Q 17. Question:聽Using the Woodward鈥�... [FREE SOLUTION]

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Chapter 27: Q 17. (page 1091)

Question:Using the Woodward鈥揌offmann rules, predict the stereochemical pathway for each cycloaddition:
(a) a [6 + 4] photochemical reaction;
(b) an [8 + 2] thermal reaction.

Short Answer

Expert verified

(a) Antarafacial

(b) Suprafacial

Step by step solution

Step 1: Woodward-Hoffmann rule for cycloaddition

The cycloaddition reactions also abide by a set of rules known as the Woodward-Hoffmann rule. In this case, the number of pi-bonds refers to the total number of pi-bonds, including the two reacting systems.

For a given system, the mode of cycloaddition bonding is different in thermal and suprafacial reaction conditions.

If the number of pi bonds is even, then thermal reaction stereochemistry is antarafacial.

If the number of pi bonds is even, then photochemical reaction stereochemistry is suprafacial.

If the number of pi bonds is odd, then thermal reaction stereochemistry is suprafacial.

If the number of pi bonds is odd, then photochemical reaction stereochemistry is antarafacial.

[6+4] Photochemical reaction

According to the Woodward-Hoffmann set of rules, an [6+4] addition refers to two systems, one having 3 pi-bonds (6) and the other having 2 pi-bonds (4). Thus, the total number of pi-bonds in the system is 5, which is odd.

Hence, the mode of bond formation for a [6+4] cycloaddition under photochemical conditions is antarafacial.

An example of a [6+4] cycloaddition is given below:

[6+4] cycloaddition

[8+2] thermal reaction

According to the Woodward-Hoffmann set of rules, an [8+2] addition refers to two systems, one having 4 pi-bonds (6) and the other having 1 pi-bonds (4). Thus, the total number of pi-bonds in the system is 5, which is odd.

Hence, the mode of bond formation for a [6+4] cycloaddition under thermal conditions is suprafacial.