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6. Conjugated Systems & Diels-Alder

When double bonds alternate with single bonds, the π electrons delocalize across the whole system. Conjugated dienes are more stable than isolated ones — and they unlock the most powerful ring-forming reaction in organic chemistry: the Diels–Alder.

Conjugation and 1,4-Addition

Adding HBr to 1,3-butadiene can give the 1,2-product (kinetic, favoured cold) or the 1,4-product (thermodynamic, favoured warm) — the allylic carbocation intermediate is resonance-stabilized, so the nucleophile can attack at either end.

The Diels–Alder Reaction

A conjugated diene plus a dienophile (an alkene with an electron-withdrawing group) forms a cyclohexene ring in one concerted step. The reaction is stereospecific, and the endo product — substituents tucked under the diene — is favoured.

Example: 1,3-Butadiene + maleic anhydride → 4-cyclohexene-1,2-dicarboxylic anhydride. The two new σ bonds form on the same face of the dienophile, so the cis relationship of the anhydride is preserved.

UV–Vis Spectroscopy

Longer conjugation means a smaller HOMO–LUMO gap and a longer wavelength of maximum absorption (λmax) — which is why β-carotene, with 11 conjugated double bonds, is orange.

Practice Problems

  1. What is the Diels–Alder product of cyclopentadiene + maleic anhydride?
  2. HBr adds to 1,3-butadiene to give two products. What are they called?
  3. Why is a conjugated diene more stable than an isolated diene?
  4. How does extending conjugation affect λmax in UV–vis?

Answers: 1. A bridged bicyclic adduct (endo) — norbornene-2,3-dicarboxylic anhydride   2. The 1,2-addition and 1,4-addition products   3. π-electron delocalization (resonance) lowers the energy   4. λmax shifts to longer wavelengths

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