Part C. Carbocation Stability A number of reactions are thought to involve carbo

Question

Part C. Carbocation Stability A number of reactions are thought to involve carbocation intermediates (positively charged carbon species). One example of such a reaction is the addition of HBr to 2-butene: HBr CH3 In some cases, more than one carbocation may form in a reaction thereby leading to the possibility of more than one product. An example would be the addition of HBr to 2-methyl-2- butene. This reaction can produce two carbocations and two products as shown below H CH CHT CH 3 Br CH3 CH3 CH H HBr + CH Br CH3 H CH3 H By investigating the stabilities of the two possible carbocations involved in this reaction, we can predict which product is preferred in the addition of HBr to 2-methyl-2-butene. Start by drawing 2-methylbutane. Remove one of the hydrogen atoms to create either the secondary or tertiary carbocation. Run an Optimization calculation being sure to first set the Charge to +1. Record the energy of your optimized structure and repeat the same procedure for the other carbocation Optimized Energy3.13au 93 232 a tertiary carbon secondary carbon Questions: Predict the products for the addition of HBr to 2-methyl-2-butene.

Explanation / Answer

If we consider the stability of carbocations the order of stability is as follows

Tertiary >secondary>primary

As well as if we consider the optimization energy for stable carbocations it has less value compared to secondary.

By using thermodynamic stability concepts highly stable compounds will form at lower energy...therefore by using these concepts we can give a conclusion as

The product for the addition of HBr to 2-methyl 2 -butene will be 2-bromo 2-methyl butane..

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