SolutionsConjugated Electrophilic Addition

1. Kinetic and thermodynamic control

Predict the result of the following processes:

1,1′-Bi(cyclohexenyl) with 1 equivalent of HBr at 90 °C and at −15 °C

At 90 °C the 1,4-addition product is obtained, with the double bond between the two rings; at −15 °C the 1,2-addition product, with bromine on the tertiary carbon, is also obtained

Protonation gives a tertiary allylic carbocation. At low temperature both products form (kinetic control); at high temperature equilibrium is reached and the 1,4-product, with the more stable tetrasubstituted double bond, predominates (thermodynamic control).

2. Where does the proton go?

In the addition of HCl to 1,3-pentadiene, why does the proton bond to C1 and not to C2 or C5?

Because this forms a secondary allylic carbocation, delocalized between C2 and C4. Protonation at C2 would give a non-conjugated primary carbocation, and at C5 a non-allylic secondary carbocation.

3. Products

Give the products of the addition of 1 equivalent of Br₂ to 1,3-butadiene at −80 °C and at 40 °C.

At −80 °C 3,4-dibromo-1-butene predominates (1,2-addition, kinetic control); at 40 °C, 1,4-dibromo-2-butene (1,4-addition, thermodynamic control), with the more stable disubstituted double bond.

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