The order of a reaction cannot be predicted from the overall stoichiometric equation. That bears repeating. The rate expression for the reaction will agree with the stoichiometric equation only if all species are present in stoichiometric ratio at the rate determining step of the reaction mechanism.

The reaction mechanism determines the rate expression, not the overall stoichiometry. If a reagent is not present at the rate determining step, its concentration will not appear in the rate expression.

Substitution and elimination reactions represent the quintessential example from organic chemistry of the impossibility of predicting the rate expression from stoichiometry without knowing the mechanism.

A common type of problem is to present reaction rate data varying the concentration of reagents. If the rate of the reaction varies when the concentrations of both the alkyl halide substrate and the nucleophile (base) are changed, the mechanism must be bimolecular (E2 or SN2). However, if only the concentration of alkyl halide appears to change reaction rate, the mechanism must be unimolecular (E1 or SN1).

The rate expressions of the bimolecular reactions contain concentration expressions for both alkyl halide and nucleophile (base), while with the unimolecular reactions, the rate expression contains only the concentration expression of the alkyl halide.

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