12. DIELS-ALDER REACTION
heat
+
O
tetraphenylcyclopentadienone
hexaphenylbenzene
diphenyl acetylene
Readings:
• Pre lecture PowerPoint Notes
• Pages 460-463 in laboratory textbook, 6e OR pages 453–456 in 5e
• Pages 430-436 in McMurry textbook 9e
Homework problems: Exp # 10 - Study questions - posted on Canvas.
Assignments to submit for grading: Quiz & Notebook (No Postlab Questions)
This experiment demonstrates the Diels-Alder [4+2] cycloaddition reaction in the preparation of
hexaphenylbenzene. Diels-Alder reaction is a one-step reaction of a diene (in the s-cis form) and a
dienophile, which is reversible. The general reaction and mechanism are depicted in Figure 1.
C2
C3
diene
C1
C4
C6
C2
C5
C3
C1
C4
C6
C5
dienophile
Figure 1. General Diels-Alder Reaction
If the diene cannot convert from its s-trans (more stable form) to its s-cis form, the reaction will not
occur. The three π bonds (two from the diene and one form the dienophile) are used to form a sixmembered ring with at least one π and two σ bonds. The reaction is usually exothermic. The reaction is
even more favorable when the diene is fixed in the s-cis form as in the case of cyclopentadienone used
in this experiment. Even though a strained intermediate is formed (see hexaphenylbenzene structure
above) the overall reaction is favored because a resonance-stabilized aromatic ring is formed. The
reaction is irreversible because carbon monoxide is lost.
CSUSM
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