To draw the product in each reaction, use curved arrows and begin at a " bond. Move the " electrons
to an adjacent carbon–carbon bond and continue in a cyclic fashion. In a ring-forming reaction, this
process forms a new ! bond that now joins the ends of the conjugated polyene. In a ring-opening
reaction, this process breaks a ! bond to form a conjugated polyene with one more " bond.
Whether the reactant or product predominates at equilibrium depends on the ring size of the
cyclic compound. Generally, a six-membered ring is favored over an acyclic triene at equilibrium.
In contrast, an acyclic diene is favored over a strained four-membered ring.