91Ó°ÊÓ

Show that the [4+2] Diels-Alder reaction is photochemically forbidden.

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

(b)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.)

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.

Question: Predict the product(s) of light-initiated reaction with NBS in CCl₄ for the following starting materials.

1. cyclohexene
2. 3,4-dimethylhex-3-ene
3. 

Question: When N-bromosuccinimide is added to hex-1-ene in $CC{l}_4$and a sunlamp is shone on the mixture, three products result.

1. Give the structures of these three products.
2. Propose a mechanism that accounts for the formation of these three products.

Question: When N-bromosuccinimide is added to hex-1-ene in $CC{l}_4$and a sunlamp is shone on the mixture, three products result.

1. Give the structures of these three products.
2. Propose a mechanism that accounts for the formation of these three products.

Addition of 1-bromobut-2-ene to magnesium metal in dry ether results in formation of a Grignard reagent. Addition of water to this Grignard reagent gives a mixture of but-1-ene and but-2-ene (cis and trans). When the Grignard reagent is made using 3-bromobut-1-ene, addition of water produces exactly the same mixture of products in the same ratios. Explain this curious result.

Show how you might synthesize the following compounds starting with bromobenzene, and alkyl or alkenyl halides of four carbon atoms or fewer.

a. 3-phenylprop-1-ene

b.5-methylhex-2-ene

c. dec-5-ene

Predict the products of the following proposed Diels-Alder reactions.

a)

b)

c)

d)

e)

f)

What dienes and dienophiles would react to give the following Diels-Alder products?

a)

b)

c)

d)

e)

f)