Conjugated Systems CHEM 4201 Fall 2014 Dr. Sheppard

Conjugated systems Benzene Commonly found in nature as pigments, hormones Isoprene

outline Structure Reactions MO Theory UV Spectroscopy

I. Structure Consider molecules with multiple double bonds… Example: pentadienes cumulated (allenes) conjugated isolated separated by: 0 single bonds 1 single bond 2 or more single bonds Which is most stable? Why? Long answer = molecular orbital theory Short answer = orbital overlap

1,3-butadiene stability Unusually strong s bond from greater s-character of C atoms

1,3-butadiene stability Overlap of p atomic orbitals In its most stable conformation, the p orbitals are parallel Allows for p electron delocalization Single bond has partial C=C character, which also contributes to stability

Structure, cont. Conformations of 1,3-butadiene: s-cis (single cis) = double bonds on same side of single bond s-trans (single trans) = double bonds on opposite sides of single bond The two conformations easily interconvert at room temperature Which conformation is more stable?

Conformations of conjugated dienes Mild steric hindrance in s-cis conformation

II. Reactions of conjugated compounds Electrophilic addition The Diels-Alder reaction Pericyclic reactions