electrion donating/withdrawing groups

[swamprat]

Can someone explain to me why an electron-donating group would increase the stability of a positive charge? What about a negative charge? And am I right in saying that an electron-withdrawing group will increase the acidity of an acid and an electron-donating group would decrease acidity or increase basisity?

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[cornpops21]

Can someone explain to me why an electron-donating group would increase the stability of a positive charge? What about a negative charge? And am I right in saying that an electron-withdrawing group will increase the acidity of an acid and an electron-donating group would decrease acidity or increase basisity?

I believe this is correct.

 

[Fulfuric]

Can someone explain to me why an electron-donating group would increase the stability of a positive charge? What about a negative charge? And am I right in saying that an electron-withdrawing group will increase the acidity of an acid and an electron-donating group would decrease acidity or increase basisity?

Electron donating group increase the stability of the positive charge because the lone pair electrons donate into the cation stabilizing it.

 

[sleepy425]

Can someone explain to me why an electron-donating group would increase the stability of a positive charge? What about a negative charge? And am I right in saying that an electron-withdrawing group will increase the acidity of an acid and an electron-donating group would decrease acidity or increase basisity?

an electron donating group increases stability of a positive charge because it donates electron density into the positively charged center. what does this mean? well, a positive charge indicates electron deficiency, and typically suggests decreased stability compared to a neutral center. by having an electron donating group, the effective positive charge is reduced, thus providing stability.

for a negative charge, it's exactly the opposite. negative charges are stabilized by the presence of electron withdrawing groups, for the exact same reasons that positive charges are stabilized by electron donating groups.

your statements about the implications of this on acidity and basicity are exactly correct. an electron withdrawing group increases acidity because an acid (by the lewis definition) is an electron pair acceptor so it's going to be accepting a negative charge (now that doesn't mean the overall charge will be negative, it can still be positive or neutral, depending on the original charge). anyway, anything that stabilizes the developing negative charge will increase acidity because it stabilizes the conjugate base preferentially over the acidic form. similar reasoning applies to basicity.

 

[nana0709]

Hi there,

I have some questions regarding electron donating group and electron withdrawing groups effects on acid strength.

1) Aniline has a -NH2 attached to a benzene ring. NH2 is an electron donating group and therefore is able to donate electrons into the benzene ring and delocalize the negative charges. This makes aniline more acidic than cyclohexylamine.

2) In the case of guanidine, the positive charges are spread out across the molecule. This makes guanidine more basic than methylamine.

Doesn't this contradict the saying that electron donating groups decrease acidity and electron withdrawing groups increases acidity? With the above 2 examples shouldn't it be the other way around?


I'm really confused suddenly. Please help. Thanks.

 

[Fifty 3rds]

When NH2 is added to a molecule/ring it is electron donating, stabilizing the molecule, increasing acidity, decreasing pH etc.

Guanidine, as I see it, is a special case. It's electronegativity/pi bond is the source of it's "problem" It is a good proton acceptor (Bronsted-Lowry base), and it's resonance stabilization, makes it the preferred form at physiological conditions.

Essentially, guanidine's structure is so supportive of a positive charge, by spreading it around through resonance, and it is composed of electronegative elements, and it has a pi bond (it's double bond) that is a wonderful candidate to be broken, that it can't help but be a wonderful base.


That is as good as I can explain. Good question. Maybe someone with a bit more O-chem knowledge will simplify that lengthy explanation.

 

[swimmingchemist]

The -NH2 group on Aniline certainly is electron donating, but this has the effect of decreasing the acidity of the hydrogens on the benzene ring. These hydrogens (already involved in the conjugated pi system) are not considered acidic under any circumstance, and -NH2 makes them less so.

The original poster started their discussion by saying: "...an electron-donating group would decrease acidity or increase basisity?" And this is absolutely correct. In the case of Aniline- we are not talking about an acid here! If we were talking about p-aminobenzoic acid, or trinitrophenol this would be an entirely different matter, because these molecules do have a potentially acidic hydrogen, whereas the only acidic hydrogens on Aniline are on the N, which we are talking about as being the substituent.

As far as guanidine- I think it is more appropriate to think about the guanidine-cation as a lewis acid- it is able to accept electrons (And not able to accept protons).

In summary, it is very important to keep in mind what is the substituent, and what are the traits of the molecule which will be afected by this group. Hope this helped some!

 

[Fifty 3rds]

Wow. I think I may have had a stroke when writing that.

Swimmingchemist's explanations seem much better.

