Question

In a Diels-Alder Reaction, 3-sulfolene produces a 1,3-butadiene and a gas. What is the gas that is produced?

Answer 1

This is actually a reverse Diels-Alder, also known as a decomposition reaction in general. We know that from how one reactant generates two.

The mechanism for this is fairly intuitive if one realizes that once 1,3-butadiene is to be formed in the mechanism, it will imply breaking the two `"C"-"S"` bonds in 3-sulfolene to generate 1,3-butadiene.

If you work through drawing this out, you would see that breaking these bonds will generate `"SO"_2`: sulfur dioxide.

This works because `"SO"_2` can be a Lewis acid (that is seen in the upper-right curved arrow).

CHALLENGE: Can you draw the forward reaction? Hint: Sulfur is the source of the donated electrons, instead of a `pi` bond on a more typical dienophile.