Chapter 21

Use the qualitative VB and MO methods to predict whether addition of \(\mathrm{HCl}\) to 1,3 -cyclohexadiene would be favored to give 3-chlorocyclohexene or 4-chlorocyclohexene.

1,3-Butadiene has a substantial stabilization energy, whereas ethene has none, yet attack of \(\mathrm{Br} \oplus\) on \(1,3-\) butadiene occurs more readily than on ethene. Explain how 1,3-butadiene can have a stabilization energy greater than ethene but still be more reactive toward reagents that donate \(\mathrm{Br}^{\oplus}\).

Suggest reasons why (a) the stabilization energy of biphenylene is less than twice that of benzene, and (b) the heat of combustion of naphthalene is less than that of azulene.

Show how one can predict the stereochemistry of the electrocyclic rearrangement of trans,cis,trans \(-2,4,6-\) octatriene to 5,6 -dimethyl-1,3-cyclohexadiene by a favorable concerted thermal mechanism.

Which compound in each of the following pairs would lose chloride ion more readily and form a carbonium ion? Explain. and C=CCCCl C=CCCl C=CC(Cl)C=C and \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2} \mathrm{Cl} \quad(21 . \mathrm{E} .2)\) C=CC=CCCl

Bicyclo \([2.2 .0]-1(4)\) -hexene is highly strained and quite unstable. When it decomposes at room temperature, tetracyclo[6.2.2.0 \(\left.^{1,8} .0^{3,6}\right]-3(6)\) -dodecene is formed. \(^{10}\) a. Write a structure formula for the product. b. Show a reasonable sequence by which it might be formed, with the knowledge that bicyclo[ \(2.2 .0]-1(4)\) -hexene is an extraordinarily reactive dienophile in \([4+2]\) cycloadditions.

Bicyclo 15-hexadiene is much less stable than its isomer, benzene, yet it does not rearrange to benzene except at elevated temperatures. Give a reason for this observation.