[realdon]

The question is to list the hydrogen from most acidic to least acidic.

This is a straight forward question and through the number of resonance forms, one can see that it is B>C>A. However, my question is about the C-H bond strength for this exact structure. In my organic chemistry textbook, it shows that the allylic hydrogen(B) has the weakest strength (of 360kJ) (which is why it is the most acidic), followed by the alkyl hydrogen (A) (400 kJ), and finally the vinylic hydrogen (445 kJ). If one were to judge by these values and assuming that a weaker bond means that the hydrogen is easier to dissociate, it would be B>A>C, not B>C>A. I am not sure why these values do not correspond to the answer if you judged by the number of resonance forms. Can someone explain this to me? Thanks

[macman104]

Acidity also increases with increasing s character as the increase in s character stabilizes the negative charge after the loss of the hydrogen.  The hydrogen of CH₃CH₂-H is less acidic than the hydrogen of CH₂CH-H with pKa values of 50 and 35 respectively

[RBF]

I believe the bond strength data is for a homolytic bond breaking process in the gas phase, which is different than an acid-base process which involves heterolytic bond breaking.

[realdon]

Thanks a lot, guys, really appreciate the help