Question

Yeah, it's straight forward. Is CH₂ = CH₂ more acidic than PH3 or not? Why so or why not?

Any help or hint would be amazing. Thanks!

PS: I think ethylene should be more acidic. It has a carbon-carbon double bond, so the density of electron cloud would be higher in the areas of the double bond while the carbon-hydrogen single bond have way less electron density. This, plus the fact that phosphorus has lesser electron negativity than carbon should mean ethylene is more acidic.

Answer 1

Ethylene (CH₂=CH₂) is more acidic than phosphine (PH3) due to the electronegativity and hybridization of carbon. Carbon can stabilize the conjugate base better than phosphorus, resulting in a stronger acid.

Step-by-step explanation:

The question of whether CH₂ = CH₂ (ethylene) is more acidic than PH3 (phosphine) centers on the ability of these compounds to release a hydrogen ion (H+). Acidity is generally determined by the stability of the conjugate base formed after the loss of a proton. In the case of ethylene, the conjugate base would involve a carbon with a negative charge, while for phosphine it would be a phosphorus anion.

Based on general trends in the periodic table, across a row the electronegativity increases and so does the acid strength. Phosphorus is less electronegative than carbon, and as we know from compounds like CH4 (pKa approximately 50) and HF (pKa 3.20), the more electronegative an atom is, the more it can stabilize the negative charge of its conjugate base, making the compound a stronger acid.

However, ethylene has a pKa around 44, which makes it less acidic than many other substances, such as HF but still more acidic than alkanes. This is due to ethylene's sp2 hybridization, which can stabilize the conjugate base better than the sp3 hybridization found in alkanes. Therefore, PH3, which lacks the stabilizing effects found in ethylene, is less acidic than ethylene.