Why is guanidine more basic than methylamine?

[realitysickness]

I was going through the EK Chemistry lesson, and they explained that Guanidine HNC(NH2)2 is a stronger base than methylamine, CH3NH2. I am confused by this because I assumed that the least stable compound would be the most basic. Guanidine is resonance-stabilized, which means it should be less basic than methylamine. Methylamine would be more reactive/nucleophilic/basic because comparatively it lacks any resonance stabilization.

Unstable "anions"/conjugate bases = strong bases

Is my reasoning faulty?

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[freak7]

Protonated Guanidine:

Does that help you conceptualize it?

 

[realitysickness]

Protonated Guanadine:

Does that help you conceptualize it?

I understand that guanidine has a more stable conjugate acid than methylamine because the guanidinium ion is resonance-stabilized while the methylammonium is not. But it's just that in class, I learned that the more basic compound is the one that is LESS stable. For example, compare -OH with -SH, and -OH is more basic than -SH because it is less stable due to the smaller size of O compared to S. Similarly, a) cyclohexane with a NHCH3 substituent is more basic than b) benzene ring with two substituents, CH3 on C1 and NH2 on C4. This is because a) is LESS stable because it cannot be resonance stabilized in the same way that b) can be. This is what gets me confused because it seems that the these trends are getting mixed up in my head.

 

[freak7]

I understand that guanidine has a more stable conjugate acid than methylamine because the guanidinium ion is resonance-stabilized while the methylammonium is not. But it's just that in class, I learned that the more basic compound is the one that is LESS stable. For example, compare -OH with -SH, and -OH is more basic than -SH because it is less stable due to the smaller size of O compared to S. Similarly, a) cyclohexane with a NHCH3 substituent is more basic than b) benzene ring with two substituents, CH3 on C1 and NH2 on C4. This is because a) is LESS stable because it cannot be resonance stabilized in the same way that b) can be. This is what gets me confused because it seems that the these trends are getting mixed up in my head.

Welcome to o-chem. Where the rules aren't rules and the points don't matter.

So guanidine is a weird case since it's pretty ok acid and base all due to its 3-fold rotational symmetry. If you take an H+ off, it can tautomerize to balance everything. If you add one, it's got that nice 3-way resonance. The "less stable" rule doesn't really hold up well here. It's more of a guideline.

 

[realitysickness]

Welcome to o-chem. Where the rules aren't rules and the points don't matter.

So guanidine is a weird case since it's pretty ok acid and base all due to its 3-fold rotational symmetry. If you take an H+ off, it can tautomerize to balance everything. If you add one, it's got that nice 3-way resonance. The "less stable" rule doesn't really hold up well here. It's more of a guideline.

Ah that makes sense. So for purposes of the MCAT exam, do you think I should follow the "stronger base is the one with the more stable conjugate acid" guideline OR "the stronger base is the less stable compound/anion/etc" guideline to inform my answer choices generally speaking?

 

[freak7]

Ah that makes sense. So for purposes of the MCAT exam, do you think I should follow the "stronger base is the one with the more stable conjugate acid" guideline OR "the stronger base is the less stable compound/anion/etc" guideline to inform my answer choices generally speaking?

Ummm honestly what I did was only think of things as Brønsted-Lowry acids/bases the whole time unless specifically told otherwise. What is more likely to be able to give a proton, and what's more likely to be able to take a proton

 

[aldol16]

Ah that makes sense. So for purposes of the MCAT exam, do you think I should follow the "stronger base is the one with the more stable conjugate acid" guideline OR "the stronger base is the less stable compound/anion/etc" guideline to inform my answer choices generally speaking?

The statement that a strong base is less stable is not necessarily true. Stability of a molecule has both kinetic and thermodynamic components whereas acidity is a thermodynamic phenomenon only. What that means is that you want to look at the equilibrium between the protonated and deprotonated states. That's what determines acidity. So for basicity, you're looking at the H+ + A- <---> HA equilibrium and asking what favors the right-hand side.

 

[freak7]

The statement that a strong base is less stable is not necessarily true. Stability of a molecule has both kinetic and thermodynamic components whereas acidity is a thermodynamic phenomenon only. What that means is that you want to look at the equilibrium between the protonated and deprotonated states. That's what determines acidity. So for basicity, you're looking at the H+ + A- <---> HA equilibrium and asking what favors the right-hand side.

Thank goodness you showed up. I was handwaving pretty hard there.