Which Of The Following Bases Can Deprotonate Acetylene

Which Bases Can Deprotonate Acetylene? A Deep Dive into Alkyne Acidity

Understanding which chemical species can remove a proton (H⁺) from acetylene (HC≡CH) is a fundamental question that reveals core principles of acid-base chemistry, specifically the concept of pKa and the strength of conjugate acids. Acetylene, a simple terminal alkyne, possesses a uniquely acidic hydrogen among hydrocarbons, but its acidity is still very weak when compared to common acids like water or acetic acid. Therefore, only a select group of exceptionally strong bases can successfully deprotonate it. This article will systematically explore the criteria for deprotonation, provide concrete examples of capable and incapable bases, and explain the underlying thermodynamic principles that govern this reaction.

The Acidic Nature of Acetylene: Why Is Its Hydrogen Removable?

The hydrogen atom attached to the sp-hybridized carbon in acetylene is acidic due to the high s-character of the sp orbital. The sp orbital holds electrons closer to the nucleus than sp² or sp³ orbitals, stabilizing the negative charge on the resulting acetylide anion (HC≡C⁻) through greater electronegativity. This stabilization makes losing a proton more favorable than for an alkene (sp²) or alkane (sp³) C-H bond. However, this does not make acetylene a "strong" acid in the everyday sense. Its pKa is approximately 25. For context, the pKa of water (H₂O) is about 15.7, and acetic acid (CH₃COOH) is about 4.76. A higher pKa value means weaker acidity. Therefore, acetylene is a very weak acid.

The key rule for any acid-base reaction is that it proceeds from a stronger acid and a stronger base to a weaker acid and a weaker base. To deprotonate acetylene, we must use a base whose conjugate acid has a pKa higher than 25. A base (B:) will deprotonate an acid (HA) only if the conjugate acid of the base (HB⁺) is weaker (has a higher pKa) than the original acid (HA).

General Equilibrium: HA + B: ⇌ A⁻ + HB⁺ The reaction favors the side with the weaker acid and weaker base.

For acetylene: HC≡CH (pKa ~25) + B: ⇌ HC≡C⁻ + HB⁺ This equilibrium lies far to the left unless HB⁺ has a pKa >> 25.

Step-by-Step Analysis: How to Determine if a Base Can Deprotonate Acetylene

To evaluate any base, follow this logical sequence:

1. Identify the Base (B:) and write its conjugate acid (HB⁺). This is the species formed when the base accepts a proton.
2. Find or estimate the pKa of HB⁺. This value is the critical determinant.
3. Compare pKa(HB⁺) to pKa(HC≡CH) ≈ 25.
   • If pKa(HB⁺) > 25, then HB⁺ is a weaker acid than acetylene. The equilibrium will favor the products, and deprotonation will occur to a significant extent. B: is a strong enough base.
   • If pKa(HB⁺) < 25, then HB⁺ is a stronger acid than acetylene. The equilibrium will strongly favor the reactants (acetylene and the base). Deprotonation will not occur to any practical extent. B: is too weak.

Examples of Bases That CAN Deprotonate Acetylene

These bases have conjugate acids with pKa values well above 25.

1. Sodium Amide (NaNH₂) in Liquid Ammonia (NH₃): This is the classic, textbook reagent.

   • Base: NH₂⁻ (amide ion)
   • Conjugate Acid: NH₃ (ammonia), pKa ≈ 38.
   • Analysis: 38 > 25. Ammonia is a much weaker acid than acetylene. The reaction goes to completion: HC≡CH + NH₂⁻ → HC≡C⁻ + NH₃. The acetylide anion is generated quantitatively in this solvent system.

2. Organolithium Reagents (e.g., n-BuLi, CH₃Li):

   • Base: RLi (where R = alkyl, e.g., butyl, methyl). The base is essentially R⁻ (carbanion) in the solvent.
   • Conjugate Acid: RH (alkane), pKa ≈ 50.
   • Analysis: 50 >> 25. Alkanes are extraordinarily weak acids. Organolithiums are so basic that they will deprotonate acetylene violently and are often used in synthesis to make acetylides, typically in hydrocarbon solvents like hexane or THF.

3. Sodium Hydride (NaH):

   • Base: H⁻ (hydride ion)
   • Conjugate Acid: H₂ (hydrogen gas), pKa ≈ 35 (in DMSO, a common reference for such comparisons).
   • Analysis: 35 > 25. Molecular hydrogen is a weaker acid than acetylene. NaH is a powerful, insoluble base commonly used in organic synthesis to generate acetylides from terminal alkynes.

4. Grignard Reagents (RMgX): While less basic than organolithiums, they can still deprotonate acetylene.

   • Base: The carbanion character (R⁻) of the Grignard reagent.
   • Conjugate Acid: RH (alkane), pKa ≈ 50.
   • Analysis: 50 >> 25. Deprotonation is favorable. However, Grignard reagents are often used in ether solvents and can be consumed by other acidic protons if present.

5. Potassium tert-Butoxide (KOtBu) in Dimethyl Sulfoxide (DMSO): This is a borderline case that highlights the importance of solvent.

   • Base: t-BuO⁻ (tert-butoxide)
   • Conjugate Acid: t-BuOH (tert-butanol), pKa ≈ 18 in water, but ~32 in DMSO.
   • Analysis: In DMSO, the pKa scale is different