# Comparison Of Lewis Acidity

Which is a stronger Lewis Acid $\ce{AlCl_3}$ or $\ce{SiCl_4}$?

I am confused as in $\ce{AlCl_3}$ , Al has an incomplete octet so it must be a stronger Lewis acid as compared to $\ce{SiCl_4}$, but the answer is just opposite.

• Well, answer is wrong so I guess there was a mistake or you misunderstood sth. Aug 11, 2016 at 20:04
• agree, AlCl3 is much certainly a stronger Lewis acid. Sep 11, 2016 at 4:24

## 1 Answer

AlCl3 + RCl → AlCl4(-) indicating that AlCl3 is a Lewis Acid but SiCl4 + RCl → SiCl5(-) ?????? So, this tells me that AlCl3 > SiCl4 in terms of electron accepting behavior. Now, don't get me wrong: pentacoordinated Si is well known. One of the failures of the Lewis Acid concept is that, as far as I know, there is no unambiguous way to measure it. It is much more qualitative than quantitative, despite years of attempts to do so. You may know that Phosphorous is typically pointed to as the first example of the breakdown of the octet rule. (see Hypervalence) After Period 3, the concept has little use, imho. Since Lewis Acidity is tangled up with Lewis structures, perhaps it is not too surprising that the concept has some serious problems. Usually, we condsider both electronic effects as well as steric effects to 'explain' Lewis Acid/Base strength. One popular alternative is Hard.Soft.Acid.Base (HSAB) "theory". Anyway, comparing a tri-coordinated species with a tetracoordinated species and predicting which will form a bond with an electron donor is really difficult. Did you know that AlCl3 doesn't really 'exist' in the solid or liquid state? And even in the gaseous state, the dimer is common. The structure of the (solid) dimer (Al2Cl6) has two Al-Cl-Al bonds. Anyway, as far as I know AlCl3 is the stronger Lewis Acid in most apple-to-apple comparisons. To be honest, I don't think that if the Lewis Acid concept weren't a historically established meme, that it wouldn't be invented today. Personally, scales should actually be founded on measurement. The Lewis Acid strength scale is just sorta kinda correlated to what we can actually measure.