Electrons in opposite phases cancel out. The lower energy of bonds comes from electrons being in between atoms, the higher energy of anti-bonds comes from a lack of electrons in between the atoms.
The method of considering how phases can overlap can be extended, such as by having three pi orbitals next to eachother, as in an allyl anion:
Only adjacent orbitals are considered to overlap. The lowest level has two in-phase overlaps and the highest has two out-of-phase overlaps. The middle MO is non-bonding, since the central carbon can point a pi orbital in either direction and it would still produce one in-phase overlap and one out-of-phase, producing a net bond order of 0.
All three carbons are sp2 hybridized, so the lone pair on the carbon is in a p orbital. This means the energy gained from overlaping three p orbitals is greater then the energy gained from having one sp3 carbon and just two pi-bonded carbons. The allyl only adopts the former structure because it is lower in energy.
Notice that the above MO energy levels also predicts the stability of an ally radical or an ally cation, because producing them just means one or two less electrons in a non-bonding orbital. Also, de-localisation of charge or radicals is in itself a stabilisation affect.
Below are two different molecules being homolytically cleaved. The second type requires less energy to cleave because of the extra stability of the product, you should be able to explain why:
The same techique is applied to butadiene below:
Why not use an allene? (http://en.wikipedia.org/wiki/Allene)
ReplyDeleteAn allene does not have three overlaping p orbitals. You can see why by considering the central carbon: it uses two p orbitals which by nature are perpendicular to eachother.
DeletePicture here: http://i.imgur.com/XQ20r.png
why is the middle orbital drawn bigger in the fourth picture?
ReplyDelete