# What aspects of a molecule's chemical makeup determine which portions of it will react?

Is there a specific way to know how two or more arbitrary molecules will react?

An example that prompted this question: $\ce{(HOC6H4)2CMe2 + 2 NaOH → (NaOC6H4)2CMe2 + 2 H2O}$ (from here). How can you know how those molecules will react? Is it just a bunch of substitutions, with certain atoms in the reacting molecules reacting better than others? What factors would determine which atom in the molecule would react with the other molecule?

• A full answer to this question is like the table of contents for an entire course in synthetic chemistry. – matt_black Feb 23 '14 at 20:57
• @matt_black True :) That's why I didn't go into great detail ;) – Klaus-Dieter Warzecha Feb 24 '14 at 21:19

Let me answer with a question (or two):

How do you identify in an electric circuit which parts are just the wiring and which are functional parts?

How's that for a mechanical engine? Can you tell cogwheels and cranks from "just the basic structure"?

It's all about pattern recognition in the first to identify the functional groups.

Next step is to know how they react under certain conditions. As a rule of thumb, a group with a high electron density will probably react with something more positively charged (think plus and minus attract each other).

But it takes quite a while and learning to predict exactly what will go on exactly in the reaction. But that not different from any other scientific/technical discipline.

I tried to keep this as general and simple as possible ;)

Edit Let me work you comment in:

Generally molecules don't contain ions though

Says who? :D Molecules may contain ionic units, think in zwitterions of amino acids, think in all the mesoionic sydnones, münchnones and whatnot. Well, no. Forget about the special cases ;) But if it has more than one atom and is electrically neutral in total it is a molecule. At least in the less rigid sense of the definition.

But yes, the electronegativity is a good point.

Forgive me that I'm not going into detail why the example with fluorine and sodium is a not the best one, but in principle, you're right: Such a setup would allow to form a new $\ce{C-C}$ bond between a carbon that is negatively polarized (the one bearing the sodium) and another one that is positively polarized (the one bearing the halogene).

• Generally molecules don't contain ions though, so you mean something like electronegativity? So if a molecule were to contain fluorine, and a second one sodium, would it (probably) react with each other, instead of with any other elements in the molecule, because of the difference in electronegativity, and therefore the increased electron density (the fluorine pulling electrons towards itself)? (Just trying to explain it to myself with the limited stuff I've learned in class.) – Goocholplex Feb 23 '14 at 9:00
• That's surprisingly simple then (it's still complicated, but it uses the same mechanisms involved in more "basic" chemistry) Thanks a lot! – Goocholplex Feb 23 '14 at 12:06
• @Goocholplex I tried to keep it as simple as possible without being wrong ;) But if you found it helpful and inspiring please consider to "accept" the answer. I'd appreciate that :) – Klaus-Dieter Warzecha Feb 23 '14 at 12:16