Out of Alkane, Alkene & Alkyne, which one is more acidic?

The acidity of hydrocarbons (alkanes, alkenes, alkynes) depends on the stability of the conjugate base formed when a hydrogen atom is removed. The more stable the conjugate base (carbanion), the stronger the acid.

1. Alkanes (sp³ carbon, pKa ≈ 50):

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  In alkanes, the carbon atom bonded to hydrogen is sp³ hybridized.

  
  


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  The carbanion formed after removing a proton is very unstable because the sp³ orbital holds electrons weakly (lowest s-character, only 25%).

  
  


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  Therefore, alkanes are the least acidic.

  
  

2. Alkenes (sp² carbon, pKa ≈ 44):

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  In alkenes, the hydrogen can be attached to an sp² hybridized carbon (33% s-character).

  
  


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  The higher s-character makes the conjugate base slightly more stable compared to alkanes.

  
  


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  Still, they are not considered acidic in general.

  
  

3. Alkynes (sp carbon, pKa ≈ 25):

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  In alkynes, the terminal hydrogen is attached to an sp hybridized carbon (50% s-character).

  
  


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  Greater s-character means the electrons are held closer to the nucleus, stabilizing the negative charge in the conjugate base.

  
  


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  This makes terminal alkynes significantly more acidic than both alkanes and alkenes.

  
  


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  Example: Ethyne (acetylene, HC≡CH) can react with strong bases like sodium amide (NaNH₂) to form acetylide ions, which are useful in organic synthesis.

  
  

Correct Order of Acidity:

Alkyne > Alkene > Alkane

Why option 3 (Alkyne) is correct?

Because alkynes have the highest s-character (50%) in their C–H bond, making them most acidic.

Why the other options are wrong?

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  Alkane (option 1): Least acidic, very high pKa.

  
  


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  Alkene (option 2): More acidic than alkane but much less acidic than alkyne.

  
  


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  All have same (option 4): Incorrect, acidity differs significantly.