[Agent-X]

I have this odd problem from this quiz I found online. Supposedly, the course uses the same book as I use; and I've been able to figure out how the professor created many of the questions. However, the book doesn't discuss resonance structures in relationship to acidity and basicity.

I could draw the resonance structures, but I just don't get the answer. And it's a rarity I simply say, "I don't get it" and not have some source to look at for help. Of the majority of book concepts, I couldn't see how concepts would be combined to derive an feasable answer. Albeit, I found a website with some information about resonance and basicity, it was torn down, and only remnants of information remain: link

It says these main things:

In general:
Resonance decreases basicity by removing electron density from atom
that shares electron pair with proton.

Avoid this common misconception: Resonance does not always decrease basicity
• Resonance may enhance basicity (uncommon)
• Resonance may have no impact on basicity (common)

Attached is the problem I don't understand and its solution.
And I guess it's an "uncommon" problem to deal with.

Irregardless, I lucked out by finding that websource. I don't know how I could have figured the attached problem out by combining concepts from the first two chapters of the book: the first discussing acid, bases, resonances; the second discussing orbital hybridization and alkanes.
Could someone describe the relationship between basicity and resonance to me?
I can't really find any decent sources.

[nj_bartel]

A simple view of basicity is as excessive electron density on an atom.  Generally, resonance spreads electrons over a molecule, decreasing the electron density on any one atom and thus decreasing basicity.  In the case of guanidine, try drawing resonance structures before you protonate it.  You'll see that the nitrogen that take part in the double bond exhibits two partial negative charges.  Resonance serves to increase the electron density around the doubly bonded nitrogen.

[Agent-X]

A simple view of basicity is as excessive electron density on an atom.  Generally, resonance spreads electrons over a molecule, decreasing the electron density on any one atom and thus decreasing basicity.  In the case of guanidine, try drawing resonance structures before you protonate it.  You'll see that the nitrogen that take part in the double bond exhibits two partial negative charges.  Resonance serves to increase the electron density around the doubly bonded nitrogen.

Are you trying to say that the reason it is more basic (before protonation) is because it has more contact areas to accept a proton than other contact areas. And the reason it has more contact areas than the other molecules (Methyl amine and Acetone mine) is because the electron delocalization allows Guanidine to create new bonds elsewhere, thus allowing the proton to position itself on different parts of the molecule? I don't see any NH3 groups on any resonance structures, so I may be wrong about what I'm interpreating that you are saying.

[ufalynn88]

a simple way to view basicity is to look at the resonance structures that are formed after the molecule has behaved as a base (ie if a stable structure is formed after a molecule behaves as a base, its likely that it will often behave that way...while if the molecule formed is unstable it likely will not)....when guanidine accepts a proton it results in the formation of 3 resonance structures...this is clearly much more favorable than either of the other 2 molecules, that result in none

[renge ishyo]

A good way to view this problem is to consider that adding a proton to guanidine makes all of the nitrogen groups attached to the central atom "look identical" which means that the double bond can resonate between all three nitrogen carbon bonds equally. This moves around the positive charge from nitrogen to nitrogen and makes the protonated molecule less reactive. Think of it as if a static positive charge was like a stationary target in a shooting gallery and a resonating positive charge was like a rapidly moving target, which would be harder to shoot? In this way it is easier in general to react a negative compound with a compound that has a "localized" positive charge than it is to react it with a compound with a "delocalized" positive charge due to resonance. Guanidine is a better base than the other alternatives for that reason. You should be able to see that it's harder to "take back" the hydrogen once you give it to guanidine than it is in those other two examples where the positive charge is just sitting there in one place.

Hope this helped.