Effect of pKa value of acid component in Mitsunobu Reaction? Mitsunobu Reaction is used to established C-S, C-C, C-N, C-O bond formation by condensation of acid component (Nuclephile) with primary and secondary alcohol in the presence of triphenyl phosphine (Ph3P) and diisopropyl azadicarboxylate (DIAD). 

Literature suggest that Mitsunobu reaction is only possible if The nucleophile (or pronucleophile) is normally a relatively acidic compound containing an O-H,
S-H, or an N-H group with pKa&lt;15, preferably below 11.

My question is that, why Mitsunobu reaction can't occur with acidic compound having Pka value more than 15 (pKa>15)?
I figured out the answe

The desired Mitsunobu reaction proceeds through path (a). On the contrary, in case of the reaction with less acidic HA (pKa>15) , the hydrazo anion (4) attacks the alkoxyphosphonium (5) directly to afford alkylated the hydrazine derivative (6) as a by-product path(b).

Since the anion (4) is not efficient in deprotonating the weakly acidic HA. In order to overcome these drawbacks, the acidic component should have pKa&lt;15 to get desire product.