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Shouldn't ethylene should be more acidic? It has a C=C bond, so the density of electron cloud would be higher in the areas of the double bond while the C-H single bond have way less electron density. This, plus the fact that carbon has higher electron negativity than phosphorus should mean ethylene is more acidic as C-H bond will be more weakened than P-H bond and thus, C-H bond will break relatively easily, releasing H+ ions more frequently.

However that seems to be not the case (as discussed in this question Is ethylene more acidic than phosphine?).

Why so? Or am I wrong? Any help or discussion is welcomed. Thanks!

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Phosphorus, a third-period element, forms a weaker bond to hydrogen than carbon; see for instance https://chem.libretexts.org/Courses/Valley_City_State_University/Chem_121/Chapter_8%3A_Chemical_Bonding_and_Molecular_Structures/8.6%3A_Bond_Enthalpies. Thus even though phosphorus has a relatively low electronegativity, the dissociation of a hydrogen ion costs less energy with phosphorus than with most hydrocarbons.

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