Acidity and the bond energy paradox in ethane, ethene, ethyne

  • #1
Krushnaraj Pandya
Gold Member
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Homework Statement


Compare bond energy of ethane/ene/yne

2. The attempt at a solution
Comparing their acidity, ethyne has sp hybridized C so more polar CH bond, therefore H+ can be released easily. Therefore acidity order is ethyne>ethene>ethane which means CH bond is weakest in ethyne and so has lowest bond energy but this is in fact reversed, I'm very confused regarding this apparent contradictions between two known facts. I'd appreciate some help
 

Answers and Replies

  • #2
Borek
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Which bond energy? CC or CH?
 
  • #3
Krushnaraj Pandya
Gold Member
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Which bond energy? CC or CH?
CH, sorry for the ambiguity
 
  • #4
mjc123
Science Advisor
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The bond energy is not the energy required to abstract a proton.
I'm pretty sure anything you see quoted as the CH bond energy in ethane, for example, is defined by
C2H6 → 2C + 6H ΔH = E(CC) + 6 E(CH)
Even if it referred to a single bond cleavage, it would be a homolytic cleavage, not ionic, i.e.
C2H6 → C6H5⋅ + H⋅
not C2H6 → C6H5- + H+
Therefore acidity is not directly related to bond energy.
 
  • #5
Krushnaraj Pandya
Gold Member
697
73
The bond energy is not the energy required to abstract a proton.
I'm pretty sure anything you see quoted as the CH bond energy in ethane, for example, is defined by
C2H6 → 2C + 6H ΔH = E(CC) + 6 E(CH)
Even if it referred to a single bond cleavage, it would be a homolytic cleavage, not ionic, i.e.
C2H6 → C6H5⋅ + H⋅
not C2H6 → C6H5- + H+
Therefore acidity is not directly related to bond energy.
isn't a weaker bond CH bond generally considered more acidic?
 

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