In a recent authoring project on organic chemistry, I came across the following statement:
A chain mechanism involves two or more repeating steps.
Is this a true statement? Well, yes and no. Yes, a chain mechanism involves the same process happening again and again. But so does a catalytic mechanism—are both mechanisms the same? If they were, we’d just call all chain mechanisms catalytic (it sounds much better, right?). In fact, the two are not the same, and there’s far more to the definition of a chain mechanism than two repeating steps.
Naively, chain initiators (let’s use radical initiators for the present discussion) look a lot like catalysts. They’re around in substoichiometric amounts and they promote the combination of reactants that would otherwise sit dormant. Clearly then, they decrease the activation energy of the reaction relative to a situation without initiator. However, radical initiators are missing a key feature of catalysts: they are consumed by the reaction. They’re about as close as one can get to a catalyst without being a catalyst!
Consider radical hydrobromination with hydrogen peroxide. During initiation the hydroperoxy radical pulls a hydrogen atom from HBr, forming a bromine radical. We thus can’t think of hydrogen peroxide or the hydroxy radical as a catalyst (it’s consumed)…but perhaps a pre-catalyst? Is bromine radical a true catalyst in this situation?
In fact, that bromine atom becomes incorporated into the product. The bromine radical reacts with the alkene reactant in the very next step of the mechanism, adding to the less substituted position to form an alkyl radical. Propagation begins with this step. If this mechanism is to be catalytic, the very selfsame bromine that attached to the alkene must pop off at a later stage. But this isn’t what happens!
In the second step of propagation, the newly formed alkyl radical abstracts a hydrogen from HBr, forming an alkyl halide (that happily floats away) and a bromine radical. This is not the same bromine atom that added to the alkene in the first place; it’s derived from a different molecule of HBr than the first. Remember that the first bromine radical came from reaction of the initiator with HBr—but after one propagation cycle, the initiator is nowhere to be found! Bromine radical, the so-called chain-carrying or propagating radical, is supplied over and over again during propagation by HBr, a reactant present in stoichiometric amounts.
How is this process fundamentally different from a catalytic mechanism? In a catalytic reaction, each catalyst molecule acts like a dedicated machine, “turning over” reactants a finite number of times (generally far greater than one) before being consumed by “off-cycle” side reactions or running out of reactants. Multiple machines can run at once, so side reactions deplete the supply of catalyst but don’t shut down the reaction entirely. That is until all of the catalyst runs out, at which point the reaction stops completely (assuming irreversible consumption of the catalyst).
In a chain mechanism, the intermediates within the cycle act more like single-use disposable gloves than dedicated reusable machines. Each particular chain-carrying radical can go through the process only once and thus full equivalents of everything in the propagation cycle are necessary. Side reactions of the propagating radicals (termination) may consume them to a point where none are left, but the reaction can restart if (and only if) initiator radicals and reactants are available to kick things off again.
The neatest thing about chain mechanisms, to me, is that the components of the cyclic process are completely made up of atoms from the reactants. While a catalytic species is something like a molecular dance floor on which the reactants rearrange themselves and depart (leaving the dance floor unchanged), a chain-carrying species is like a reactant Frankenstein whose mission in life is regenerating its creator. This is the most surefire way to tell a chain mechanism from a catalytic one: if no group of atoms stays completely within the cycle, the mechanism is a chain. If a group of atoms stays within the cycle, the mechanism is catalytic and we generally refer to that common group of atoms as the “catalyst per se.”