1. One Equivalent of HBr Gives a Conjugated Diene Two Products
Adding a single HBr across 1,3-butadiene produces two constitutional isomers at once — the 1,2-adduct and the 1,4-adduct — not one clean product.
1,3-Butadiene + one equivalent of HBr — the same reaction also yields the 1,4-adduct shown below.
2. The Split Comes From a Resonance-Stabilized Allylic Cation
After H+ adds to an end carbon, the resulting cation is allylic, so its positive charge is delocalized over two carbons by resonance.
3. Bromide Traps the Nearer Carbon for the 1,2-Adduct
When Br- attacks the carbon next to where H added, the addition spans one original double bond and gives 3-bromo-1-butene.
4. Bromide Traps the Far Carbon for the 1,4-Adduct
When Br- attacks the far end of the shared system, the H and Br land on carbons 1 and 4, leaving a new double bond in the middle.
5. Temperature Chooses: Cold Gives Kinetic, Hot Gives Thermodynamic
Low temperature favors the faster-forming 1,2-adduct (kinetic), while high, reversible temperature favors the more-substituted, more stable 1,4-adduct (thermodynamic).
6. Summary
One HBr, two products · allylic cation shares charge over two carbons · 1,2 traps the near carbon · 1,4 leaves the double bond in the middle · cold → kinetic 1,2 · hot → thermodynamic 1,4 · builds on conjugation and stability and the Diels–Alder reaction.
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Because H+ adds first to make an allylic cation whose positive charge is delocalized over two carbons by resonance. Bromide can then attack either of those two carbons, so you get two constitutional isomers — the 1,2- and 1,4-adducts.
The 1,4-adduct (1-bromo-2-butene). H and Br add to carbons 1 and 4 of the conjugated system, which leaves a double bond between carbons 2 and 3 — right in the middle. The 1,2-adduct instead keeps the double bond at the end.
The 1,2-adduct — it is the kinetic product. At low temperature the reaction is effectively irreversible, so the product that forms faster (bromide traps the nearer, higher-charge-density carbon) accumulates, even though it is slightly less stable.
At high temperature the addition becomes reversible, so the system equilibrates to the more stable product. The 1,4-adduct has a more-substituted (internal) double bond, which is thermodynamically more stable, so it wins under thermodynamic control.
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