Organic Polarity vs. Acidity

[TawMus]

Is polarity and acidity related in any way in o. chem???

I'm having a hard to differentiating whether a compound is more / less
polar that another compound...

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[Zerconia2921]

Is polarity and acidity related in any way in o. chem???

I'm having a hard to differentiating whether a compound is more / less
polar that another compound...

Yes polarity and acidity are mentioned in many chapters of organic chemistry the few that come to mind are SN1/E1 and SN1/E2 that is where polar protic and polar aprotic distinctions come up in ochem.

Sn1 -1st order kinetics deals with carbon cation stability in the follow order 3>2>1>CH3 A subsitution is likly to occur if the solvent is polar protic to solvate the carbon cation intermediate.

SN2- 2nd order kinetics deals with the substrate (alkyl halid) and the nucleophile. The nucleophile must be strong and the substrated less hindered. For the subsitution to take place the solvent would have to be polar aprotic because you would not want your solvent attacking your base.

This may be out of scope of your question but if you can ask a specific question ill try my best to explain.

 

[G1SG2]

Is polarity and acidity related in any way in o. chem???

They most definitely are related. The more polarized the bond (due to EN differences), the weaker it is, and therefore, the stronger the acid. For example, water has a pKa around 15.7, as opposed to HF which has a pKa around 3.2 This is because the H-F bond is more polarized/weaker than the H-O bonds in water, making HF the stronger acid. Hope this helps.

 

[TawMus]

They most definitely are related. The more polarized the bond (due to EN differences), the weaker it is, and therefore, the stronger the acid. For example, water has a pKa around 15.7, as opposed to HF which has a pKa around 3.2 This is because the H-F bond is more polarized/weaker than the H-O bonds in water, making HF the stronger acid. Hope this helps.

I thought the more stable the compound, the more acidic. If the polarization causes bond weakness, isn't that the opposite of stability and thus acidity??

 

[andafoo]

I thought the more stable the compound, the more acidic. If the polarization causes bond weakness, isn't that the opposite of stability and thus acidity??

I'm not sure where you heard, the more stable a compound was the more acidic it is.

You should check back on that.

Strongly acidic species tend to not be very stable. Take for example H2SO4, the protonated form will not be commonly seen. You'll most likely see the deprotonated form, HSO4-.

 

[TawMus]

I'm not sure where you heard, the more stable a compound was the more acidic it is.

You should check back on that.

Strongly acidic species tend to not be very stable. Take for example H2SO4, the protonated form will not be commonly seen. You'll most likely see the deprotonated form, HSO4-.

I see what you are saying. What's up with stabilizing group increasing acidity then? (ex: electron donating groups increase phenol/benzene acidity by stabilizing the molecule)

 

[G1SG2]

I thought the more stable the compound, the more acidic. If the polarization causes bond weakness, isn't that the opposite of stability and thus acidity??

Consider this reaction:

H-F --> H+ + F-

This reaction requires breaking the H-F bond. Since F is highly EN, it will pull the electrons toward it. So think of it this way: since this bond is polarized, with the EN F pulling the electrons toward it, it's already on it's way to becoming H+ and F-. The polarity of the bond due to the EN F makes the bond weak in terms of making it easier to break, thus forming the products H+ and F-. The same also applies to bond length-the longer the bond is, the weaker it is, and the easier it is to break and dissociate into the respective ions. Remember, the closer the electrons are to the nucleus, the more stable the bond. If a strong acid is to dissociate into it's ions, there usually is some type of factor such as EN or bond length that makes it easier to break that bond.

What's up with stabilizing group increasing acidity then? (ex: electron donating groups increase phenol/benzene acidity by stabilizing the molecule)

Benzene isn't acidic-it's nucleophilic. For example, during electrophilic aromatic substitution, the pi bonds of the ring attack an electrophile. When this happens, you get a resonance stabilized arenium ion, which contains a positive charge. Electron donating groups donate their electrons (via resonance or induction) and stabilize the positive charge.

 

[TawMus]

Consider this reaction:

H-F --> H+ + F-

This reaction requires breaking the H-F bond. Since F is highly EN, it will pull the electrons toward it. So think of it this way: since this bond is polarized, with the EN F pulling the electrons toward it, it's already on it's way to becoming H+ and F-. The polarity of the bond due to the EN F makes the bond weak in terms of making it easier to break, thus forming the products H+ and F-. The same also applies to bond length-the longer the bond is, the weaker it is, and the easier it is to break and dissociate into the respective ions. Remember, the closer the electrons are to the nucleus, the more stable the bond. If a strong acid is to dissociate into it's ions, there usually is some type of factor such as EN or bond length that makes it easier to break that bond.



