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

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 $\text{C"-"S}$ 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 ${\text{SO}}_{2}$: sulfur dioxide.
This works because ${\text{SO}}_{2}$ 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.