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What is the reason behind it?

"Among sp3 hybridized systems, methane has the strongest C-H bond. C-H bonds on primary carbons are stronger than those on secondary carbons, which are stronger than those on tertiary carbons."

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Nature always try to acheive least potential energy for stability . Any organic reaction which makes the product less stable is not favourable .

If the C-H bonds are cleaved homolytically in all cases , the resulting stability of the free radicals will tell us about the feasibility of the reaction .

The primary free radical is the most unstable and to bring about its C-H bond breaking would mean increasing its energy comparitively . Hence its bond energy is the most and so its the stronger bond of the three .

Tertiary free radical is the most stable while secondary lies in between them . The same explanation can be used to draw out their comparitive strength of the C-H bonds .

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    $\begingroup$ Bond dissociation energy is defined based on homolytic cleavage, so you can't use carbocation stability as a rationale (although it turns out that the trend is the same). $\endgroup$ – jerepierre Feb 16 '15 at 2:35
  • $\begingroup$ @jerepierre does the free radical so formed follow the logic ? $\endgroup$ – Del Pate Feb 16 '15 at 19:41

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