1,2- vs 1,4-addition showed that HBr and buta-1,3-diene give mostly the 1,2-product at −80 °C (about 80:20) and mostly the 1,4-product at 40 °C (about 15:85), from one allylic cation by one mechanism. This page turns that result into a rule for any reaction in which one intermediate can give two products.
Faster or more stable: two ways to win
The two diene products differ in speed and in stability. C2 of the allylic cation carries more of the positive charge than C4, so bromide bonds to C2 faster and the 1,2-product forms faster. The 1,4-product is more stable, because its C=C sits inside the chain with two carbons attached (disubstituted), while the 1,2-product's C=C sits at the end with only one (monosubstituted). The previous section worked out both reasons.
Energy diagrams previewed a name for each way of winning. The product that forms faster is the kinetic product; here it is the 1,2-product. The product that is more stable, meaning lower in free energy, is the thermodynamic product; here it is the 1,4-product. The names come from kinetics, the study of reaction rates, and thermodynamics, the study of energy and equilibrium.
An energy diagram shows both facts at once. (Energy diagrams called its high and low points peaks and wells; here they are hills and valleys.) Put the allylic cation in the middle, with one route down to each product. The route to the 1,2-product climbs a lower hill. The route to the 1,4-product climbs a higher hill, but it ends in a deeper valley.
A hill and a valley are different measurements
As Energy diagrams & the Hammond postulate showed, speed and stability are read from different parts of the diagram. The height of a hill, from the starting point up to the transition state, is the activation energy, ΔG‡. It sets the rate: the lower the hill, the faster the route. The depth of a valley, measured from where the route started, is ΔG°. It sets the equilibrium: the deeper the valley, the more of that product is present once the mixture has settled.
The lower hill does not have to lead to the deeper valley. Often it does, as the Hammond postulate suggests, but in the diene case it does not, so the kinetic and thermodynamic products are different compounds.
Small energy gaps make large differences, because rates and equilibrium constants both depend exponentially on energy. At room temperature, every 6 kJ/mol or so (the 1.4 kcal/mol rule from Energy diagrams) is a factor of about ten. A hill 10 kJ/mol (2.4 kcal/mol) lower makes a route roughly fifty times faster, and a product 12 kJ/mol (2.9 kcal/mol) more stable is favored at equilibrium by about 130 to 1. So speed alone, or stability alone, usually gives one main product. In the diene case they pick different ones.
Reversibility decides which comparison counts
Go back to the diene at −80 °C. Once bromide bonds, at C2 or at C4, the product stays put: at that temperature the new C–Br bond almost never breaks. Each cation gets one chance, so the faster route claims more of them. The 80:20 ratio reflects the two rates.
At 40 °C the C–Br bond can break again. Bromide leaves, the allylic cation re-forms, and bromide bonds to either end. Each molecule makes this trip many times. A product in a shallow valley leaves easily and a product in a deep valley rarely does, so over time the material collects in the deeper valley. The 15:85 ratio reflects the two stabilities.
These two situations have names. When the step that forms the products cannot reverse, the reaction is under kinetic control, and the product ratio matches the ratio of the two rates. When that step can reverse, and the mixture has had time to reach equilibrium, the reaction is under thermodynamic control, and the product ratio matches the equilibrium ratio.
Temperature is the usual way to choose between them. In the cold, the products lack the energy to climb back out, so kinetic control wins. Warmth supplies that energy, so thermodynamic control wins. Warming barely changes the hills and valleys, and it does not change which product forms faster or which is more stable. What it changes is whether the products can return to the intermediate.
The kinetic product is the first to return, and the diagram shows why. To go back, a product climbs from its own valley up to the hill it came over. That climb equals the forward hill plus the depth of the valley. The kinetic product has both the lower hill and the shallower valley, so its climb back is the shorter one. Material that leaves it splits between the two routes again, and what reaches the deep valley tends to stay there.
