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

AI Thread Summary
Ethyne exhibits higher acidity than ethene and ethane due to its sp hybridized carbon, resulting in a more polar C-H bond that facilitates proton release. The confusion arises from the relationship between bond energy and acidity; while weaker C-H bonds typically indicate higher acidity, the bond energy does not directly correlate with the energy required to abstract a proton. The bond energy for C-H bonds is defined through homolytic cleavage rather than ionic processes. Consequently, the bond energy analysis does not straightforwardly explain the observed acidity trends. Understanding this distinction clarifies the apparent contradiction in the bond energy and acidity relationship for these hydrocarbons.
Krushnaraj Pandya
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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
 
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Which bond energy? CC or CH?
 
Borek said:
Which bond energy? CC or CH?
CH, sorry for the ambiguity
 
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.
 
mjc123 said:
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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