Unimolecular Reaction
An elementary step in which a single molecule rearranges or decomposes.
Formula / equation
A → products, rate = k[A]
Unit
s⁻¹
In depth
A unimolecular step is always first order. The molecule must first acquire enough energy through collisions to reach the transition state, which is why apparently unimolecular gas reactions become second order at very low pressure — the Lindemann–Hinshelwood mechanism.
Examples in the real world
Cyclopropane isomerising to propene; the ionisation of a tertiary haloalkane in the SN1 mechanism.