# How to find the strongest base using stability arguments

Answers: The most acidic is D and the most basic is A.

So first off I compared A with B:

I'm kinda confused on this. I looked at the base NH one and saw that when it was protonated it lost its aromaticity. So as a result, the NH²⁺ one must be an unstable acid, thus it is the stronger acid. The stronger acid must have the stronger base on the same side, thus I concluded that A is stronger than B. Is this the correct way to think about it?

When I compare the two bases on each side (the O⁻ and OH⁺ one), I see that the O⁻ one is more stable (on the left side), thus it is the weaker base, meaning the OH⁺ one (on the right side) is the stronger base. However, that is not right as A is the strongest.

However, when I compare the two acids on each side. I see that the OH one (right side) is more stable, thus it must be the weaker acid, so its conjugate base which is the O⁻ one must be stronger. This way of thinking got me the correct answer. However, I'm still unsure if this is the reason why as I was always told to compare the charged species.

Also E doesn't have a lone pair to donate, so I neglected that in the conversation of the strongest base. For C, it just didn't seem likely to me.

• welcome to Chemistry SE. Thanks for showing your work. A few points: You are correct about the first comparison. A is a stronger base than B. Your comparison of A with D is wrong (check the structure of D). As for E, you are right, but cations are almost never basic. – Michael Lautman May 31 at 21:22
• You have to be careful with the "stability" argument. If the species looks "ok" and looks "instable" when protonated, it is not a good base. If the species looks "instable" but looks "ok" when protonated, it is a good base. If both conjugate base and conjugate acid look "wacky", all bets are off. – Karsten Theis May 31 at 21:24
• @MichaelLautman Ethylenediamine? – Karsten Theis May 31 at 21:29
• @KarstenTheis you got me! I should have been more specific. I wasn't thinking about diamines. The ammonium group isn't basic. – Michael Lautman May 31 at 21:36
• @MichaelLautman Well, you said almost never. And I did not come up with a situation where a cationic functional group accepts a proton. On the flip side, there are many anions that can lose another proton (sulfuric acid, phosphoric acid). – Karsten Theis May 31 at 21:39

A vs. B

I will label your first reaction scheme, using the letters A-E for the deprotonated (i.e. as shown in the problem) form and the symbol $$\ce{AH+}$$ etc for the protonated form (after acting as a base and accepting a proton):

$$\ce{A (stronger base) + BH+ (stronger acid) <=> AH+ (weaker acid) + B (weaker base)}$$

Your argument about aromaticity is great; imidazolium ($$\ce{BH+}$$) is the stronger acid compared to cyclohexanol ($$\ce{AH+}$$), so the alkoxide ($$\ce{A}$$) is the stronger base compared to imidazole.

A vs. D

The protonated tetrahydrofuran (D) is highly reactive ("unstable") in water because it would want to get rid of a proton. That does not make it a stronger base. In your reaction scheme, you add another proton to a species that already is a strong acid (not sure if it even exists, see Why doesn't H₄O²⁺ exist?).

$$\ce{A (stronger base) + DH+ (stronger acid) <=> AH+ (weaker acid) + D (weaker base)}$$

The cyclohexoxide ($$\ce{A}$$) is the stronger base than protonated tetrahydrofuran ($$\ce{B}$$) because we know phenol ($$\ce{AH+}$$) exists, but we don't think twice-protonated tetrahydrofuran ($$\ce{BH+}$$) exists.

the O- one is more stable (on the left side), thus it is the weaker base, meaning the OH+ one (on the right side) is the stronger base

You have to specify stable in terms of what reaction. Protonated tetrahydrofuran is very stable against protonation, so it is not a base.

However, when I compare the two acids on each side. I see that the OH one (right side) is more stable, thus it must be the weaker acid, so its conjugate base which is the O- one must be stronger.

Now you are comparing the two in terms of either one losing a proton, which is the right thing to consider when comparing two acids.

C and E

You are right about E, the nitrogen already makes four bonds, so it can't accept another proton. I'm not sure where a proton would go in C. In any case, it is not a good base.

• I see, now if I compare A and D by saying which is the strongest acid first. – Lat May 31 at 21:47
• AH+ (weaker acid) + D (without the H on the O, so the base form of it) --><-- A (stronger base) + D (stronger acid). In this way, D is more unstable than AH+, so D is the stronger acid, thus weaker base. However, I was always told to compare charged species. A with an H is neutral. So does it not matter if you compare a charged species with a neutral species? – Lat May 31 at 21:54
• @Zach This sounds like a new question: Are you asking whether A or D is the stronger acid? Or are you mixing and matching - trying to protonate A and deprotonate D? "D stronger acid thus weaker base" does not work. It is "DH+ stronger acid thus D weaker base". If you want to compare A and D, they have to compete for a proton, i.e. appear on different sides of the chemical equation. – Karsten Theis May 31 at 22:10
• @Zach About "I was always told to compare charged species". You might run into trouble with that rule. What if both acid and conjugate base are charged? What if the neutral species is highly reactive and the charged species is what you usually find in solution (e.g. HCl in water)? – Karsten Theis May 31 at 22:13
• @Karsten Theis: You mentioned A as a phenolate ion, but is not. It is the conjugate base of cyclohexanol. – Mathew Mahindaratne Jun 1 at 6:39