Electrophilic Addition of Hydrogen Bromide to 2-Methylpropene

Electrophilic Addition of Hydrogen Bromide to 2-Methylpropene THE OVERALL REACTION  

Step 1 This is the rate-determining step and is bimolecular. Protonation of the double bond occurs in the direction that gives the more stable of two possible carbocations. In this case the carbocation is tertiary. Protonation of C-2 would have given a less stable secondary carbocation. 

Step 2 This step is the combination of a cation (Lewis acid, electrophile) with an anion (Lewis base, nucleophile) and occurs rapidly. 

Higher energy transition state gives primary carbocation 

Energy diagram comparing addition of hydrogen bromide to 2-methyipropene according to Markovnikov s ruie (soiid red curve) and opposite to it (dashed biue curve). is iess, and the reaction is faster for the reaction that proceeds via the more stabie tertiary carbocation. 

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Mechanism 6.2 outlines the two-step sequence for the electrophilic addition of hydrogen bromide to 2-methylpropene. 

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