Lecture 7 Saturday January 23 2016 10 55 AM Birch reduction Benzene does not want any more electrons If you look the LUMO of benzene is an anti bonding orbital So to put an electron there we need a strongly reducing conditions like Na in ammonia The product of the birch reduction is always a nonconjugated diene Mechanism Synthesis of alkyl benzenes we need to make the indicated C C bond There is only one way you know how to make a C C bond reaction of acetylide with an electrophile Based on this we can conclude that the desired product can be formed from For export Page 1 Lecture 7 Saturday January 23 2016 10 55 AM To make the bromide we do radical bromination of the benzylic position To finish we need Z selecrtive reduction of the alkyne to an alkene Lindlar catalyst will do that This is a good demonstration of how special reactivity at the benzylic position and low reactivity of benzene can be used in organic synthesis Antiaromaticity Cyclooctatetraene is cyclic conjugated polyalkene But it is not aromatic Reduction of it produces more than the reduction of 4 isolated alkenes This suggests that the four bonds in conjugation in the cyclooctatetraene are less stable than 4 isolated double bonds In fact this molecule avoids conjugation by odopting 3D structure that prevents conjugation Similarly cyclobutadiene is so unstable that it has not been isolated It has only been observed as an reactive intermediate In this molecule we can clearly identify single and double bonds Remember in bezene all bonds are the same length Both molecules are cyclic and can be fully conjugated However they have none of the special stability that comes from aromaticity that we saw in benzene In fact if they were flat both would be less stable than isolated alkenes For these molecules we say that they are anti aromatic Why are some cyclic conjugated systems aromatic and some anti aromatic For export Page 2 Lecture 7 Saturday January 23 2016 10 55 AM Huckel s rule If a number of electrons in a monocyclic conjugated pi system is 4n 2 the system is aromatic If it is 4n the system is anti aromatic In the case of 4pi or 8pi systems the distribution of pi electrons into pi molecular orbitals leads to an open shell electronic configuration with 2 unpaired electrons This leads to high reactivity and low stability of these molecules In most cases 4n pi systems will actually adopt 3D structure that will make them not be conjugated That can be achieved if p atomic orbitals are not parallel to each other We saw an example of this in cyclooctratetraene In cyclobutadiene molecule simply cannot adopt anything but a nearly planar arrangement of atoms and that is why it is particulalrly unstable The qualitative prediction of the energy distribution of molecular orbitals in cyclic conjugated polyenes can be obtained using Frost cycle Put the cyclic molecule into a cycle with one of the carbons at 12 o clock and where ever the circle touches the molecules there will be an orbital Aromatic compounds Annulenes from the book Aromatic cations and anions The high acidity of the cyclopentadiene is a result of the fact that the anion is aromatic The anion has a p atomic orbital in the same plane on 5 consecutive carbons making a conjugated cyclic system with 6 electrons in it As a result the anion is aromatic So contribution to the pi system does not have to be from pi bonds Anions can contribute as well For export Page 3 Lecture 7 Saturday January 23 2016 10 55 AM However the cation is aromatic Similarly Heteroaromatics In this case the anion is antiaromatic 8 electrons in the pi system conjugated cyclic pi system with 6 electrons Cation is the key for ensuring the cyclic conjugated system It does not contribute electrons but and empty p atomic orbital that ensures conjugation We have seen that electrons in the aromatic pi system may come from double bonds or anions they can also come from free electron pairs of hetero atoms N O S In each case the molecule is aromatic For export Page 4