# Nucleophilicty and Basicity of OH and F

Why is $\ce{OH-}$ more basic than $\ce{F-}$?

From what I understand, $\ce{O}$ has larger radius so it should be more stable (meaning less basic) than $\ce{F}$. When they form anions, since $\ce{O^{2-}}$ is larger, it can distribute its electrons over larger volume than $\ce{F-}$, meaning $\ce{OH-}$ should be less basic. Is this because of partial negtaive charge on oxygen?

In a polar aprotic solution, why is $\ce{F-}$ a stronger nucleophile than $\ce{I-}$?

I learned that basicity (not nucleophilicity) is about stability. So can this be explained by electron density? Is explaining it with polarizability right?

In a lecture, my teacher said $\ce{RO-}$ is relatively stable because of the high electronegativity of $\ce{O}$.

I think it should be unstable because of high electron density caused by high electronegativity. What's right?

When people say something is stable, are they referring to its acid/base characteristics, or can stability also be referring to nucleophilicity/electrophilicity?

Why is $\ce{OH-}$ more basic than $\ce{F-}$?

To answer this question, let us consider the $\mathrm{p}K_\mathrm{a}$'s of the conjugate acids of these anions: $\ce{H2O}$ and $\ce{HF}$. $$\mathrm{p}K_\mathrm{a}(\ce{H2O})=15.7\ \\ \mathrm{p}K_\mathrm{a}(\ce{HF})=3.17$$

You didn't need me to tell you that $\ce{HF}$ is a stronger acid than water though, but it is -- $10^{12.53}$ times stronger to be exact. This means that this is also how many times stronger $\ce{OH-}$ is than $\ce{F-}$

In a polar aprotic solution, why is $\ce{F-}$ a stronger nucleophile than $\ce{I-}$?

Small, electron-dense ions are always great nucleophiles, except when the solvent crowds the nucleophile with hydrogen bonds, as in the case of $\ce{F-}$. It is so heavily surrounded by solvent molecules in polar protic solvents that it can't make an attack very well. $\ce{I-}$ is not as heavily solvated in these solvents, and it is very easily polarizable, making it a better nucleophile in these solvents.

In polar aprotic solvents, the small $\ce{F-}$ ion can easily slip between solvent molecules and make attacks. Now in comparison, $\ce{I-}$ is a much poorer nucleophile.

In a lecture, my teacher said $\ce{RO-}$ is relatively stable because of the high electronegativity of $\ce{O}$.

Again, we can answer this by looking at $\mathrm{p}K_\mathrm{a}$'s. The $\mathrm{p}K_\mathrm{a}$ of ethanol, for example, is $15.9$! Only $10^{0.2}=1.58$ times weaker an acid than water! That means that ethoxide will deprotonate everything $\ce{OH-}$ will and then a little more, so it's not really that much more unstable.

When people say something is stable, are they referring to its acid/base characteristics, or can stability also be referring to nucleophilicity/electrophilicity?

The two are pretty well related. You have exceptions like how $\ce{F-}$ is a poor nucleophile in water despite being a good base, but as a rule of thumb, I would say people mean acid/base reactivity when they talk about "stability."

• Thank you. it makes sense when you explain it using pKa. but can't you explain another way using movement of e- like I wrote in my question? – NK Yu Mar 8 '16 at 0:52
• Yes, but both $\ce{F-}$ and $\ce{OH- /OR-}$ have unpaired electrons. What is more important is to understand how reactive these electron pairs are. – ringo Mar 8 '16 at 0:54
• and you said "Small, electron-dense ions are always great nucleophiles", but I don't understand. high polarizability means good Nu, thus small, elctron-dense ion should be poor Nu. where am I confused? – NK Yu Mar 8 '16 at 0:54
• Charge density actually makes for a better nucleophile, and $\ce{F-}$ is much smaller than $\ce{I-}$. If you think about it, which will be better at fitting into a molecules structure to force a reaction, something small or something big? – ringo Mar 8 '16 at 1:02
• I actually thought bigger atom would be better Nu. because bigger ones pulls their e- less. If you are saying it's wrong, then where does the polarizability come from? – NK Yu Mar 8 '16 at 1:11