F>Cl>Br>I in aprotic and protic

[gegogi]

Okay, I know F is the most electronegative. F>Cl>Br>I. But, HF is weak acid because its bond length is short due to the smallest size of F. HF is hard to give off H, so it's weak acid. Therefore, the conjugate base of weak acid is strong base, so F is strong base. So, bond length(size) beats electronegativity?

In aprotic, kaplan says F is strong base and I is weak base as I stated. In protic, F is less nucleophile and I is better nucleophile. Anyone can reason this?😕

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[MNova]

In aprotic solvents, you don't have to worry about solvation of the nucleophile. Solvation impedes nucleophilicity. Therefore, the strongest base wins out (F- in this case).

In protic solvents, a solvent shell is created around the nucleophiles, hindering their ability to attack. I-, which is the largest of the halides, is less heavily solvated (since it's larger and it's charge is not as concentrated) and therefore, a better nucleophile in protic solvents.

Solvation refers to a protic solvent "donating" an H+. The H+s are attracted to the negative nucleophiles and surround them, forming hydrogen bonds. Since the hydrogens strongly interact with the anionic nucleophiles, they reduce the efficacy of the nucleophiles.

Hope this helps!

 

[fsddsms]

Okay, I know F is the most electronegative. F>Cl>Br>I. But, HF is weak acid because its bond length is short due to the smallest size of F. HF is hard to give off H, so it's weak acid. Therefore, the conjugate base of weak acid is strong base, so F is strong base. So, bond length(size) beats electronegativity?

In aprotic, kaplan says F is strong base and I is weak base as I stated. In protic, F is less nucleophile and I is better nucleophile. Anyone can reason this?😕

I- is a better nucleophile in polar protic solvents (like H20) because it has a larger radius and size compared to F.
In polar protic solvents, I- can better shed its water of hydration (partial positive Hs surrounding that negative charge on I-). Think of it this way: Imagine I surrounded by a sphere of electrons while F is a smaller sphere. I has more area to shed off the attractive dipole interactions between the electrons and the surrounding polar protic medium. Because it has more instances where it can "expose that negative charge" compared to F-, it is a better nucleophile than F-. HF = weak acid/F- = strong base vs HI = strong acid/ I- weak base.

In polar APROTIC solvents, there aren't dipole molecules trying to interact with the charges on F- and I-. In this aprotic scenario, nucleophilicity is based straight up on basicity. F- will be a better nucleophile simply because it is a stronger base.

In regard to acidity, HF is a weak acid because although it is very electronegative, the covalent bond formed between HF is strong in that it doesnt want to lose that H in comparison to the other halides.