I thought NH3+ was electron donating

[deleted647690]

Wouldn't it be? If it is electron deficient, doesn't that mean that electrons are pushed away from it? I was comparing the pka of H3C(CH2)2CO2H and H3N+CH2CO2H and I thought that NH3+ would be more electron donating that the alkyl groups....
I guess if it's deficient it pulls electrons towards it and acts as an electron withdrawing group.

But for some reason I was thinking that, since it is deficient in electrons, it would be pushing the electrons it previously had away...lol

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[confidentandgood]

Alkyl groups are considered ring-activating, though weakly so, due to hyperconjugation. An NH3+ group is strongly deactivating due to its positive charge.

 

[deleted647690]

Alkyl groups are considered ring-activating, though weakly so, due to hyperconjugation. An NH3+ group is strongly deactivating due to its positive charge.

Ehhh....I honestly don't remember what activating/deactivating means haha. I need to look at EAS

 

[confidentandgood]

Ehhh....I honestly don't remember what activating/deactivating means haha. I need to look at EAS

For convenience, here's the chart I always use.

 

[aldol16]

Wouldn't it be? If it is electron deficient, doesn't that mean that electrons are pushed away from it? I was comparing the pka of H3C(CH2)2CO2H and H3N+CH2CO2H and I thought that NH3+ would be more electron donating that the alkyl groups....

Use chemical intuition. No need to memorize a list. Ask yourself: how would -NH3+ donate electrons? Where do those electrons come from?

 

[Necr0sis713]

If it's a positive charge, it's most likely going to want negative charges to stabilize itself. Chemistry is all about stability. When it doubt, think in terms of stability.

 

[wutwutwut]

^ like they said

if something is positive it lacks electrons because if it had extra electrons then it would be negative
therefore NH3+ ends up being electron withdrawing because it needs electrons in order to be stable (i.e. neutral)

if I remember correctly alkyl groups are electron donating because they are not very electronegative not because they are positively charged or anything but it has been a while since organic so I could be wrong

Edit: oh I just read what some of the other people said I guess the alkyl thing is due to hyper conjugation not electronegativity so ignore me haha

 

[deleted647690]

Am I correct with this:
I am comparing the strength of the OH bond in an alcohol vs. an OH bond in carboxylic acid. In carboxylic acid, the electronegative O on the carbonyl acts as an EWG and pulls electron density away from the hydroxyl. This makes the OH group more electron deficient.
In an alcohol, the methyl group attached is less electronegative than the O in OH, so it donates electron density to the OH, making the OH more electron rich. Since it has more electrons, it's more willing to act as an acid.

Therefore, the OH bond in an alcohol is more likely to break than the OH bond in a carboxylic acid.

 

[BerkReviewTeach]

Everything was fine up until the "Since it has more electrons, it's more willing to act as an acid" part.

Think about a base and an acid in their crudest from. An acid is H+, which has absolutely no electrons. A base is a lone pair donor, so it must have electrons. If you make the OH group electron deficient, you have made it more positive (more like H+), so it will be more acidic. If you make the OH group electron rich, you have made it more negative (more like OH-), so it will be more basic.

 

[deleted647690]

Everything was fine up until the "Since it has more electrons, it's more willing to act as an acid" part.

Think about a base and an acid in their crudest from. An acid is H+, which has absolutely no electrons. A base is a lone pair donor, so it must have electrons. If you make the OH group electron deficient, you have made it more positive (more like H+), so it will be more acidic. If you make the OH group electron rich, you have made it more negative (more like OH-), so it will be more basic.

I think I meant to say that, since the OH in an alcohol has more electron density, it is less willing to accept electrons, therefore it would be less acidic. Also, in terms of the strength of the bond, does extra electron density mean that the OH bond can be more stabilized for some reason? The extra electrons present make the bond stronger?

Sorry, I know these are basic fundamentals, but I seem to keep confusing them whenever I come across a question about it.

 

[aldol16]

In an alcohol, the methyl group attached is less electronegative than the O in OH, so it donates electron density to the OH, making the OH more electron rich. Since it has more electrons, it's more willing to act as an acid.

The fact that the methyl carbon is less electronegative than O is not sufficient for donating electron density. You need hyperconjugation and that's the thing that causes electron donation.

