In a published synthetic procedure, acetone is treated with ethenyl (vinyl) magnesium bromide, and the reaction mixture is then neutralized with strong aqueous acid. The product exhibits the ¹H NMR spectrum shown below. What is its structure? When the reaction mixture is (improperly) allowed to remain in contact with aqueous acid for too long, an additional new compound is observed. Its ¹H NMR spectrum has peaks at δ = 1.70 (s, 3 H), 1.79 (s, 3 H), 2.25 (broad s, 1 H), 4.10 (d, J = 8 Hz, 2 H), and 5.45 (t, J = 8 Hz, 1 H) ppm. What is the structure of the second product, and how did it get there?