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Chapter 28: Q44P (page 1239)
A student found that heating any one of the isomers shown here resulted in scrambling of the deuterium to all three positions on the five-membered ring. Propose a mechanism to account for this observation.
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Problem:a. Under thermal conditions, will ring closure of (2E,4Z,6Z,8E)-2,4,6,8-decatetraene be conrotatory or disrotatory?
b. Will the product have the cis or the trans configuration?
c. Under photochemical conditions, will ring closure be conrotatory or disrotatory?
d. Will the product have the cis or the trans configuration?
Account for the difference in the products obtained under photochemical and thermal conditions:
a. Propose a mechanism for the following reaction. (Hint:An electrocyclic reaction is followed by a Diels–Alder reaction.)
b. What would be the product if trans-2-butene were used instead of ethene?
Propose a mechanism for the following reaction:
Problem:a. For conjugated systems with two, three, four, five, six, and seven conjugated bonds, construct quick MOs (just draw the lobes at the ends of the conjugated system as they are drawn on pages 1220 and 1221) to show whether the HOMO is symmetric or antisymmetric.
b. Using these drawings, convince yourself that the Woodward–Hoffmann rules in Table 28.1 are valid.
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