Which bond in ethane breaks by homolysis at thermal conditions?

The carbon–carbon bond in ethane is most likely to break under homolytic thermal cleavage because its bond energy is lower than the carbon–hydrogen bond energy, so it requires less energy to break.

Think of the molecule like a pair of sticks tied together, where each stick represents a bond. The carbon–carbon stick has a smaller glue strength than the carbon–hydrogen sticks, so when you heat the molecule you are more likely to break the weaker glue first. Homolytic cleavage means the bond splits evenly so each atom takes one electron and becomes a radical, so breaking the carbon–carbon bond gives two carbon radicals while breaking a carbon–hydrogen bond gives a carbon radical and a hydrogen atom. Because the carbon–carbon bond needs less energy, heating favors its breakage over breaking one of the stronger carbon–hydrogen bonds. That is why, given the numbers, the carbon–carbon bond is the one to watch for thermal homolysis.

Students often think every bond in a molecule has the same chance to break, but weaker bonds break more easily, so compare bond energies rather than just counting bonds, and if you are unsure check the bond energy values before predicting reactivity. Another common mistake is confusing homolytic with heterolytic cleavage; homolytic gives one electron to each atom while heterolytic gives both electrons to one atom, so use the correct cleavage type when reasoning about radicals. Sometimes people assume temperature alone guarantees a particular break, but remember the required energy must be supplied and competing pathways or stabilization of radicals can change outcomes, so consider radical stability alongside bond strength.

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Predicting Which Bond Breaks in Ethane

1. Compare bond strengths
2. Weaker bonds break first
3. Homolytic cleavage gives radicals
4. C–C break yields two carbon radicals
5. Heating favors lower energy breaks

Common Mistakes Predicting Bond Cleavage

1. Assuming all bonds break equally
2. Confusing homolytic with heterolytic cleavage
3. Ignoring radical stability and competing pathways

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