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.


1 Answer 1


It seems to be extraordinarily difficult to find a figure for the pKa of a phosphine proton (P-H). However the preparation of alkylsilylphosphines has been reported by nBuLi deprotonation of phosphine here and sodium dimethyl phosphide has been prepared by deprotonation of dimethyl phosphine using sodamide here. This leads me to feel that the pKa is somewhere beyond 30, but less than that of ammonia which is estimated to be 38 source . This is less than ethylene which is estimated to be ~45-50 source here

  • $\begingroup$ Thank you, sir for this piece of information. However, I'm very curious about the theoretical aspect of this. Why is the pKa of ethylene so high? Due to the low density of electron present in the C-H bond in electron, shouldn't it be the opposite? $\endgroup$
    – 74H54N3
    Commented Jan 17, 2023 at 14:52
  • 2
    $\begingroup$ Maybe someone else can answer this, I cannot. Otherwise it would make a good follow-up question. $\endgroup$
    – Waylander
    Commented Jan 17, 2023 at 15:12

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