I think I meant to say that, since the OH in an alcohol has more electron density, it is less willing to accept electrons, therefore it would be less acidic. Also, in terms of the strength of the bond, does extra electron density mean that the OH bond can be more stabilized for some reason? The extra electrons present make the bond stronger?

Whether a bond is strong or weak depends on where the electron density lies in the bond. For a carboxylic acid, most of the electron density lies on the alcoholic oxygen and thus the O-H bond is highly polarized. Thus, heterolytic bond cleavage is easy. Conversely, for an aliphatic alcohol, that electron density does not lie as close to the O as in a carboxylic acid. Thus, heterolytic bond cleavage there is harder.

You can also think of it in terms of acidity. Carboxylic acid is more acidic than alcohol and thus the O-H bond will be more easily broken in the former than in the latter.

 

[deleted647690]

The fact that the methyl carbon is less electronegative than O is not sufficient for donating electron density. You need hyperconjugation and that's the thing that causes electron donation.



Whether a bond is strong or weak depends on where the electron density lies in the bond. For a carboxylic acid, most of the electron density lies on the alcoholic oxygen and thus the O-H bond is highly polarized. Thus, heterolytic bond cleavage is easy. Conversely, for an aliphatic alcohol, that electron density does not lie as close to the O as in a carboxylic acid. Thus, heterolytic bond cleavage there is harder.

You can also think of it in terms of acidity. Carboxylic acid is more acidic than alcohol and thus the O-H bond will be more easily broken in the former than in the latter.

I thought that the fact that the carboxylic acid had another oxygen in the molecule would mean that there would be less electron density on the OH group because the other oxygen is electron withdrawing, while the hydroxyl in the alcohol has more ability to draw electrons because its only competing with the methyl. Would it be incorrect to think about it that way?

 

[aldol16]

I thought that the fact that the carboxylic acid had another oxygen in the molecule would mean that there would be less electron density on the OH group because the other oxygen is electron withdrawing, while the hydroxyl in the alcohol has more ability to draw electrons because its only competing with the methyl. Would it be incorrect to think about it that way?

The alcoholic O is not "competing" with the other atoms in the sense that you are implying. Draw these molecules. The alcoholic O is on the same side as these other groups relative to the hydrogen and thus will also withdraw electron density from its own bond with the hydrogen because it's more electronegative than the hydrogen. You get this same effect in both cases so it's not a good distinguishing factor. You can think about this in a similar way to the one you mentioned. In one case, you have a methyl group pushing electron density onto the oxygen, which would make the oxygen less electron-withdrawing. In the other case, you have a carbonyl withdrawing electron density from the alcoholic oxygen, which makes it want to pass that effect onto the hydrogen - that is, it will withdraw more electron density from its own bond with the hydrogen, thus polarizing the bond.

 

[deleted647690]

The alcoholic O is not "competing" with the other atoms in the sense that you are implying. Draw these molecules. The alcoholic O is on the same side as these other groups relative to the hydrogen and thus will also withdraw electron density from its own bond with the hydrogen because it's more electronegative than the hydrogen. You get this same effect in both cases so it's not a good distinguishing factor. You can think about this in a similar way to the one you mentioned. In one case, you have a methyl group pushing electron density onto the oxygen, which would make the oxygen less electron-withdrawing. In the other case, you have a carbonyl withdrawing electron density from the alcoholic oxygen, which makes it want to pass that effect onto the hydrogen - that is, it will withdraw more electron density from its own bond with the hydrogen, thus polarizing the bond.

By "polarizing the bond" you mean introducing a sort of dipole within the bond?

Sorry for all the questions by the way. I've found that, once I think about something one way, it's very difficult to see it differently.

 

[aldol16]

By "polarizing the bond" you mean introducing a sort of dipole within the bond?

A dipole already exists as a permanent apart of the O-H bond due to electronegativity difference. That's how it can even show up on an IR spectrum. What I mean is that all a bond is is electron density. That density, in a perfectly covalent bond, lies exactly at the midpoint of the distance between the two atoms participating in the bond. However, no bond with two different atoms is perfectly covalent and so that electron density usually lies closer to one or the other of the atoms, governed in large part by electronegativity but also by inductive, conjugation, and hyperconjugation effects. If that electron density is so close to one atom that most of it lies right next to that atom, then that bond is said to be extremely polarized, or ionic. The intermediate range results in so-called covalent bonds but since it's a spectrum, some bonds are more covalent than the others.