A reaction between substances X and Y shows the following kinetic behavior at 298 K:
• Doubling [X] while holding [Y] constant doubles the initial rate.
• Doubling [Y] while holding [X] constant increases the initial rate by a factor between 3.6 and 4.0.
Three mechanisms have been proposed (overall reaction X + 2 Y → products):
Mechanism 1
Step 1 (slow): X + Y → Z
Mechanism 2
Step 1 (slow): Y + Y → W
Mechanism 3
Step 1 (slow): X + 2 Y → products
Which mechanism (1, 2, or 3) cannot be ruled out by the experimental rate data?