Why does larger size = weaker base?

[MTD52]

F > Cl > Br > I

Why is a smaller molecule a better base? Wouldn't a smaller atom have a shorter distance to electrons and therefore a stronger pull towards them? That would mean that they are less electron donating, and it would be the opposite of what is really correct.

Explain?

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[mx41]

Remember the rule: the weaker the acid, the stronger the conjugate base?

HF is a weak acid, its not among the 7 strong acids. Therefore, its conjugate base is very strong.
HI is a very strong acid (The larger the molar mass the stronger it is), so its conjugate base will be weak.

 

[MTD52]

Okay, that makes sense. But why does what I said not make sense?

 

[ohdearhopeipass]

a larger molecule= weaker base because it can stabilize a charge better, in other words, when a larger molecule loses/gains an electron, the charge is spread out better, and the molecule is less reactive, thus it does not react well as a base. a strong base is a very reactive compound that donates electrons

 

[hunterpostbacst]

See here.
Your explanation is basically right, but only in a protic solven

In a protic solvent, large atoms = better nucleophiles as they can shed their solvent molecules and are more polarizable. (polarity is a matter) So, F<Cl<Br<I is right.

But, in aprotic solvents, basicity is a matter as they are naked. So, F>Cl>Br>I is right in an aprotic ones.

 

[miedvied]

F > Cl > Br > I

Why is a smaller molecule a better base? Wouldn't a smaller atom have a shorter distance to electrons and therefore a stronger pull towards them? That would mean that they are less electron donating, and it would be the opposite of what is really correct.

Explain?

For one thing, think of charge density. Fluorine is just as negative as iodine - but compacted into a much smaller area. In physics these are equivalent since you tend to use particle models, but in reality it makes a difference.

Second, longer bonds tend to be weaker bonds. Iodine, relative to fluorine, is going to form weaker/longer bonds.

Also, what everyone else said, more or less. The other guy gave you order of nucleophilicity in protic/aprotic solvents, which was correct, except let's not mistake basicity with nucleophilicity (i.e., tert-butoxide is an awesome base, but regardless of solvent it's going to be a **** nucleophile because of steric hindrance). They're correlated, but not equivalent.

 

[MTD52]

For one thing, think of charge density. Fluorine is just as negative as iodine - but compacted into a much smaller area. In physics these are equivalent since you tend to use particle models, but in reality it makes a difference.

Second, longer bonds tend to be weaker bonds. Iodine, relative to fluorine, is going to form weaker/longer bonds.

Right, so if Iodine has weaker bonds, wouldn't it be more likely to give up it's electrons, making it a stronger base?

 

[DoctaWalnuts]

Right, so if Iodine has weaker bonds, wouldn't it be more likely to give up it's electrons, making it a stronger base?

Bases do not give up electrons. Bases are proton (electron) acceptors. Acids give up electrons, thus the larger the better (less pull from the nucleus).

Nucleophiles and bases are very similar except, A larger nucleophile is stronger. That is something that contradicts bases and you should be aware of this.

 

[MTD52]

Bases do not give up electrons. Bases are proton (electron) acceptors. Acids give up electrons, thus the larger the better (less pull from the nucleus).

Nucleophiles and bases are very similar except, A larger nucleophile is stronger. That is something that contradicts bases and you should be aware of this.

According to the Lewis definition, bases are electron pair donors

 

[DoctaWalnuts]

According to the Lewis definition, bases are electron pair donors

thank you for catching that. I need to go back and figure out what I don't understand as well.

 

[MTD52]

Can anybody else explain this? I still don't see why a smaller molecule would be more willing to give up electrons?

 

[Extirpator]

Smaller atoms are less stable and thus want to get rid of their charge/share their charge, in order to become more stable.

While larger atoms, having a larger e- cloud, have better charge delocalization and thus do not want to get rid of their electrons as much as smaller atoms.

Someone else already said this, but I figured I would restate it and see if it helps your understanding.

 

[MTD52]

Smaller atoms are less stable and thus want to get rid of their charge/share their charge, in order to become more stable.

While larger atoms, having a larger e- cloud, have better charge delocalization and thus do not want to get rid of their electrons as much as smaller atoms.

Someone else already said this, but I figured I would restate it and see if it helps your understanding.

Yeah I didn't catch that up there before. Makes perfect sense. Thanks a lot

 

[ttran9229]

F > Cl > Br > I

Why is a smaller molecule a better base? Wouldn't a smaller atom have a shorter distance to electrons and therefore a stronger pull towards them? That would mean that they are less electron donating, and it would be the opposite of what is really correct.

Explain?

Your comments partially explain the phenomenon. First we need to review the nature of the acidic proton and bond strength. The more polar the molecule is, the more the hydrogen behaves like an acidic proton. This would suggest that HF would be the strongest acid, but if we review the bond strength, it tells us something different. As you have already mentioned, HF bond strength is quite strong due to atomic size, so HF does not fully disassociate in an aqueous solution which diminishes its acidic strength. One can empirically determine this by determining or looking up the Ksp.

 

[nze82]

Can anybody else explain this? I still don't see why a smaller molecule would be more willing to give up electrons?

I'm not sure if this is the correct explanation or not, but here's how I would justify this for myself.
Let's focus on the role of the basic entity in Elimination Reactions.
In such reactions the base is essentially abstracting a proton, which is H+. Now, all the basic species you mentioned have a -1 charge. However, they differ in size. The smaller they become the greater the charge density will be, and the greater the density of the negative charge, the more easily the base can abstract the positively charged proton. Hence, smaller species are stronger bases (at least when we consider their role in Elimination Reactions).