Vladimir markovnikov 1838 1904 9 in the rate

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Vladimir Markovnikov (1838-1904)
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9 In the rate determining step: C H 2 H B r - f a s t B r H We could have obtained two structurally different carbocations; however, A is formed much faster than B . This immediately reacts with Br - in the 2nd step. H B r s l o w C H 2 H + C H 2 H A B The preference for product A is explained by the mechanism : Carbocation Stability: Hyperconjugation More substituted carbocations are more stable empty p orbital The C-H σ -bond on the neighboring carbon lines up with the empty p orbital and can donate electron density to the carbocation. (How about the methyl cation??) Also: Alkyl groups are electron- donating and can stabilize carbocations by donating electron density through the σ -bond. (sp 2 center more electronegative than sp 3 )
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10 Carbocation Stability 3° allylic > 2° allylic 3° alkyl > 1° allylic 2° alkyl > 1°alkyl > methyl These trends are important to remember! Overall reaction coordinate for 3º halide rate determining step + HBr CH 3 Br CH 3 Br
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11 The carbocation stabilities affect their rates of formation H H H H H H
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