Kinetic vs Thermo control (Org Chem)

[dl9006]

I understand kinetic control is the product from the fastest reaction and thermo control is the product from the most stable reaction.

But in the DAT destroyer 2008, the answer to question # 56 shows B as the transition state that leads to the product from thermo control.
(please refer to the rough picture that I drew in MS paint)

Isn't B a secondary cation and thus less stable than A which is a tertiary cation?

How can the most stable product result from a secondary cation vs tertiary cation?

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[nze82]

I understand kinetic control is the product from the fastest reaction and thermo control is the product from the most stable reaction.

But in the DAT destroyer 2008, the answer to question # 56 shows B as the transition state that leads to the product from thermo control.
(please refer to the rough picture that I drew in MS paint)

Isn't B a secondary cation and thus less stable than A which is a tertiary cation?

How can the most stable product result from a secondary cation vs tertiary cation?

First of all, let's make this clear. At low temperatures such as -40C, the dominant product is the kinetically controlled product (1,2 addition).

Regarding your question about the stability of a secondary carbocation vs. that of a tertiary carbocation, I must say that in this particular example, the positive charge on the secondary carbocation is being stabilized via resonance. So, you still get a pretty stable intermediate, which leads to production of the thermodynamically controlled product (1,4 addition).

 

[loveoforganic]

The thermodynamic product has nothing to do with the stability of the intermediate - only to do with the stability of the product. The 1,4-addition leads to the more substituted alkene, and thus the more stable product.