1. Conjugated Systems
CHEM 4201
Fall 2014
Dr. Sheppard

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

3. outline
Structure
Reactions
MO Theory
UV Spectroscopy

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

5. 1,3-butadiene stability
Unusually strong s bond from greater s-character of C atoms

6. 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

7. 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?

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

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