Lecture06outline - I Review In considering cyclic-systems...

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I. Review • In considering cyclic π -systems for an analysis of aromaticity, all atoms in the ring must contain a p-orbital and all orbitals must be properly aligned (parallel) • Lone pairs count toward aromaticity if they reside in a p-orbital that is parallel with the rest of the system, but do not count if they are in an orthogonal sp 2 orbital • Hückel's 4n+2 rule extends to polycyclic molecules such as naphthalene (10 e ) and pentacene (8 e ) • The molecular orbitals of cyclic molecules can be drawn qualitatively with the help of a Frost diagram • To draw a Frost diagram, ennscribe the shape of the monocyclic molecule in a circle with one vertex at the absolute bottom. Relative orbital energies will be located at the points where the ring structure and the circle meet • Aromatic compounds have all of their electrons on orbitals that are low in energy while antiaromatic molecules have unpaired electrons in higher energy orbitals (at the equator of the Frost diagram)
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