[tyler1]

I'm working through organic chem as a second language. When studying proton acidity, the conjugate base's stability is considered. This book states that, in general, the factors that affect conjugate base stability are (in descending importance): type of atom, resonance, induction and orbital type. I came across hexen-5-yne, for which I must find the most acidic proton:
C1 and C6 have sp² and sp orbitals, respectively, which makes the sp proton the most acidic, followed by sp², and then sp³ being the least acidic. This makes sense. Though when I considered other possible hydrogens on the molecule, I ran into an issue. C3 and C4 should exhibit resonance once the corresponding protons are removed. Since resonance plays a more important role in acidity than orbital type (as stated above), I'd expect the protons of C3 and C4 to be more acidic than the protons of C1 and C6.

The answer says that the proton on C6 is the most acidic, which contradicts the conclusion I came to from applying the general principles. Is there a flaw in my reasoning? Is this just an exception, and if so, what explanation could be given? To add: how can resonance and orbital type be simultaneously considered to rank the protons of C1, 3, 4 and 6 in terms of acidity?

[pgk]

………….This book states that, <in general>, the factors that affect conjugate base stability are (in descending importance): <type of atom>, <resonance>, <induction> and <orbital type>……………….
But:
<in general> does not mean <always>,
<the type of atom> is the same in this case, regardless if being participant in sp, sp2 or sp3 orbitals,
<resonance> effect is the almost same in this case and
<induction> effect is the almost same in this case.
Consequently, the <orbital type> is the critical factor in this case.
Thus, the order of acidity is C6 > C1 > C4 ≈ C3 in this case.

[pgk]

Contrary, in the case of 4-pentyne-2-one or but-3-ynoic acid, the order of protons acidity is completely different because all the above factors are different.