Question

how to identify acidic hydrogen

Answer 1

To identify an acidic hydrogen, assess the stability of the conjugate base formed after its removal as a proton ($H^+$). The more stable the conjugate base, the more acidic the hydrogen.

Answer 2

To identify an acidic hydrogen, the primary principle is to assess the stability of the conjugate base formed after the removal of that hydrogen as a proton ($H^+$). The more stable the conjugate base, the more acidic the hydrogen.

Here are the key factors that enhance the stability of the conjugate base, thereby increasing the acidity of the hydrogen:

1. Electronegativity of the Atom Bonded to Hydrogen:

   • Acidity increases across a period as the electronegativity of the atom bonded to hydrogen increases, allowing it to better accommodate the negative charge.
   • Order: $H-F > H-O > H-N > H-C$. For example, carboxylic acids (O-H) are more acidic than amines (N-H), which are more acidic than alkanes (C-H).

2. Hybridization of the Atom Bonded to Hydrogen (for Carbon):

   • For C-H bonds, acidity increases with higher s-character in the hybrid orbital. An orbital with more s-character is closer to the nucleus, providing better stabilization for the negative charge.
   • Order: $sp > sp^2 > sp^3$. Terminal alkynes ($sp$ C-H) are more acidic than alkenes ($sp^2$ C-H) or alkanes ($sp^3$ C-H).

3. Resonance Stabilization:

   • If the negative charge on the conjugate base can be delocalized over multiple atoms through resonance, it is significantly stabilized.
   • Examples:
     • Hydrogens alpha to carbonyl groups (e.g., ketones, aldehydes, esters) form resonance-stabilized enolate anions.
     • Protons in carboxylic acids and phenols lead to resonance-stabilized carboxylate and phenoxide ions, respectively.
     • Allylic or benzylic protons can be acidic if the resulting carbanion is resonance-stabilized.

4. Inductive Effects:

   • Electron-Withdrawing Groups (EWGs): Groups that pull electron density away from the negative charge (e.g., halogens, nitro, cyano) stabilize the conjugate base, increasing acidity. The effect decreases with distance.
   • Electron-Donating Groups (EDGs): Groups that push electron density towards the negative charge (e.g., alkyl groups) destabilize the conjugate base, decreasing acidity.

5. Aromaticity of the Conjugate Base:

   • If the removal of a proton leads to a conjugate base that gains aromatic stability (i.e., it becomes cyclic, planar, fully conjugated, and contains (4n+2) π electrons following Hückel's rule), the hydrogen is exceptionally acidic.
   • Example: The $sp^3$ protons of cyclopentadiene are highly acidic (pKa ≈ 16) because its conjugate base, the cyclopentadienyl anion, is aromatic.

In summary, to identify an acidic hydrogen, look for:

• Hydrogen atoms bonded to highly electronegative atoms (O, N, S).
• Hydrogen atoms on carbons with higher s-character (e.g., terminal alkynes).
• Hydrogen atoms that, upon removal, lead to a conjugate base that is resonance-stabilized, inductively stabilized by EWGs, or gains aromaticity.