Explain why the other diastereomeric cation is not formed...

Explain why the other diastereomeric cation is not formed (Hint: You may find it helpful to construct a molecular model.). A. only one face of the empty p orbital is accessible to the migrating carbon atom. B. there is lack of conformational freedom imposed by the tricyclic system. A only B only Both A and B None

When cyclohexadiene (A) reacts with $\mathrm{Br}_{2^{\prime}}$ a mixture of cis- and trans-1, 2 -addition products is formed (in addition to other products). However, when cyclohexene (B) reacts with $\mathrm{Br}_{2}$ under identical conditions, only trans product is observed. What is the best explanation for the observed difference in stereochemistry of the addition? (a) The cis and trans products are the result of aromaticity in the cyclic TS for reaction of A. In $B$ there are only four electrons in TS, and cyclic TS is destabilised. (b) Reaction of A proceeds through an intermediate that has an $\mathrm{sp}^{3}$ -hybridised carbocation, while the analogous intermediate in reaction of B has sp-hybridised carbocation. (c) Both reactions occur through bromonium ions, but because of planarity enforced by neighbouring double bond, cis addition is not sterically hindered in A. (d) B reacts through a bromonium ion intermediate, while A does through an allyl cation.

Advanced Problems in Organic Chemistry for Competitive Examinations

Hydrocarbons

Level I

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