I thought I understood this, but don't: Acidity vs Nucleophilicity

[chaser0]

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Hey guys, I need some help understanding this.

HF is less acidic than HCl, because they are in the same column and hence should be judged on size. Cl is a larger atom, and hence a better acid.
Acidity has a very simple trend to follow: Columns- size, Rows-EN.


In terms of Nucleophilicity, what exactly is the trend?
It is not size, because Cl- is a better nucleophile than F-. It just seems to be straight up Electronegativity and ignores size? It makes sense because it seems to be FONClBrISCH backwards in a protic solution.


They also say that smaller nucleophiles are better than larger ones. (in reaction terms, I understand as this allows easier access to the target). But I don't see this reflected anywhere in the
SH CN I OR OH BR NH3 C6H5........
It just seems to be near completely EN, aside from a few switches due to H-bonding.

 

[Buttafuoco]

At least how Kaplan teaches it, in a polar, aprotic solvent, nucleophilicity = basicity, so F- would be the strongest nucleophile. In a polar protic solvent, the strongest bases can't free themselves of protons so they can't attack. Therefore, the trend is reversed and I- is the strongest nucleophile.

 

[chaser0]

oh my god, this is hurting my chest.

I see the disparity in the solvents.
If nucleophilicity=basicity, wouldn't CN- be the strongest nucleophile? Considering that N is less electronegative than F-, and hence a stronger base.

 

[MedPR]

Hey guys, I need some help understanding this.

HF is less acidic than HCl, because they are in the same column and hence should be judged on size. Cl is a larger atom, and hence a better acid.
Acidity has a very simple trend to follow: Columns- size, Rows-EN.


In terms of Nucleophilicity, what exactly is the trend?
It is not size, because Cl- is a better nucleophile than F-. It just seems to be straight up Electronegativity and ignores size? It makes sense because it seems to be FONClBrISCH backwards in a protic solution.


They also say that smaller nucleophiles are better than larger ones. (in reaction terms, I understand as this allows easier access to the target). But I don't see this reflected anywhere in the
SH CN I OR OH BR NH3 C6H5........
It just seems to be near completely EN, aside from a few switches due to H-bonding.

Be careful about that last part. Smaller molecules are better nucleophiles than larger molecules, but larger atoms are better nucleophiles than smaller atoms.

For example, methoxide is a better nucleophile than tertbutoxide. Methoxide is smaller than tert-butoxide.

Iodide is a better nucleophile than all of the halogens, but Iodide is bigger than all of the halogens.

 

[MedPR]

oh my god, this is hurting my chest.

I see the disparity in the solvents.
If nucleophilicity=basicity, wouldn't CN- be the strongest nucleophile? Considering that N is less electronegative than F-, and hence a stronger base.

I don't understand what you all mean by nucleophilicity = basicity. There is no direct relationship.

 

[chaser0]

I figured it out~
After hours of research using EK and BR together.

Nucleophilicity increases with atomic radius, going lower left on the periodic table.
Larger atoms (trend of lower left) make better nucleophiles because they are easily polarizable (No one knows why this helps though). This is a stable relationship in aprotic polar solvents, but a few of the stronger ones (OR-, OH-) are weakened in protic polar solvents.

So, it follows atomic radius better than basicity.

 

[chaser0]

I don't understand what you all mean by nucleophilicity = basicity. There is no direct relationship.

yeah, I don't think there's anything there either.

I was just repeating what Buttafuco said.

 

[Jarudy]

I don't understand what you all mean by nucleophilicity = basicity. There is no direct relationship.

Nucleophiles = Lewis bases

 

[Jarudy]

I figured it out~
After hours of research using EK and BR together.

Nucleophilicity increases with atomic radius, going lower left on the periodic table.
Larger atoms (trend of lower left) make better nucleophiles because they are easily polarizable (No one knows why this helps though).

Look at transition states to see why polarizable molecules make better nucleophiles. If a transition state is lower in energy (Sn2 with partial bonds being formed with polarizable molecule = lower energy) then by definition said molecule is a better nucleophile.

 

[ipodtouch]

I would say just memorize this:

SH->NC->I->OR->OH->Br->NH3>C6H5O->CH3CO2->Cl->F->ROH>H2O
in a protic polar solvent

and OR- and OH- jump to the front in aprotic polar solvents.



Along with your relationship to ionic size (which also happens to be the decreasing Electronegativity trend), you should be set.

For MCAT purposes, this is literally ALL you need to know.

 

[hellocubed]

I would say just memorize this:

SH->NC->I->OR->OH->Br->NH3>C6H5O->CH3CO2->Cl->F->ROH>H2O
in a protic polar solvent

and OR- and OH- jump to the front in aprotic polar solvents.



