Unsymmetrical alkene
Br2 becomes polarised by nearby 𝛑 bond due to e- repulsion & becomes an electrophile Brẟ+- Brẟ- 𝛑 bond acts as a nucleophile and attacks the ẟ+ Br; 𝛑 bond breaks to form a C-Br covalent bond Heterolytic fission of Br2 and formation of carbocation intermediateUnstable carbocation intermediate attacked by Br- ion
Markovnikov’s Rule
• Hydrogen will attach to the carbon bonded to the greater number of hydrogens. (allow +ve charge on carbon with more alkyl groups)
• Formation of the more stable cation intermediate is favored
1. 𝛑 bond acts as a nucleophile and attacks the ẟ+ H; 𝛑 bond breaks to form a C-H covalent bond
2. heterolytic fission of HBr and formation of carbocation intermediate
3. Unstable carbocation intermediate attacked by Br- ion (propene onwards)
4. Theoretically, two carbocation intermediates (1o vs 2o) can be formed because of carbon’s position #
5. Hence, two products are formed (1-bromopropane & 2-bromopropane)
6. Experimentally, 2-bromopropane is formed (the major product) because it is made from a secondary carbocation intermediate which is more stable than the primary carbocation involved in making 1-bromopropane because it has a greater positive inductive effect.
*greater stability of carbocation : 3o>2o>1o
• due to the positive inductive effect (alkyl groups can donate electrons to stabilize positive charge)
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