Arrange the following labelled hydrogens in decreasing order of acidity:

1. Carboxylic Acid Hydrogen (b)

• Location: Part of the -COOH group attached to the aliphatic ring.
• Analysis: When H_b is removed, the negative charge is delocalized over two highly electronegative oxygen atoms via resonance. Generally, carboxylic acids are significantly more acidic than phenols and alkynes because of this equivalent resonance stabilization.
• Conclusion: H_b is the most acidic hydrogen.

2. Phenolic Hydrogens (c and d)

Both H_c and H_d belong to phenolic -OH groups attached directly to the benzene ring. When they are removed, the negative charge is stabilized by resonance into the aromatic benzene ring.

To determine which one is more acidic between c and d, we must analyze the electronic effects of the strongly electron-withdrawing nitro group (-NO2):

• Hydrogen c: * The -OH group holding H_c is located ortho (adjacent) to the -NO2 group.
  • The nitro group exerts both a strong negative inductive effect (-I) and a strong negative resonance effect (-M) at the ortho position. This dramatically pulls electron density away, stabilizing the resulting phenoxide ion exceptionally well.
• Hydrogen d:
  • The -OH group holding H_d is located meta to the -NO2 group.
  • Resonance effects (-M) do not operate at the meta position; the nitro group can only stabilize this conjugate base through its weaker, distance-dependent negative inductive effect (-I).
• Conclusion: Because the ortho position receives both -M and -I stabilization while the meta position only receives -I, H_c is more acidic than H_d (c > d).

3. Terminal Alkyne Hydrogen (a)

• Location: Attached to an sp-hybridized carbon atom (C triple bond C).
• Analysis: While an sp-hybridized carbon is relatively electronegative for a carbon atom (holding onto a negative charge better than sp2 or sp3 carbons), carbon is still fundamentally far less electronegative than oxygen. Consequently, a terminal alkyne is much less acidic than both carboxylic acids and phenols.
• Conclusion: H_a is the least acidic hydrogen.

Final Order of Acidity

Combining all our findings from strongest to weakest:

b > c > d > a