Thiols: stronger acids AND nucleophiles?

[pfaction]

Trends of acids: within same group, the further down you go, the stronger.
Nucleophiles: same trend.
So I compared an alcohol and thiol, and I got that thiols are stronger acids AND nucleophiles? 😕

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[gettheleadout]

S- is less electronegative than O-, but more nucleophilic due to its larger size. So yes, thiols (RSH) are more acidic and more nucleophilic than alcohols (ROH.)

 

[pfaction]

Cool, just wanted to make sure. Thanks.

 

[SaintJude]

Why does this matter? How do you think this would play out in a question?

I'm not being skeptical...I just don't understand the significance

 

[pfaction]

TPR asked a question regarding nucleophilicity of a thiol and an alcohol as a free standing during one of their tests, if I remember. Also, I associated bases with nucleophiles (lewis bases) so I wasn't sure if this was possible. But I guess since oxygen is definitely acidic and nucleophilic I should have realized sulfur is as well.

 

[SaintJude]

S- is less electronegative than O-, but more nucleophilic due to its larger size.

I understand that a nucleophile is an electron-rich species (with at least one free lone pair of electron) that is attracted to positive charges. So I understand O and S can both fit the bill.

But what's size got to do with nucleophicity?

 

[pfaction]

The larger the atom, the more diffuse the negative charge can be. That's what makes a strong acid good; the ability to spread out its neg charge.

So theoretically, larger sizes will be worse nucleophiles because nucleophiles are negative atoms, the larger the atom the less neg it'll be. But I think what I said is really wrong based on nucleophile trends.

Does anyone want to help with the anomaly? Example:

HI vs HCl: HI is the stronger acid because it stabilizes the negative charge better, due to a larger atomic radius. Therefore, we can say that chlorine would have a greater negative charge on its surface.
I vs Cl: I is the stronger nucleophile...but nucleophiles have large negatives?

 

[gettheleadout]

I understand that a nucleophile is an electron-rich species (with at least one free lone pair of electron) that is attracted to positive charges. So I understand O and S can both fit the bill.

But what's size got to do with nucleophicity?

The larger the atom, the more diffuse the negative charge can be. That's what makes a strong acid good; the ability to spread out its neg charge.

Exactly. This is why HF is a weak acid despite the high electronegativity of fluorine. The small area over which the negative charge in F- can be distributed is less stable than with a large atom (as in HBr, HCl, and HI.)

So theoretically, larger sizes will be worse nucleophiles because nucleophiles are negative atoms, the larger the atom the less neg it'll be. But I think what I said is really wrong based on nucleophile trends.

The overall negative charge of a larger atom won't be different compared to a small one (e.g. I- vs F-) but the size of the electron cloud is relevant for bonding (as in nucleophilic attack.) The trend of increasing size -> increasing nucleophilicity can be reversed in aprotic solvents, but here's how I learned it: A smaller nucleophile like F- is going to be thoroughly surrounded during solvation by a protic solvent like water, blocking the lone pairs from easily attacking an electrophile. A larger nucleophile like I- exists less hindered by solvation by water molecules, allowing for easier attack by its lone pairs. Another explanation is that a larger electron cloud is more polarizable, allowing for lower energy transition state during nuc. attack.

 

[SaintJude]

Edit: Oops! didn't see getheleadout's post!

Ok, just referred to my Kaplan book and it had a section called "size and polarizability". wonderful.

Turns out, the trends are different for protic (e.g. SN1) and aprotic (e.g. SN2) solvents

in a protic solvent large atoms tend to better nucleophiles b/c they can shed the solvating protons surrounding them and are more polarizable.

This make sense. A very small, electronegative atom such as fluorine will pick up a proton the second it could and thus wouldn't even make it to the electrophile. But a bigger nucleophile, such as a big old iodide atom can picks up a H+ proton it can then shift its electrons around, such that it can still "attract: an electrophile.
So in protic solution solvents, the trend is is CN- > I- > RO-> HO -> halogens....
Basically electron species that have more space to rearrange their electrons while accommodating proton-pick ups are stronger nucleophiles

But in APROTIC solvents:
There is no concern about size & polarizability. So nucleophicity will tus directly correlate with base strength (in sense of Bronsted def: tendency do accept protons) so F- > Cl - > Br - > I>

Kaplan noted this is the opposite of the trend in protic solvents.

