Nucleophile Strength

[YYC]

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Hey all,

Knowing that during normal condition (neither in protic or aprotic solvent) a nucleophile's strength is dependent upon its size(atomic radius) and eletronegativity. Such that the smaller the nucleophile and the less electronegative the nucleophile makes the nucleophile stronger (i.e. more reactive).

With that been said, unless what I wrote above is wrong, which of the following is a stronger nucleophile and why? Is it H_2O or H_2S?

The textbook which I used gave the answer that H_2S is a stronger nucleophile. Does that mean electronegativity dominates over size in determining nucleophile' strength such that sulfur, although larger than oxygen, is a better nucleophile for it's less electronegative than oxygen?

 

[aqz]

You're wording it wrong.

Sulfur is a better nucleophile BOTH because it's a -larger- atom and -less electronegative- than oxygen.

Being nucleophilic directly translates to "likes nucleus", meaning you'd want make it easy to give away your electrons. So larger radius and less electronegativity makes it easier to lose them.

 

[YYC]

I see, so it is my wording! So in the case of I- and Br- and Cl- ions as nucleophiles, I- would be better nucleophile because it has largest radius (more shielding allow to donate e-) and also less electronegativity (less force to prevent it from donating)?

 

[412smb412]

I see, so it is my wording! So in the case of I- and Br- and Cl- ions as nucleophiles, I- would be better nucleophile because it has largest radius (more shielding allow to donate e-) and also less electronegativity (less force to prevent it from donating)?

yes the I- is the best nucleophile. but, elements in a group have the same shielding...

for example if u compare S-, O-, F- the best nucleophile is s then o then f. if u compare halogens to each other, again the least electronegative is the better nucleophile. but compared to other elements with the same charge they will not be as good due to stability and therefore not as reactive. if i were u just go with least electronegative = best nucleophile (assuming there is negative charge etc)

 

[YYC]

Okay, I see what you are saying. And my question strictly refers to the comparison between those 3 halogens so I- would be stronger nu- out of the 3.

So I just wanted to double check about nucleophile strength and please correct me if I'm wrong again this time:

-Normal condition (neither in protic nor aprotic solvent): smaller EN and greater atomic radius = stronger nu-

-Aprotic solvents: smaller EN and smaller atomic radaius = stronger nu-

-Protic solvents: smaller EN and greater atomic radius = stronger nu-

 

[412smb412]

Okay, I see what you are saying. And my question strictly refers to the comparison between those 3 halogens so I- would be stronger nu- out of the 3.

So I just wanted to double check about nucleophile strength and please correct me if I'm wrong again this time:

-Normal condition (neither in protic nor aprotic solvent): smaller EN and greater atomic radius = stronger nu-

-Aprotic solvents: smaller EN and smaller atomic radaius = stronger nu-

-Protic solvents: smaller EN and greater atomic radius = stronger nu-

as electronegativity DECREASES, the size INCREASES therefore chose one or the other its the same either way (you can't have the electronegativity decreasing and the size decreasing at the same time, also don't pay attention to the solvent). in ANY condition you have the molecule/atom with the negative charge and lowest electronegativity will be the best nucleophile.

 

[aqz]

I don't know about the aprotic/protic rule, but I never had to use it anyways.

But I'd like to add that bigger radius is also better because smaller nucs often get "hidden" by the solvent around it and have a harder time to "attack".

And no...elements in a group (that's column) do NOT have the same shielding. As you go down the group, you get more electron shells and more shielding, meaning less electronegative.

 

[412smb412]

I don't know about the aprotic/protic rule, but I never had to use it anyways.

But I'd like to add that bigger radius is also better because smaller nucs often get "hidden" by the solvent around it and have a harder time to "attack".

And no...elements in a group (that's column) do NOT have the same shielding. As you go down the group, you get more electron shells and more shielding, meaning less electronegative.

wahts teh relationship btwn effective nuclear charge and shielding?

 

[YYC]

The last two trends for the protic and aprotic solvents were directly from Chad's video.

He said something along the line that "water, for example, is a protic solvent that stabilizes smaller ion a lot making them a lot weaker, and stabilizes large ion a little making them just a little weaker." The ion is the nuceophile with a negatice charge in his example. So weaker EN with no doubt makes a stronger nucleophile, but size does make a difference between nucleophiles that are in protic and nucleophiles that are in aprotic solvents.

 

[412smb412]

The last two trends for the protic and aprotic solvents were directly from Chad's video.

He said something along the line that "water, for example, is a protic solvent that stabilizes smaller ion a lot making them a lot weaker, and stabilizes large ion a little making them just a little weaker." The ion is the nuceophile with a negatice charge in his example. So weaker EN with no doubt makes a stronger nucleophile, but size does make a difference between nucleophiles that are in protic and nucleophiles that are in aprotic solvents.

the solvent is for helping a rxn go sn1 or sn2. but to determine which nucleophile is best u probably will never see them tell u the solvent in a question that asks u to determine which rxn would proceed the fastest... do some qvault and dat destroyer to get used to how they phrase the questions.

 

[aqz]

the solvent is for helping a rxn go sn1 or sn2. but to determine which nucleophile is best u probably will never see them tell u the solvent in a question that asks u to determine which rxn would proceed the fastest... do some qvault and dat destroyer to get used to how they phrase the questions.

Yeah, that's what I thought for the solvents.

Polar aprotic = SN2

Polar protic = SN1

Totally forgot the reasons though.

But asides from that, all you need to know is that less electronegative = more nucleophilic and bigger is better. In fact, if they wanted to test if you knew that size mattered, they'd probably only test you on the halogens...e.g. "which of the following will perform the following SN2 reaction the fastest?" choices are Cl, Br, F, I, and you're going to pick I.