The combustion of kinetically stable fuel releases lots of energy by a very complex mechanism. To understand how energy is involved in the progress of a reaction we will need to take a much more simple and familiar mechanism. The reduction of a ketone to an alcohol with sodium borohydride will do. You met this reaction in Chapters 5 and 6 and it should by now be a familiar part of your chemical vocabulary. An example is shown in the margin: in this particular case, the ketone is rather hindered by the adjacent tert-butyl group, and the reaction must be heated to form the product. Evidently, then, there is an activation barrier that must be overcome. Let’s think about what that barrier might be. Although the final product is an alcohol, as you know well, the fi rst step is transfer of a hydrogen atom from boron to the carbonyl group, as shown in the mechanism below. Overall, as shown in the energy profile diagram, the products of this step are more stable than the starting materials (ΔG is negative), but to get there the reaction has to pass through the activation energy barrier (ΔG‡). This barrier—the highest energy point on the profile—must correspond to some structure (which we have shown in square brackets) in which the hydrogen atom is only partly transferred from B to C, and the carbonyl group is only partly broken. We call this structure—the highest energy form through which the molecules must pass to get from reactants to products—the transition state. It is often represented in square brackets, frequently with a double dagger symbol ‡ (to match the activation energy ΔG+).

Notice that the transition state has some features of the reactants and some features of the products. The B–H bond is partly broken, so we represent it as a dotted line, and the new H–C bond is partly formed, so likewise that is dotted too, as is the breaking C=O bond. The negative charge, which starts associated with B and ends on the oxygen atom, is shown in brackets in both locations, to indicate that it is shared between them. It takes energy to get to the transition state because the H has to move away from the B without significant compensation.

But once the transition state is passed, the formation of a stable C–H bond and the migration of a charge to electronegative oxygen means that stability is regained. A transition state is always unstable and can never be isolated: if the reaction proceeds just a little more forwards or backwards, the energy of the system is lower. Isolating a transition state would be like balancing a marble on top of a bowling ball.

● Transition state

A transition state is a structure that represents an energy maximum on passing from reactants to products. It is not a real molecule in that it may have partially formed or broken bonds and may have more atoms or groups around the central atom than allowed by valence bond rules. It cannot be isolated because it is an energy maximum and any change in its structure leads to a more stable arrangement. A transition state is often shown by putting it in square brackets with a double-dagger superscript.

مواضيع ذات صلة

Catalysis

Reaction intermediates

Why some reactions are done at low temperature

How fast do reactions go? Activation energies

Some reactions are reversible on heating : cracking

Equilibrium constants vary with temperature

Enthalpy versus entropy—some examples

Energy, enthalpy, and entropy: ΔG, ΔH, and ΔS

Typical method for making an ester

How to make the equilibrium favour the product you want The direct formation of esters

A small change in ΔG makes a big difference in K

The sign of ΔG tells us whether products or reactants are favoured at equilibrium