BIOL 3305 Lecture Notes - Lecture 21: Cyclobutadiene, Annulene, Antiaromaticity
Document Summary
The next highest energy mo"s are 2 and 3. Their overall energy is zero, which is non-bonding ( 2 and 3 both have two bonding and two anti-bonding interactions). The highest energy mo is 4, and comprises solely of antibonding interactions (and two nodal planes). The four electrons which have to be accommodated are arranged putting 2 electrons in 1, and one each in 2 and 3 (hund"s rule chapter 1). The mo picture predicts that cyclobutadiene should display diradical character (two unpaired electrons) in its ground state. This arrangement is not a closed bonding shell. Therefore mo theory correctly predicts that cyclobutadiene should be very reactive, and therefore unstable. Mo theory offers an explanation for the increased stability of benzene (6 electrons) and the increased instability of cyclobutadiene (4 electrons). The patterns of the mo"s for benzene and cyclobutadiene are similar to those found for the other annulenes and are shown above.
