Hi guys,
I was looking over the EK Orgo book , and i realized that when talking about bond energies the higher the bond energy the more stable the bond is, however when it goes on to talk about sigma and pi bonds it says that a sigma bond is stronger due to the electrons having less energy, I found this contradictory, is there anybody that can explain this please?
Thanks!
You confusion has to do with bond energy (the heat required to break the bond) and thermodynamic free energy (the amount of work a system can perform).
Sigma bonds are
lower in free energy (i.e. lesser ability to do work due to greater stability), so it takes a
greater input of energy to break that bond to over come that
greater stability; therefore, sigma bonds have a
greater bond energy.
Pi bonds are
greater in free energy (i.e. greater ability to do work to lesser stability), so it takes a
lesser input of energy to break that bond to over come the
lesser stability; therefore, pi bonds have a l
esser bond energy.
Although a sigma bond has a higher bond energy (kJ/mol) than a pi bond, as their names indicate, double and triple bonds constitute multiple bonds, so the summation of each individual bond energy creates a bond energy continuum as was earlier stated:
Bond Energy: triple (1sigma, 2 pi) > double (1 sigma, 1 pi) > single (1 sigma)
The lengths of the bonds have to due with molecular orbital theory (mathematical model for the mixing of orbitals). The trend for orbital size is as follows:
Orbital Size: s < sp < sp2 < sp3
In other words, a s-s molecular orbital (diatomic hydrogen) has the shortest bond length, and a sp3-sp3 molecular orbital (i.e. C-C bond in ethane [H3C-CH3] would have a longer bond length. The less p character a molecular orbital has (more s-character), the less the separation between the two atoms, which produces a shorter bond. It helps to look at the hybridization states of each atom when determining bond lengths, but the rule of thumb for bond length, single > double > triple, is true none the less.
Be careful with terminology as in the case of bond energy vs. free energy. I know the MCAT doesn't require one to understand the complete vernacular of a subject, but in the case of passage driven questions, a confusion such as that could throw off the entire meaning of the passage and lead to a few errors with the questions or taking too much time to answer them.
I hope this helps!
