Química Orgânica 1 · Adição a alcenos e regioquímica
Grow a strained ring during HCl addition
Condições: 1-Methyl-1-vinylcyclobutane (1-ethenyl-1-methylcyclobutane) with HCl in a nonpolar solvent under ionic conditions. Follow the ring-expanded branch to 1-chloro-1,2-dimethylcyclopentane. Direct capture of the first cation and a competing methyl shift are not requested. Connectivity only: the cis/trans relationship of the two product methyl groups is not assessed.
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Protonate the vinyl group
HCl (pKa approximately −7) protonates the terminal carbon so that the cation forms on the internal carbon next to the ring.
As setas curvas
- The vinyl π pair bonds the terminal carbon to the acid proton, leaving an empty orbital on the carbon next to the ring.
- The H–Cl pair stays on chlorine as chloride.
Expand the ring
The secondary cation sits beside a strained four-membered ring. A ring C–C bond donates its electrons to form a bond to the cationic carbon. This makes a five-membered ring and leaves a tertiary cation on the methyl-bearing ring carbon.
Nota: Use a ring C–C bond at the carbon attached to the side chain. Either of its two equivalent ring bonds gives the same ring-expanded cation.
As setas curvas
- The ring C–C bond electrons form a new bond to the cationic carbon. That carbon becomes part of the ring, and the methyl-bearing ring carbon becomes the cation.
Capture the ring-expanded cation
Chloride from the first step completes the addition at the new cationic center.
As setas curvas
- Chloride donates a lone pair to the tertiary ring carbocation.
Ring-expanded alkyl chloride
The ring now contains five carbons. One ring carbon carries chlorine and the original methyl group. Its neighbor carries the methyl group formed by protonating the terminal alkene carbon. The product is 1-chloro-1,2-dimethylcyclopentane. Two driving forces act together: about 26 kcal/mol of cyclobutane ring strain is largely relieved, and the cation becomes tertiary.
O que você vai praticar
Use a ring-bond migration that relieves four-membered-ring strain and upgrades a secondary cation to a tertiary one.
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