1,2- versus 1,4-addition
Add a reagent like HBr across an ordinary alkene and there is only one place for the two pieces to land. But add it across a conjugated diene and something richer happens: the diene offers two different landing patterns, and you get two products. This split — 1,2-addition versus 1,4-addition — is one of the most instructive demonstrations of how delocalization steers a reaction.
Here is the mechanism. The electrophile (say H+) adds first to one end of the diene, generating not a localized carbocation but an allylic cation whose positive charge is shared over carbons 2 and 4. Now the nucleophile (Br-) can attack either of those charged carbons. If it bonds to carbon 2 — right next to where H landed — you get the 1,2-addition product, with the leftover double bond at 3-4. If it bonds to the far carbon 4, you get the 1,4-addition product, with the double bond having shifted to the middle, at 2-3. Same starting diene, same reagent, two outcomes, because the intermediate cation is delocalized.
Which one wins depends on conditions, and this is the headline lesson in kinetic versus thermodynamic control. At low temperature the 1,2-product usually dominates: it forms faster (the nucleophile attacks the nearer, more positively charged carbon) — it is the kinetic product. At higher temperature, or given time to equilibrate, the 1,4-product takes over: it is usually more stable because its double bond is more substituted — it is the thermodynamic product. So by simply choosing the temperature, a chemist can dial the same reaction toward the fast product or the stable one — a beautiful, controllable example of one intermediate, two destinies.
Adding HBr to 1,3-butadiene at -80 degrees C gives mostly 3-bromo-1-butene (the 1,2-product), but warming the same mixture, or running it at +40 degrees C, gives mostly 1-bromo-2-butene (the more stable 1,4-product).
One allylic cation, two attack points: low temperature favours the fast 1,2-product, heat favours the stable 1,4-product.
The kinetic product is not necessarily the less stable one by rule; it is simply the one that forms faster — here the two often happen to differ, so this pair is the textbook illustration, but always reason from the actual rates and stabilities, not a memorized shortcut.