# Is there a specific term for reactions of the form A + B → C + D?

I'm working with an abstract model of chemical reaction networks. In this model, some of the reactions have the form $\ce{A + B <=> C}$ and others have the form $\ce{A + B<=>C + D}$. I'm wondering if there is some existing terminology I can use to refer to these two cases.

I know that the terms "unimolecular" and "bimolecular" refer to the number of reactants, but in this case the important thing is the number of products. The ideal thing would be if there's a general term describing whether a reaction has the same number of products as it does reactants.

Alternatively, since the species I'm modelling are abstract polymers, I can use the terms "cleavage" and "ligation" to refer to the $\ce{A + B <=> C}$ case, depending on the direction in which the reaction proceeds. If there is a corresponding term for $\ce{A + B<=>C + D}$ type reactions in polymers, that would also be useful to know.

I suppose at a stretch I could call it an exchange or a substitution reaction, but (as far as I know) that tends to imply that the thing being substituted or exchanged is a single atom, rather than a whole section of polymer. (In my case the reaction might be something along the lines of $\ce{aaabb + ccdddd -> aaadddd + ccbb}$, where $\ce{a}$, $\ce{b}$, $\ce{c}$ and $\ce{d}$ are different types of monomer.)

There are 4 'base-types' of chemical reactions:

1. Synthesis reactions ($\ce{A + B -> AB}$)
2. Decomposition reactions ($\ce{AB -> A + B}$)
3. Substitution reactions ($\ce{A + BC -> B + AC}$)
4. Metathesis reactions ($\ce{AB + CD -> AD + CB}$)

Given that you talk about polymers that in one case combine to form a new (single) polymer: $\ce{A + B <=> C}$ you could call this a synthesis reaction.

You other example, $\ce{A + B <=> C + D}$, is an exchange of parts of the polymers so that would be a metathesis reaction.

For polymers there are a couple of additional options to name the types of reactions, but these depend a lot on the specific chemistry (e.g. hydrolysis if water is one of the reactants) so that is probably difficult if you have 'abstract' polymers.

• Thanks, this is helpful. One slight problem though is that some of my $\ce{A + B<=>C + D}$ type reactions would be classed as substitution reactions according to this scheme and others would be metathesis reactions - ideally I'd like a term that encompasses both possibilities. Do you think it would be reasonable to use the word "metathesis" to include substitution reactions as well? I suppose I could just say "substitution/metathesis" throughout the paper, but it's a bit of a mouthful. – Nathaniel Mar 29 '13 at 7:55
• Both substitution and metathesis reactions are also often called single resp. double replacement reactions, so I would call them replacement reactions then. – Michiel Mar 29 '13 at 8:01