Why is there no set of rules that lets you predict the product of a reaction the way physics predicts a trajectory?

There is also a hard computational reason, which is the one I find most satisfying.

A reaction is not two molecules meeting once. It is an enormous number of molecules colliding in every orientation, at a distribution of energies, with solvent molecules involved, exploring a landscape of possible arrangements. The number of dimensions in that landscape scales with the number of atoms, so it explodes almost immediately.

Physics gets clean answers where it can isolate a small number of degrees of freedom. A thrown ball is essentially one object with a handful of relevant variables. Chemistry rarely gets that luxury — even a modest organic molecule has dozens of atoms, and the interesting question is about the transition state, which is a saddle point on a surface you cannot draw.

This is why computational chemistry is a large field that gives genuinely useful answers and still cannot simply be asked "what will this produce".

It is partly teaching and mostly real, and the honest answer has two halves.

Chemistry does have predictive theory. Thermodynamics tells you which products are favoured at equilibrium and it is quantitative and reliable. Quantum chemistry can compute a molecule's structure and energy from first principles to useful accuracy.

But which product you actually get is usually not decided by equilibrium. It is decided by kinetics — which pathway happens fastest — and the fastest pathway is often not the one leading to the most stable product. So the thermodynamic answer tells you where the system would like to end up and the reaction gives you something else entirely.

That gap is the real subject. Most of what a working chemist knows is about controlling which pathway wins, using temperature, solvent, catalyst, order of addition and concentration. Those are not fudge factors around a missing theory; they are the actual controls.

The comparison to physics is a bit unfair to chemistry, and it is worth saying why.

Physics predicts a trajectory beautifully for the cases where it can. Ask it to predict the weather in two weeks, or exactly where a specific leaf will land, and it gives you the same kind of answer chemistry gives: general behaviour, statistical confidence, sensitivity to conditions you cannot measure precisely enough.

Chemistry's problems are almost all of the second type. Every reaction is a many-body problem with sensitive dependence on conditions. The subject is not less rigorous — it is working in a regime where the rigorous answer is a distribution rather than a point.

On the teaching half: introductory courses do make it look worse than it is, because they front-load the memorisation before the frameworks that organise it.

A great deal of what looks like arbitrary rules in a first course turns into a small number of principles later — where electrons are, where they want to go, what makes a good leaving group, what stabilises a charge. Once those land, most "exceptions" stop being exceptions and become consequences.

It still will not let you calculate an unknown reaction from scratch. But it moves you from memorising outcomes to reasoning about them, which is a different experience of the subject entirely.