# Which is the strongest base?

When I was in lower grades, I was taught $\ce{NaOH and KOH}$ are the strongest bases. But I realised later that there can be more stronger ones. After a bit of research on the internet, I found these three to be spoken more about.

1. Lithium diisopropylamide (LDA)
2. Methyllithium (MeLi)
3. Butyllithium (BuLi)

Wikipedia on LDA says that the $\mathrm{p}K_\mathrm{a}$ of its conjugate acid is $36$, which means the $\mathrm{p}K_\mathrm{b}$ value at $\pu{298K}$ is $-22$. (Since $\mathrm{p}K_\mathrm{a} + \mathrm{p}K_\mathrm{b} = 14$).
Likewise, for BuLi and MeLi I found the $\mathrm{p}K_\mathrm{b}$ value to be $-36$ and $-34$ respectively from the $\mathrm{p}K_\mathrm{a}$ values of their conjugate acids. So, can we conclude that BuLi is the strongest base? Is this absolute basic strength defined?

Sources for $\mathrm{p}K_\mathrm{a}$ values:

• Depends on the solvent system to some extent – Waylander Mar 26 '18 at 14:12
• pKb as measure of basic strength takes into account the solvent and the proton afinity of the species. pKb is effectively a free energy of reaction. So it takes into account the whole system: what the species is, what conentration, temperature, the solvent. Proton affinity by itself is a measure of the energy associated with being a base -- i.e. extracting protons, it's somewhat more universal. But then again the environment of a species effects it ability to extract those protons so if you wanted to make a basic solution you'd need to take into account the pKb too. – tmgriffiths Mar 27 '18 at 3:42

Looks like it's neither of the three; ortho-diethynylbenzene dianion has the highest calculated proton affinity.

From Wikipedia

according to calculations, ortho-diethynylbenzene dianion is the strongest base, with a proton affinity of $\pu{1843 kJ \cdot mol^{−1}}$.

This base claimed the title of the strongest base 2016 ish, another base may have grabbed the spot but I haven't been able to find it.

B L J Poad et al, Chem. Sci., 2016, DOI: 10.1039/c6sc01726f

• Any mention on the pKb value? Also I asked another question reg. the absolute basic strength. – MollyCooL Mar 26 '18 at 13:49
• Ofc no absolute basic strength is not defined . No idea regarding the pKb values – Avnish Kabaj Mar 26 '18 at 13:57
• Oh! Why is it not defined when we can say a single base is the strongest as of now? And is proton affinity the only factor? – MollyCooL Mar 26 '18 at 14:05
• How can you get the di-anion to start with? Neutral compounds or even mono-anions are more accessible. – Oscar Lanzi Mar 26 '18 at 18:16
• @AvnishKabaj I read the link twice/thrice. But I couldn’t find an explanation for “why proton affinity is the decisive factor?” when it comes to base strength. Thoughts? – MollyCooL Mar 26 '18 at 23:07