Along with your relationship to ionic size (which also happens to be the decreasing Electronegativity trend), you should be set.

For MCAT purposes, this is literally ALL you need to know.

This is quite useful.

Also note the following trends:


1.) Acidity Increases with
Increasing atomic Radius
Increasing EN

2.) Basicity Increases with
Decreasing atomic Radius
Decreasing EN

3.) Nucleophilicity Increases with
Increasing atomic Radius
Decreasing EN


The nucleophilicity trend is sort of a hybrid of the acid and basicity....
The better base is not always the better nuclephile.

 

[pfaction]

http://forums.studentdoctor.net/showthread.php?t=902810

Also made a topic about this 🙂 This one seems a lot better though.

 

[circulus vitios]

3.) Nucleophilicity Increases with
Increasing atomic Radius

Don't you mean decreasing atomic radius? A smaller atomic radius means a more dense electron cloud, which is more likely to attack an electrophile.

 

[hellocubed]

Don't you mean decreasing atomic radius? A smaller atomic radius means a more dense electron cloud, which is more likely to attack an electrophile.

This is what I thought at first, and what the OP has obviously been struggling with and later came to my conclusion.


What you described is a property of basicity.
I do not know why this differs in nucleophiles compared to bases, I would have thought it would behave the same way. But increased polarizability is inversely proportional to basicity, and directly proportional to nucleophilic power.

--> Larger size= weaker base, stronger acid, stronger nucleophile.

 

[circulus vitios]

This is what I thought at first, and what the OP has obviously been struggling with and later came to my conclusion.


What you described is a property of basicity.
I do not know why this differs in nucleophiles compared to bases, I would have thought it would behave the same way. But increased polarizability is inversely proportional to basicity, and directly proportional to nucleophilic power.

--> Larger size= weaker base, stronger acid, stronger nucleophile.

http://faculty.southseattle.edu/sen...files/How to determine a good nucleophile.pdf

Atomic radius (size): An atom is driven to share an electron pair in order to decrease its charge density (electron density per unit of surface area). Everything else being equal, smaller atoms have a higher charge density and thus a stronger drive to form new covalent bonds. Therefore, smaller atoms will be better nucleophiles.

We don’t generally see this trend with the halides (I- > Br- > Cl- > F-) in a polar protic solvent. This is because the solvent affects the nucleophilicity. I- is not as strongly solvated in a polar protic solvent as F- is and is therefore a stronger nucleophile even though it is a weaker base. In an aprotic solvent, the smaller atom will be the stronger

http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch08/ch8-5.html

1. If one is comparing the same central atom, higher electron density will increase the nucleophilicity,
   e.g. an anion will be a better Nu (lone pair donor) than a neutral atom such as HO- > H2O. This is the same order as for basicity.

 

[hellocubed]

We don’t generally see this trend with the halides (I- > Br- > Cl- > F-) in a polar protic solvent. This is because the solvent affects the nucleophilicity. I- is not as strongly solvated in a polar protic solvent as F- is and is therefore a stronger nucleophile even though it is a weaker base. In an aprotic solvent, the smaller atom will be the stronger

So I did some more research in the past hour,
There is a difference between aprotic/polar, protic/polar, and aprotic/non polar solvents.


http://forums.studentdoctor.net/showthread.php?t=902810

This topic gives more information on it. I made a post at the end...

 

[syoung]

"Within a group in the periodic table, increasing polarisation of the nucleophile as you go down a group enhances the ability to form the new C-X bond and increases the nucleophilicity, so I- > Br- > Cl- > F-. The electron density of larger atoms is more readily distorted i.e. polarised, since the electrons are further from the nucleus."

 

[hellocubed]

"Within a group in the periodic table, increasing polarisation of the nucleophile as you go down a group enhances the ability to form the new C-X bond and increases the nucleophilicity, so I- > Br- > Cl- > F-. The electron density of larger atoms is more readily distorted i.e. polarised, since the electrons are further from the nucleus."

Yess, this is definitely the case.

But it seems, from consensus of the board, that the trend actually changes for aprotic/non-polar solvents.



This will be all the nucleophilic trends compiled here:

1.) Acidity Increases with
Increasing atomic Radius
Increasing EN


2.) Basicity Increases with
Decreasing atomic Radius
Decreasing EN


3.) Nucleophilicity of POLAR solvent Increases with
Increasing atomic Radius
Decreasing EN

In protic:
SH->NC->I->OR->OH->Br->NH3>C6H5O->CH3CO2->Cl->F->ROH>H2O

In aprotic:
OR->OH->SH->NC->I->Br->NH3>C6H5O->CH3CO2->Cl->F->ROH>H2O


4.) 3.) Nucleophilicity of Non-POLAR solvent Increases with
Decreasing atomic Radius
Decreasing EN
=basicity