Destroyer OCHEM #23

[Demps]

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in 2011 edition of destroyer ochem #23,

it asks you to pick out the most acidic proton.

answer is "a" - hydrogen off of phenol,

wouldn't allylic hydrogen, choice d be more acidic?

am I misunderstanding something here?

 

[PooyaH]

Allylic hydrogens are acidic but not as acidic as the hydrogen off of phenol. You can check their Ka's and you'll see. Also, the alpha hydrogens are more acidic than allylic hydrogens.

 

[rmm30]

There's a couple reasons why the phenol (pka apx 10) H is more acidic than allylic. While both have resonance stabilizing the negative charge, phenol is stabilized through a benzene ring. Which is both more resonance and higher quality than that of an allylic compound. The other reason is more obvious. Look at the atoms where the acidic proton is attached. The Oxygen atom is MUCH MUCH more electronegative than a carbon atom. THis means the negative charge will be stabilized better on the Oxygen than carbon, making the deprotonated phenyl a much weaker base, hence a stronger acid.

 

[rmm30]

you know what would really help. If you drew out the phenol and deprotonated it and drew the resonance structures. You would see there are 4 total. One has neg charge on O. The other three have the neg charge in the Orthos and Para position