How do pi bonds make a molecule more basic?

[iwillbeatthis]

TBR Ochem Figure 1-34 (page 35), has this molecule below. It says that the molecule is more BASIC if there was a pi bond between the 2nd and 3rd carbon by having the methoxy donate to acid group through lone pair.... how so? wouldn't the molecule w/ the extra pi bond be more acidic since it'd have resonance ?
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[icedragon]

Going by the Lewis definition, an acid is an electron-pair acceptor and a base is an electron pair donor. Adding more electrons would make it more basic because it has more electrons to donate.

 

[sps27]

I have another related question. Why is an Alkyl Amine more basic than Ammonia. The pKb value for an Alkyl Amine is like 4 and that for Ammonia is like 4.6 or close to that. Is it the same reason i.e., an alkyl group is EDG (electron donating group) which makes the molecule more basic / nucleophilic. So does that make a tertiary amine more basic than secondary more basic than primary?

 

[Jepstein30]

TBR Ochem Figure 1-34 (page 35), has this molecule below. It says that the molecule is more BASIC if there was a pi bond between the 2nd and 3rd carbon by having the methoxy donate to acid group through lone pair.... how so? wouldn't the molecule w/ the extra pi bond be more acidic since it'd have resonance ?
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if there was a double bond between C2 and C3, we now have a resonance system that's going to be a decent contributor to the overall electronic structure:

So now we have a negative charge on the oxygen in the carboxylic acid group... meaning the actual molecule would have a partial negative there. Because it has some added e- density, it's "easier" for it to donate an electron pair and accept a proton (i.e. acting like a base).. because it has access to those "extra" electrons. At the same time, it would lower the acidity of the carboxylic acid group because now the proton will be more tightly bound (attracted) to the increased negative charge.

You have to consider what the resonance would do.. here it would add electron density to the carboxylic acid group.

 

[iwillbeatthis]

if there was a double bond between C2 and C3, we now have a resonance system that's going to be a decent contributor to the overall electronic structure:

So now we have a negative charge on the oxygen in the carboxylic acid group... meaning the actual molecule would have a partial negative there. Because it has some added e- density, it's "easier" for it to donate an electron pair and accept a proton (i.e. acting like a base).. because it has access to those "extra" electrons. At the same time, it would lower the acidity of the carboxylic acid group because now the proton will be more tightly bound (attracted) to the increased negative charge.

You have to consider what the resonance would do.. here it would add electron density to the carboxylic acid group.

Thanks for the response - but I thought we were focusing on the atom attached to the HYDROGEN that is being donated (in this case, there are no partial charged on the OH).. Thus the partial negative on the Oxygen if the carbonyl wouldn't matter..?

Also, I was always under the impression that RESONANCE = INCREASE ACIDITY. Would this be a lame case where it does the opposite: decrease acidity?

 

[Jepstein30]

Thanks for the response - but I thought we were focusing on the atom attached to the HYDROGEN that is being donated (in this case, there are no partial charged on the OH).. Thus the Oxygen on the carbonyl wouldn't matter..?

atom attached to the hydrogen? not sure what you mean here

if you want to just consider the effect on the ability of the carboxylic acid to act as an acid, this still works the same.

when you add in the double bond, you get the resonance I pictured above. Now, that proton (hydrogen) is held tighter to the molecule because of the partial negative nearby it. Hence, it won't be able to donate the proton as easily = less acidic = more basic.

Nearby partial charges still affect the hydrogen.. not as much as closer partial charges, though.

 

[1TB4RKSB4CK]

I just think of it as a general EWG or EDG, where one increases acidity and one increases basicity (decreases acidity), respectively.

 

[iwillbeatthis]

atom attached to the hydrogen? not sure what you mean here

if you want to just consider the effect on the ability of the carboxylic acid to act as an acid, this still works the same.

when you add in the double bond, you get the resonance I pictured above. Now, that proton (hydrogen) is held tighter to the molecule because of the partial negative nearby it. Hence, it won't be able to donate the proton as easily = less acidic = more basic.

Nearby partial charges still affect the hydrogen.. not as much as closer partial charges, though.

Oh, I just meant that in TBR and my ochem textbook it says to look at how resonance, atomic size, inductive effect, etc affects the atom attached to the hydrogen that will be donated (aka "the acidic hydrogen"). In this case, the acidic hydrogen is attached to an Oxygen. This oxygen does NOT have any sort of partial charge and appears to not be affected by resonance..... But you're saying that even though the partial neg charge is on the OTHER oxygen it matters? And also, doesn't resonance always INCREASE acidity? Sorry, I am just annoyed because that has been emphasized in my ochem classes and now they're confusing me.

Also, the first structure could have a resonance structure with a partial neg charge on the oxygen as well (see below).. just the arrow to the Oxygen. O now has 3 lp and a neg charge. So I still don't see how the second structure is more basic than the first... I know this can qualify as a resonance structure (albeit no a stable one) because TBR does the same on page 30).

 

[Jepstein30]

Oh, I just meant that in TBR and my ochem textbook it says to look at how resonance, atomic size, inductive effect, etc affects the atom attached to the hydrogen that will be donated (aka "the acidic hydrogen"). In this case, the acidic hydrogen is attached to an Oxygen. This oxygen does NOT have any sort of partial charge and appears to not be affected by resonance..... But you're saying that even though the partial neg charge is on the OTHER oxygen it matters? And also, doesn't resonance always INCREASE acidity? Sorry, I am just annoyed because that has been emphasized in my ochem classes and now they're confusing me.

Also, the first structure could have a resonance structure with a partial neg charge on the oxygen as well (see below).. just the arrow to the Oxygen. O now has 3 lp and a neg charge. So I still don't see how the second structure is more basic than the first... I know this can qualify as a resonance structure (albeit no a stable one) because TBR does the same on page 30).

resonance doesn't always increase acidity.. not sure why that point would be emphasized.

but yes, nearby atoms can influence acidity/basicity ... this IS the inductive effect. Here we have resonance leading to an increased inductive effect.

the first structure has that resonance, sure.. but so does the second... so the second structure will still have a stronger partial negative on the nearby oxygen. all resonance structures aren't created equal and since molecules don't actually follow resonance (i.e. they don't flip between each possible resonance structure but are actually a combination of all of them weighted by how much they contribute).. they can be 'additive' like this.

 

[Schenker]

Going by the Lewis definition, an acid is an electron-pair acceptor and a base is an electron pair donor. Adding more electrons would make it more basic because it has more electrons to donate.

Introducing a double bond between C2 and C3 would remove electrons (yep, an oxidation!) from the parent compound. Though, you can still use the Lew definition to justify the increased basicity of the 2,3-saturated derivative by saying that the carbonyl oxygen's lone pairs become more nucleophilic. Jepstein30 did a nice job of explaining why this is so.

I have another related question. Why is an Alkyl Amine more basic than Ammonia. The pKb value for an Alkyl Amine is like 4 and that for Ammonia is like 4.6 or close to that. Is it the same reason i.e., an alkyl group is EDG (electron donating group) which makes the molecule more basic / nucleophilic. So does that make a tertiary amine more basic than secondary more basic than primary?

You are correct that primary amines are more basic than ammonia and that secondary amines are more basic than primary amines, for the reason that you said. However, tertiary amines are less basic than secondary amines due to increased steric hindrance from the R groups.