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Chapter 15: 15-19P (page 782)

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

Short Answer

Expert verified

In the Diels-Alder cycloaddition, the LUMO of the dienophile and the excited state of the HOMO of the diene (HOMO*) generate one bonding and one antibonding interaction. Hence, it is a photochemically forbidden process.

Step by step solution

01

Diels-Alder reaction

Diels-Alder reactions are a kind of cycloaddition reaction. Two molecules link in a single step to generate a new ring in this reaction.

02

Forbidden cycloaddition

If a cycloaddition creates an overlap of positive-phase p orbitals with the negative-phase orbitals, antibonding interactions are formed. These interactions increase the activation energy and are symmetry forbidden.

03

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

When ultraviolet light is used instead of heat for [4+2] Diels-Alder reaction, it is photochemically not allowed. A photon with the appropriate energy strikes 1, 3-butadiene, one of the pi electrons gets excited to the next higher molecular orbital. This higher molecular orbital is occupied and is the new (HOMO*) of the excited state.

Bonding and the antibonding interaction of the Diels-Alder cycloaddition

In the Diels-Alder cycloaddition, the LUMO of the dienophile and the excited state of the HOMO of the diene(HOMO*) generate one bonding and one antibonding interaction. Hence, it is a photochemically forbidden process.

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Most popular questions from this chapter

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.

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

a)

b)

c)

d)

e)

f)

(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.)

Question: When Br2 is added to buta-1,3- diene at -150C , the product mixture contains 60% of product A and 40% of product B. When the same reaction takes place at -600C, the product ratio is 10% A and 90% B.

a. Propose structures for products A and B (Hint: In many case, an allylic carbocation is more stable than a bromonium ion.)

b. Propose a mechanism to account for formation of both A and B.

c. Show why A predominates at -150C and B predominates at -600C .

d. If you had a solution of pure A, and its temperature were raised to -600C , what would you expect to happen ? Propose a mechanism to support your prediction.

Question: The central carbon atom of an allene is a member of two double bonds, and it has an interesting orbital arrangement that holds the two ends of the molecule at right angles to each other.

  1. Draw an orbital diagram of allene, showing why the two ends are perpendicular.
  2. Draw the two enantiomers of penta-2,3-diene. A model may be helpful.
See all solutions

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