Benzene isn't acidic-it's nucleophilic. For example, during electrophilic aromatic substitution, the pi bonds of the ring attack an electrophile. When this happens, you get a resonance stabilized arenium ion, which contains a positive charge. Electron donating groups donate their electrons (via resonance or induction) and stabilize the positive charge.

So what you are saying is that when my text books are giving me an Acidity ranking (ex. phenol > methanol > primary OH > secondary OH > tertiary OH) it is refering not to the stability, but rather the reactivity of the molecule??

 

[G1SG2]

So what you are saying is that when my text books are giving me an Acidity ranking (ex. phenol > methanol > primary OH > secondary OH > tertiary OH) it is refering not to the stability, but rather the reactivity of the molecule??

No, it is referring to stability. When you say "stable", you have to think about what the stability is referring to. In the example of electrophilic aromatic substitution, the electron donating groups stabilize the arenium ion in terms of stabilizing the positive charge.

Primary alcohols are more acidic than secondary and tertiary alcohols because the secondary and tertiary alcohols have more alkyl groups, which are electron donating. The more alkyl groups you have on the conjugate base, the greater the negative charge on the conjugate base, which makes it a stronger base (i.e., the tertiary OH has 3 alkyl groups donating electrons to it, which would make it's conjugate base stronger than the conjugate base of the secondary alcohol, which has only 2 alkyl groups donating electrons to it). So, the electron donating groups effectively destabilize the carbanion (as opposed to a carbocation, as they would stabilize it).

In the case of phenol, benzene can reduce the negative charge on the oxygen in the conjugate base through resonance, which would lower the basicity of the conjugate base. And remember, the acid of a strong conjugate base would be a weaker acid, and the acid of a weak conjugate base would be a stronger one (i.e., H-Cl is a strong acid, while it's conjugate base Cl- is a poor base). So, in this example, the electron donating groups lower acidity (weaker acid) because they destabilize the conjugate base and increase basicity (stronger conjugate base), but they are stabilizing in terms of benzene because they stabilize the positive charge of the arenium ion (and can even give everyone an octet via resonance). EDG donating to positive = stability, EDG donating to negative = instability which = stronger conjugate base which = weaker acid.

 

[G1SG2]

Hope that helps.

 

[TawMus]

Yea, it helps alot man. I guess the book using the general term "Acidity Ranking" completely threw me off.

 

[userah]

question. I understand that electron donating groups decrease acidity because of the stabilization of the arenium ion but relatively speaking, are ED groups like alcohols and amines more acidic than such groups by themselves?

like which is more acidic, an amine by itself or an amine attached to a benzene ring?

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also, i thought the higher the dipole moment, the stronger the acid. So then, with alcohols, which are pretty polar, if you add EW groups like alkyl groups, wouldn't that increase the polarity and thus increase acidity?



the ED group is increasing the dipole moment here no? so then wouldn't the acidity be greater? if that's the case, wouldn't tertiary alcohols be most acidic?

sorry i'm just really confused 😕

 

[G1SG2]

like which is more acidic, an amine by itself or an amine attached to a benzene ring?

Let's take a look at who's the better base. Amines are weak bases. However, if an amine is attached to a benzene ring (like aniline), the amine can donate it's lone pairs to benzene through resonance (of course, then, aromaticity is loss). So the way I see it, the aniline would be a weaker base, because the lone pairs on the N can "goof off" with the benzene, whereas the amine by itself can only grab a proton. So, the amine with the benzene would be a better acid.

also, i thought the higher the dipole moment, the stronger the acid. So then, with alcohols, which are pretty polar, if you add EW groups like alkyl groups, wouldn't that increase the polarity and thus increase acidity?

Yes, the higher the dipole moment, the stronger the acid. The more polarized the bond, the stronger the acid. But which bond? The one concerning the atoms that will dissociate into the ions (like H+). Alkyl groups are ED, not EW. EWG would increase the acidity, and EDG decrease acidity.

if that's the case, wouldn't tertiary alcohols be most acidic?

They wouldn't be because tertiary alcohols have EDG, not EWG. The tertiary alcohol has three alkyl groups donating electrons to it. Electron donating groups very much strengthen the conjugate base (the alkoxide ion is a strong base). And with strong conjugate bases (the alkoxide ion), you'll have weaker acids (alcohols). Does this answer your question in any way?