 

[pfaction]

Great explanation guys, just wanted to sort that out. Thanks!!

TBR does not mention any difference in aprotic vs protic solvents.

See, St. Jude? Good thing I asked!

 

[hellocubed]

Hey guys,
This is a great topic, but I believe it requires some modification according to BR.

BR states there are 3 different solvents:
Protic/Polar SN1
Aprotic/polar SN2
Aprotic/Nonpolar Elimination reactions


Basically, for Protic/Polar AND Aprotic/polar, the trend is still dependent on size.
But the strength of the most powerful nucleophiles are significantly reduced in the protic.

Protic/Polar
SH- > CN- > I- > RO- > HO- >.....

Aprotic/polar
RO- > HO- > SH- > CN- > I- >


It is in the Aprotic/Non polar solvents that has the nucleophilicity=basicity parallel trend.
For E reactions...

 

[chiddler]

Alright here's a question that I got wrong following the information from this, and previous, threads. I'm sick of seeing questions about this topic too, so help me and lets get it over with!

Which of the following correctly ranks the halides in order of increasing rate of addition to 3-hexene in a non-polar aprotic solvent?

A. HCl < HBr < HI
B. HCl < HI < HBr
C. HBr < HCl < HI
D. HI < HBr < HCl

Answer: A.

As was written above, a polar protic solvent makes it easy for small molecules like F to be solvated and thus the F- gets stuck in solution. It is a poor nucleophile.

Then if we switch it up, nonpolar aprotic, we should get the reverse because F is not as easily solvated so it can approach the electrophile more easily.

answer says otherwise!

 

[hellocubed]

Alright here's a question that I got wrong following the information from this, and previous, threads. I'm sick of seeing questions about this topic too, so help me and lets get it over with!

Which of the following correctly ranks the halides in order of increasing rate of addition to 3-hexene in a non-polar aprotic solvent?

A. HCl < HBr < HI
B. HCl < HI < HBr
C. HBr < HCl < HI
D. HI < HBr < HCl

Answer: A.

As was written above, a polar protic solvent makes it easy for small molecules like F to be solvated and thus the F- gets stuck in solution. It is a poor nucleophile.

Then if we switch it up, nonpolar aprotic, we should get the reverse because F is not as easily solvated so it can approach the electrophile more easily.

answer says otherwise!

Ah, i see where you are getting tripped up.

This topic is about nucleophiles. The question you presented is about Acidity.
Alkene double bonds are electron rich, and hence are Lewis Bases. It is acids that add to the bases... hence the question is asking about the relative acidity of the compounds.


As far as I know, larger molecules are ALWAYS better acids (within column comparisons).
Hence A

 

[chiddler]

Ah, i see where you are getting tripped up.

This topic is about nucleophiles. The question you presented is about Acidity.
Alkene double bonds are electron rich, and hence are Lewis Bases. It is acids that add to the bases... hence the question is asking about the relative acidity of the compounds.


As far as I know, larger molecules are ALWAYS better acids (within column comparisons).
Hence A

oh, duh! that's right.

thanks very much i didn't realize the difference.

 

[pfaction]

Wait, what? It said polar, aprotic, so I guessed D. Why wasn't it D??

 

[SaintJude]

nvm.

 

[pfaction]

Explain it to me because I put D. I thought nonpolar aprotic means F is the best, so continuing that trend, I<Br<Cl

 

[hellocubed]

Explain it to me because I put D. I thought nonpolar aprotic means F is the best, so continuing that trend, I<Br<Cl

hey, no prob.


The trend we were discussing was about nucleophilicity.


This question is about acidity.
Within a column, Acids strength is proportional to size, always.

 

[chiddler]

Explain it to me because I put D. I thought nonpolar aprotic means F is the best, so continuing that trend, I<Br<Cl

the difference is that we look for the strongest acid regardless of how strong the corresponding nucleophile is. HI is a much stronger acid than Cl and is therefore a better reagent for getting more reaction speed.

the slow step is forming the carbocation and therefore stronger acids will provide more H+ = faster speed.