breaking double bonds?

[Philip Lee 1L]

Suppose a reactant molecule has a double bond between two carbon atoms, and the product molecule has only a single bond between these two carbon atoms. When doing calculations with bond enthalpies, why does the double bond break completely and then reform into a single bond (rather than just one bond breaking in the double bond)?

[Matthew Tran 1H]

This is because bond enthalpies are not additive; that is, the bond enthalpy of a double bond is not double that of a single bond. You can verify this by looking at the table of bond enthalpies.

[Iris Bai 2K]

Dr. Lavelle said in lecture that double bonds have to break completely in order to form a single bond between the same two atoms. It's probably because a single bond isn't actually half the energy of a double bond, so you can't just break one of the bonds in a double bond.

[Kevin ODonnell 2B]

Just to give some evidence to what everyone else is saying. A carbon-carbon single bond has a length of 154 and energy of 348, a carbon-carbon double bond has a length of 134 and energy of 614, while a carbon-carbon triple bond has a length of 120 and energy of 839. Here we can see that a double bond is not just double the energy of a single bond, and a triple bond is not just triple the energy of a single bond.

[Jordan Lo 2A]

If you imagine a double bond as the sigma and pi bond that we learned in 14A, I think it makes sense that you can't just halve the value of a double bond to get the value of a single bond because the first and second bonds hold the atoms together in different ways

[005113695]

single bonds are not just the half of a double bond. instead to break a double bond and form a single bond, the double bond has to break completely

[Aili Ye 4L]

Double and triple bonds don't have the bond enthalpy equal to double or triple that of a single bond, so you must calculate single and double bond breaks differently.