Which of the following compounds will show acidic character?

The acidic character of alkynes arises from the presence of a hydrogen atom attached to an sp-hybridized carbon atom. This hydrogen, known as a terminal hydrogen, is more acidic than the hydrogens in alkanes or alkenes because of the increased s-character of the sp orbital.

In an sp-hybridized carbon, the s-character is 50%, which means the electrons are held closer to the nucleus. This causes the C–H bond to become more polarized, making the hydrogen more easily removable as a proton (H⁺).

Ethyne (C₂H₂), propyne (CH₃–C≡CH), and 1-butyne (CH₃–CH₂–C≡CH) all have this terminal hydrogen atom directly bonded to an sp-hybridized carbon. When treated with strong bases such as sodium amide (NaNH₂) or sodium metal (Na), these alkynes react to form acetylide ions (RC≡C⁻), demonstrating their acidic nature.

Examples:

1. 
   

   CH₃–C≡CH + Na → CH₃–C≡C⁻Na⁺ + ½H₂

   
   


2. 
   

   HC≡CH + NaNH₂ → HC≡C⁻Na⁺ + NH₃

   
   

In contrast, internal alkynes like 2-butyne (CH₃–C≡C–CH₃) do not have a terminal hydrogen and therefore do not exhibit acidity.

Relative acidity order:
Alkyne (sp) > Alkene (sp²) > Alkane (sp³)

This is because sp-hybridized carbons hold electrons closer, making the hydrogen more ionizable.

Key Points:

• 
  

  Terminal alkynes have acidic hydrogen atoms.

  
  


• 
  

  The acidity is due to the high s-character of sp-hybridized orbitals.

  
  


• 
  

  Reaction with strong bases forms metal acetylides.

  
  


• 
  

  Internal alkynes lack acidic hydrogens.

  
  

Therefore, ethyne, propyne, and 1-butyne all exhibit acidic character, while alkenes and alkanes do not.

✅ Correct Option: D) All of the above