Química Orgânica 2 · Conjugação e cicloadição
Connect delocalization to bond formation
Condições: Follow HBr addition to 1,3-butadiene with H added to the left terminal carbon C1 and Br ultimately bonded to the right terminal carbon C4. This is a specified connectivity pathway, not a prediction of exclusive E/Z selectivity; a final check compares the 1,2- and 1,4-adducts when the additions are reversible.
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Form the allylic intermediate
Draw both arrows for protonation; retain bromide as the counterion.
As setas curvas
- The left π pair forms the C1–H bond, leaving an adjacent allylic cation.
- The acid bond pair stays with bromine, forming bromide.
Represent the same delocalized ion
This is resonance within one allylic intermediate, not a timed conversion into a different species. The next reaction is shown using this alternative contributor.
As setas curvas
- Move the right π pair toward C2 to display the contributor with positive charge at C4.
Trap the specified terminal site
Capture at C4 gives the specified 1,4 connectivity; capture at C2 would instead give a different addition pattern.
As setas curvas
- Bromide donates a pair to C4, making the C–Br bond and neutralizing both ions.
Compare the 1,2- and 1,4-adducts
Both adducts come from one allylic cation. Cold, irreversible addition favors the faster 1,2-adduct (kinetic control). When the adducts can re-ionize, as at about 40 °C, they interconvert through the allylic ion pair and the more stable 1,4-adduct, whose C=C is internal and disubstituted, accumulates (thermodynamic control). No exact ratio is asserted.
The remaining double bond
H and Br have added at the original diene ends, and the remaining double bond is C2=C3. All atoms and net zero charge are retained. The drawing represents one geometry of the connectivity product; it does not establish an exclusive E/Z outcome or a 1,2:1,4 ratio.
O que você vai praticar
Trace the specified 1,4-addition pathway through an allylic intermediate while distinguishing resonance from a reaction.
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