Definition

A reaction that is second (or higher) order overall but behaves as first order because one reactant is present in such large excess that its concentration remains essentially constant.

How It Works

True rate law: $\text{rate} = k[\text{A}][\text{B}]$

If [B] >> [A]: [B] ≈ constant = [B]_{0}

$\text{rate} = k'[\text{A}] \quad \text{where } k' = k[\text{B}]_0$

k' is the pseudo first-order rate constant.

Classic Example: Ethyl Acetate Hydrolysis

$\text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O} \xrightarrow{\text{H}^+} \text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH}$

SMILES: CC(=O)OCC + O → CC(=O)O + CCO

• True rate: k[ester][$H_{2}O$] (second order)
• [$H_{2}O$] ≈ 55.5 M >> [ester] → [$H_{2}O$] practically constant
• Observed rate: k′[ester] (pseudo first-order)
• k′ = k × 55.5 M

Other Pseudo First-Order Examples

• Sucrose (cane sugar) hydrolysis in dilute HCl
• Methyl acetate hydrolysis in excess water
• Many enzyme-substrate reactions at low substrate concentration

Experimental Consequence

The reaction shows first-order kinetics in practice (linear ln[ester] vs t), even though the true mechanism is bimolecular. The measured k is k′ = k[$H_{2}O$].