Question

Find most acidic hydrogen?


Answer 1

Identify familiar functional groups.

• `p` is an amide proton. `"pK"_a ~~ 15`
• `Y` is an alkyl proton para to a carbonyl. `"pK"_a > 50`
• `X` is an alkyl proton adjacent to a carbonyl. `"pK"_a ~~ 20`
• `Z` is an amine proton. `"pK"_a > 30`

Therefore, `p` should be the most acidic. Its conjugate base is the weakest base here, and is thus the most stabilized.

http://cactus.dixie.edu/smblack/chem2310/summary_pages/pka_chart.pdf

Answer 2

The most acidic hydrogens are (d) p.

Explanation:

(b) Y

Protons Y are alkane hydrogens. They are the least acidic.

`"p"K_text(a) ≈ 50`

(c) Z

Protons Z are amine hydrogens.

They are slightly more acidic than alkanes because `"N"` is more electronegative than `"C"` and an `"N-H"` bond is weaker than a `"C-H"` bond.

`"p"K_text(a) ≈ 35`

(a) X

Protons X are alpha to a carbonyl group.

They are more acidic because the electrons of the conjugate base can be delocalized onto the adjacent carbonyl group to form a resonance-stabilized enolate ion.

`"p"K_text(a)` values vary widely but for cyclohexanone `"p"K_text(a) ≈ 17`.

(d) p

Protons p are amide hydrogens.

The lone pair electrons can be delocalized onto the adjacent carbonyl group to form a resonance-stabilized anion.

`"p"K_text(a) ≈ 15`

Thus, protons p are the most acidic protons.