Kinetic vs thermodynamic control — Conjugation & Pericyclic Reactions

The diene in the last section gave one product cold and a different one warm, from the same intermediate by the same mechanism. That is not a quirk of dienes. It is a general situation that arises whenever a reaction has two competing pathways, and it is worth stating on its own because the same reasoning returns in enolate chemistry, in sulfonation of aromatics, and anywhere a reaction can be run reversibly.

Two different questions, two different answers

When two products can form from a common intermediate, there are two entirely separate things you might mean by "which one forms":

The kinetic product is the one that forms faster — the one whose transition state is lower. Its formation has the smaller activation energy, ΔG‡.
The thermodynamic product is the one that is more stable — the one that sits lower in energy once formed. Its formation has the more negative ΔG.

There is no rule that these are the same compound, and the interesting cases are exactly the ones where they are not. A reaction can run quickly to a product that is not the most stable available, and it will stay there as long as it has no way back.

The distinction is between a barrier and a well. Kinetics is about how high the hill is on the way out; thermodynamics is about how deep the valley is when you land. Nothing requires the lower hill to lead to the deeper valley, and when it does not, the conditions decide which one you end up in.

Reversibility is the switch

This is the part worth holding onto, because it is what turns the distinction from a piece of vocabulary into something you can control at the bench.

If the first step cannot reverse, the reaction is under kinetic control. Whatever forms first is trapped, and the product ratio simply reflects the relative rates. Low temperature is what enforces this: there is not enough thermal energy to climb back out of either product well.

If the first step can reverse, the reaction is under thermodynamic control. Both products form and both go back, repeatedly, and the system explores its options until it settles in the lowest well available. The ratio then reflects relative stabilities and has nothing to do with rates. Higher temperature is what permits this, by supplying the energy to reverse the step.

So temperature is not changing which product is more stable, and it is not changing which forms faster. It is changing whether the system is allowed to find out.

The diene case, read off the diagram

Put the allylic cation from the last section at the top of an energy diagram with two routes down from it.

The route to the 1,2-product has the lower barrier, because bromide attacks the carbon carrying more positive charge — the secondary terminus of the allylic cation. Lower hill, faster.
The route to the 1,4-product ends in the deeper well, because that product's alkene is more substituted. Higher hill, more stable landing.

At −80 °C nothing can climb back out, so the ratio is set by the barriers and you get mostly the 1,2-product. At 40 °C bromide can leave again, so both products keep returning to the cation, and the mixture accumulates in the deeper well: mostly the 1,4-product. Warming the pure 1,2-product gives the same equilibrium mixture, which is the experiment that proves the mechanism.

A reaction under thermodynamic control has to be reversible, and reversible means the product can go back to starting material. If a question tells you the product is formed irreversibly, thermodynamic control is off the table no matter how long you heat it.

Where else this appears

Two routes down from one intermediate, and they disagree. The left route has the lower hill because bromide attacks the carbon carrying more positive charge; the right route ends in the deeper valley because its alkene is more substituted. Neither fact has anything to say about the other.Temperature does not move a single line on this diagram. It decides only whether the system is allowed to climb back out of the shallow well on the left — and that is the entire content of "kinetic versus thermodynamic control." Read it as a test you can apply anywhere: if the first step cannot reverse, compare the hills; if it can, compare the valleys and ignore the hills completely.

The pattern is worth recognizing because it explains several results that otherwise look arbitrary:

Enolates. A ketone with two different α positions gives the kinetic enolate — from the less hindered side — with a bulky base like LDA at −78 °C, and the thermodynamic enolate — the more substituted, more stable one — under warmer, equilibrating conditions. Same molecule, same two options, same switch.
Aromatic sulfonation. The reaction is reversible, which is exactly why a sulfonic acid group can be installed to block a position and then removed again later. Most electrophilic aromatic substitutions cannot do this.
Elimination. Zaitsev's more substituted alkene is the thermodynamic product; a bulky base giving the less substituted Hofmann product is a kinetic outcome imposed by sterics.

Worked example — reading the conditions

A reaction gives products A and B. A forms four times faster; B is 12 kJ/mol more stable. What do you get?

At low temperature, irreversibly: mostly A. The rates decide, and A wins on rate.

At high temperature, reversibly: mostly B. The stabilities decide, and B wins on stability. A still forms first — it is simply not able to stay.

The trap: "B is more stable, so B is the product" is only true if the reaction can reverse. Without that, B's extra stability is irrelevant, because nothing is ever going to visit it.

What carries forward

Two habits are worth building now. First, when a question specifies a temperature — and especially a strikingly low one like −78 °C — treat that as a deliberate signal that kinetic control is being asked for. Second, when you are asked which of two products dominates, decide first whether the reaction is reversible, because that question determines which of the two comparisons you should even be making.