Show that the [4 + 4] cycloaddition of two butadiene molecules to give cycloocta-1,5-diene is thermally forbidden but photochemically allowed.

There is a different, thermally allowed cycloaddition of two butadiene molecules. Show this reaction, and explain why it is thermally allowed. (Hint: Consider the dimerization of cyclopentadiene.)

One milligram of a compound of molecular weight 160 is dissolved in 10 mL of ethanol, and the solution is poured into a 1-cm UV cell. The UV spectrum is taken, and there is an absorption at λ_max = 247 nm. The maximum absorbance at 247 nm is 0.50. Calculate the value of e for this absorption.

Match four of the following UV absorption maxima (λ_max) with the corresponding compounds: (1) 232 nm; (2) 256 nm; (3) 273 nm; (4) 292 nm; (5) 313 nm; (6) 353 nm.

Phenolphthalein is an acid–base indicator that is colorless below pH 8 and red above pH 8. Explain briefly why the first structure is colorless and the second structure is colored.

Classify the following dienes and polyenes as isolated, conjugated, cumulated, or some combination of these classifications.

(a) cycloocta-1,4-diene

(b) cycloocta-1,3-diene

(c) cyclodeca-1,2-diene

(d) cycloocta-1,3,5,7-tetraene

(e) cyclohexa-1,3,5-triene (benzene)

(f) penta-1,2,4-triene

Predict the products of the following reactions.

(a) allyl bromide + cyclohexyl magnesium bromide

Predict the products of the following reactions.

(b) cyclopentadiene + anhydrous HCl

Predict the products of the following reactions.

(c) 2-methylpropene + NBS, light

Predict the products of the following reactions.

(d) furan + trans-1,2-dicyanoethylene

Predict the products of the following reactions.

(e) buta-1,3-diene + bromine water

Predict the products of the following reactions.

(f) hexa-1,3,5-triene + bromine in CCl₄

Predict the products of the following reactions.

(g) 1-(bromomethyl)-2-methylcyclopentene, heated in methanol

Predict the products of the following reactions.

(h) cyclopentadiene + methyl acrylate, CH₂=CH–COOCH₃

Predict the products of the following reactions.

(i)

Show how the reaction of an allylic halide with a Grignard reagent might be used to synthesize the following hydrocarbons.

a. 5-methylhex-1-ene

Show how the reaction of an allylic halide with a Grignard reagent might be used to synthesize the following hydrocarbons.

b. 2,5,5-trimethylhept-2-ene

Show how the reaction of an allylic halide with a Grignard reagent might be used to synthesize the following hydrocarbons.

c. 1-cyclopentylpent-2-ene

Draw the important resonance contributors for the following cations, anions, and radicals.

(a)

Draw the important resonance contributors for the following cations, anions, and radicals.

(b)

Draw the important resonance contributors for the following cations, anions, and radicals.

(c)

Draw the important resonance contributors for the following cations, anions, and radicals.

(d)

Draw the important resonance contributors for the following cations, anions, and radicals.

(e)

(f)

A solution was prepared using 0.0010 g of an unknown steroid (of molecular weight around 255) in 100 mL of ethanol. Some of this solution was placed in a 1-cm cell, and the UV spectrum was measured. This solution was found to have λ_max = 235 nm, with A = 0.74.

(a) Compute the value of the molar absorptivity at 235 nm.

(b) Which of the following compounds might give this spectrum?

The diene lactone shown in part (a) has one electron-donating group (-OR) and one electron-withdrawing group (C=O). This diene lactone is sufficiently electron-rich to serve as the diene in a Diels–Alder reaction.

b. The Diels–Alder product A is not very stable. Upon mild heating, it reacts to produce CO₂ gas and methyl benzoate (PhCOOCH₃), a very stable product. Explain how this strongly exothermic decarboxylation takes place. (Hint: Under the right conditions, the Diels–Alder reaction can be reversible.)

Predict the products of the following proposed Diels–Alder reactions. Include stereochemistry where appropriate.

(a)

Predict the products of the following proposed Diels–Alder reactions. Include stereochemistry where appropriate.

(d)

Predict the products of the following proposed Diels–Alder reactions. Include stereochemistry where appropriate.

(e)

(f)

Predict the products of the following proposed Diels–Alder reactions. Include stereochemistry where appropriate.

(a)

(b)

(c)

Show how Diels–Alder reactions might be used to synthesize the following compounds.

(a)

(b)