The warming experiment from 1,2- vs 1,4-addition confirms this picture: warm the pure 1,2-product to 40 °C with a trace of HBr, and it turns into the same 15:85 mixture. The kinetic product has gone back through the cation and ended up as the thermodynamic product.
One intermediate can give product A or product B. In the cold, A forms four times as fast as B, and B is 12 kJ/mol more stable than A. What do you get?
Match each number to what it measures. “Four times as fast” describes the hills. A factor of four is a difference of only a few kJ/mol in ΔG‡ (about 3 kJ/mol, 0.8 kcal/mol, at room temperature; less in the cold). “12 kJ/mol more stable” describes the valleys, and at room temperature that is an equilibrium ratio of about 130 to 1. The two numbers cannot be compared with each other, because one belongs to a hill and the other to a valley.
Cold, with no way back: about 80% A, from the 4:1 ratio of rates. The hills decide, and A wins.
Hot, with a way back: almost all B. The valleys decide, and a 12 kJ/mol gap is large. A still forms first, but it does not stay.
Two products can form: X forms faster and Y is more stable. Before asking which is more stable, ask whether the reaction can reverse.
−78 °C, cannot reverse: X. It is cold and there is no way back, so only the hills count.
80 °C, can reverse: Y. It is warm and there is a way back, so the mixture keeps sorting itself until it collects in the deeper valley.
80 °C, but neither product-forming step can reverse: X, still. Students often miss this one, because the high temperature looks like the signal. Reversibility is the signal. Heating makes both products form sooner and the ratio a little less lopsided, but it never turns X into Y, however long you wait.
A different pair, where the faster product is also the more stable one: you get that product cold or warm. The kinetic and thermodynamic products are the same compound more often than not. The two names matter only when they differ.
Where the switch does not apply
An E2 never switches. In E2, a small base gives mainly the more substituted Zaitsev alkene, and a bulky base gives more of the less substituted Hofmann alkene. With an ordinary base, the Zaitsev alkene is both the faster-forming and the more stable product, so there is no contest.
With a bulky base, the Hofmann alkene forms faster, while the Zaitsev alkene is still the more stable one. But an E2 cannot reverse, so it is always under kinetic control: heating a bulky-base E2 never turns the Hofmann alkene into the Zaitsev alkene. The bulky base raises the hill to the Zaitsev alkene; it does not change which valley is deeper.
“Kinetic” also does not mean “less crowded.” The reaction in the next section, The Diels–Alder reaction, has a kinetic product that is the more crowded of its two possible arrangements, as that section shows. The word only ever means “forms faster.”
A preview: two cases from later chapters
This part is a preview. Nothing else on this page depends on it, and later chapters teach both cases properly. The same switch turns up well beyond dienes.
Enolates, in Enolate Chemistry: a ketone with two different alpha carbons can lose a proton from either side, and a bulky base in the cold gives the faster-formed (kinetic) anion, while conditions that let the proton move back and forth give the more stable (thermodynamic) one.
Aromatic sulfonation, from Aromatic Chemistry. Strong sulfuric acid attaches a sulfonic acid group, –SO₃H, to a benzene ring, and hot dilute acid takes it off again. Few substitutions on a benzene ring can be undone this easily, which is why chemists use –SO₃H to block one ring position and remove it once its job is done.
Because the reaction reverses, it can run under either kind of control. Naphthalene, two benzene rings fused along one edge, shows both. At about 80 °C the –SO₃H group goes mainly to C1, next to the shared edge: the kinetic product. At about 160 °C it ends up mainly on C2, the next carbon along, where it does not crowd the hydrogen on C8: the thermodynamic product.
What carries forward
When a question asks which of two products dominates, first decide whether the product-forming step can reverse. That answer tells you which comparison to make: the hills if it cannot, the valleys if it can. Watch the stated conditions too. A strikingly low temperature, such as −78 °C, usually signals that the question wants the kinetic product.