Find most acidic hydrogen?

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Feb 10, 2018

Answer:

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.

www.chem.ucalgary.ca

#"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.

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Write your answer here...
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Explanation

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Feb 10, 2018

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

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