 

[Phantastic]

I think it is more appropriate to think about the guanidine-cation as a lewis acid- it is able to accept electrons (And not able to accept protons).

I agree. When incorporating Lewis Acid/Base concepts, and drawing out the movement of the electrons, it's easier to see how this destabilization of the conjugate base is caused by an electron donating group. Just try to draw acceptable resonance structures, and assess there relative energies.

 

[nana0709]

Wow..you guys are awesome..Thank you very much!!

 

[starr0329]

Hi,

I just had a quick question. Im looking over the orgo section right now. And I noticed that all of the electron donating groups that were the strongest also had some of the stongest electronegative atoms(ex: -O, -OH). Is is safe to assume that in general that strong electronegative atoms also will be strong electron donators? This might be a dumb question, but I had to ask ne way.

 

[orgohacks]

Hi,

I just had a quick question. Im looking over the orgo section right now. And I noticed that all of the electron donating groups that were the strongest also had some of the stongest electronegative atoms(ex: -O, -OH). Is is safe to assume that in general that strong electronegative atoms also will be strong electron donators? This might be a dumb question, but I had to ask ne way.

No, not necessarily. Fluorine, chlorine, etc. are strongly electronegative but are only weak electron donors.

There are really two factors at play here, which cause confusion.

1) sigma-acceptor ability : this correlates directly with electronegativity, and is a matter of how much electron density the element "sucks out" of a covalent sigma bond it's attached to. F, Cl, N, O, etc. all have electronegativities greater than that of C and therefore are net sigma acceptors.

2) pi-donation ability: this corresponds to an element having a Lewis basic electron pair that is able to donate into a neighbouring atom and form a pi bond. This is a donating effect. -NH2, O- , OH, OR, etc. are all good pi donors. The lone pairs of halogens (F, Cl, Br, etc) are weak Lewis bases and only weak pi donors.

In aromatic substitution, pi donation is a more powerful effect than sigma donation. Hence, amines and alcohols are strongly activating, and they direct electrophilic aromatic substitution ortho-para. Halogens are deactivating (due to sigma acceptor ability) but are ortho-para directing (due to pi donation ability). hope this helps.

 

[yesnomaybe]

Sorry for pulling up an old thread! I am studying for organic chemistry and my books don't do a decent job of explaining this situation....

I am given three acid and need to rank their strength.

1. CH3CH20CH3

2.
.......O
.......||
CH3-C-CH3

3.
.......O
.......||
CH3-C-NH2


#3 is the one that's giving me the most confusion. It has the O=C electron withdrawing group which usually makes a compound more acidic than one without (if in right location). When the "H" is removed, the negative charge on "N" is stabilized through resonance. The O will "give" N one of it's bonds and take the negative charge. I would ASSUME this would mean #3 is the most acidic. However, my book also states that "N" is an electron donating. So it appears there is both an electron donating and withdrawing group. How do you determine the acidity of this in comparison to the other two?

#2 compound makes sense to me. I know the ability to form resonance structure helps in making it more acidic, but the carbon having a negative charge makes it not as acidic as it could be.

I also don't know where the ether fits into things. If I'm right, the O brings forth induction and stablizes the negative charge on the C, but as far as ranking goes, where does this fit into the scheme of things?


Thanks everyone! I REALLY appreciate any help. And I swear this isn't for a homework assignment. Studying for a test!

 

[yesnomaybe]

I FIGURED PART OF IT OUT! Nevermind!

Between #2 and #3, #3 is the stronger acid because a negative charge on "N" is more stable than a negative charge on "C"

Now to figure out how CH3CH20CH3 plays a roll!

 

[integralx2]

I FIGURED PART OF IT OUT! Nevermind!

Between #2 and #3, #3 is the stronger acid because a negative charge on "N" is more stable than a negative charge on "C"

Now to figure out how CH3CH20CH3 plays a roll!

Just by looking I would say 3 is more acidic then 2 which is more acidic then 1. You know #3 is strongest cause of resonance, and if there is resonance that means conjugate base is weaker (since resonance distributes charge), and conjugate acid is stronger. #2 is is stronger I think because it can be pronated at the alpha carbon due to the fact that the negative charge can be stabalized by resonance, and #1 is weakest aster cause its an ether, which i think is a stronger base because it has those free electrons on the oxygen, which would make it more basic (nucleophiles have electrons to donate) and looking to bond to an hydrogen. I hope someone can verify what I said is correct, dont wanna give you wrong info, and then I would know if I dont understand something properly also .