance effect is greater? Is that something I should know? I may have to look up hyperconjugation to try and understand this

 

[wizzed101]

What is incorrect about seeing the methyl group as an EDG that would increase available electrons for S donate?

It's wrong because the methyl group is not donating anything to S. Answer this: what makes methyl an "electron donating group?"

That aside, -SH is a better nucleophile due to the same reason that -OH is a better nucleophile than an ether.

 

[deleted647690]

It's wrong because the methyl group is not donating anything to S. Answer this: what makes methyl an "electron donating group?"

That aside, -SH is a better nucleophile due to the same reason that -OH is a better nucleophile than an ether.

I thought I read in Klein's Orgo as a second language that alkyl groups were electron donating

http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch12/ch12-8b.html

 

[wizzed101]

But why? Why don't you question that but question that fact the -SH is a better nucleophile?

Electron donating to what?

 

[deleted647690]

But why? Why don't you question that but question that fact the -SH is a better nucleophile?

Electron donating to what?

I initially thought, as aldol mentioned above, that through hyperconjugation, CH3 pushes electron density onto the SH

 

[aldol16]

It's wrong because the methyl group is not donating anything to S. Answer this: what makes methyl an "electron donating group?"

I initially thought, as aldol mentioned above, that through hyperconjugation, CH3 pushes electron density onto the SH

It does donate, but this is a very weak effect because of poor orbital overlap. In fact, hyperconjugation between carbon and sulfur commonly goes the opposite way (negative hyperconjugation) although this effect is also weak for the same reason. If you're interested, you can see how weak it is in action here: http://pubs.acs.org/doi/abs/10.1021/jo9719463?journalCode=joceah

 

[wizzed101]

I mean like there is push and pull. The contribution of CH3 is to the resulting cation after the nucleophilic attack if it happens at all. That is the push or product stablizing effect. I don't think it's right to say that CH3 contribute anything to the ability of S to do a nucleophilic attack. It is applicable to O because the electronegativity of O is much higher than C while S has almost the same electronegativity as C. Not to mention S is much larger than C and like you said this would lead to reduced hyperconjugation. But in the most simple terms, I think that just by looking at electronegativity is enough.