Read Conjugation Before Predicting a Diene's Addition Product
Conjugated dienes react differently from isolated ones because the two double bonds share a delocalised π-system — this single structural fact explains both their unusual bond lengths and why they give a MIXTURE of addition products.
Dienes: Conjugation, Stability and 1,2- vs 1,4-Addition is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Conjugation shows up directly in bond length
In 1,3-butadiene, the C₁-C₂ and C₃-C₄ bonds (nominally 'double') are slightly LONGER than a typical isolated C=C, while the C₂-C₃ bond (nominally 'single') is slightly SHORTER than a typical isolated C-C — partial delocalisation across all four carbons averages the bond orders somewhat, a direct structural signature of conjugation.
1,2- and 1,4-addition compete because the intermediate cation is delocalised
Adding one equivalent of Br₂ to 2,4-hexadiene at 0°C gives a mixture of two products: the 1,2-addition product (Br₂ across one specific double bond) and the 1,4-addition product (Br atoms end up on C₁ and C₄, with the remaining double bond shifted to C₂-C₃) — both come from the SAME delocalised allylic cation intermediate, which has partial positive charge at both C₂ and C₄, letting bromide attack either position.
Allylic and benzylic radicals are stabilised by the same delocalisation
Ranking free-radical stability follows the same logic as cation stability: a radical that can delocalise into an adjacent π-system (allylic, next to a C=C; or benzylic, next to an aromatic ring) is far more stable than a simple alkyl radical with no adjacent π-system to share the unpaired electron with.
Cumulated dienes (allenes) are non-planar
In an allene (C=C=C), the two π-systems at the central carbon are perpendicular to each other (each terminal CH₂ plane is rotated 90° relative to the other) — this makes the WHOLE molecule non-planar, unlike a conjugated diene (all p-orbitals parallel, fully planar) or an isolated alkene.
OlympIQ exam lens
- ✓Conjugation lengthens the 'double' bonds and shortens the 'single' bond in a conjugated diene, relative to isolated single/double bonds.
- ✓1,2- and 1,4-addition compete for conjugated dienes because the intermediate allylic cation is delocalised across both possible attack positions.
- ✓Allylic and benzylic radicals are stabilised by delocalisation into an adjacent π-system, just like allylic/benzylic cations.
- ✓Cumulated dienes (allenes) are non-planar — their two perpendicular π-systems force the terminal CH₂ groups out of a shared plane.
Don't assume a conjugated diene gives only the 1,2-addition product from limited electrophilic addition — the delocalised intermediate genuinely gives a mixture of 1,2- and 1,4-products, and which one dominates can depend on temperature (lower temperature favours the kinetic 1,2-product in many cases).
Ready to try the pattern?
100% coverage across the 5 linked questions.
Practise 5 linked questions