Our mechanism for reduction of the ketone with borohydride is of course not yet complete: there is another step to follow—the protonation of the alkoxide by the ethanol solvent. We can add this step to our energy profile diagram.

Now the product of the first stage of the reaction is the starting material for the second, which follows on straightaway because the activation energy for the second reaction, the proton trans fer, is smaller than for the first one. Notice that we have now labelled the middle set of structures, which includes the alkoxide ‘intermediate’. An intermediate is a staging post in a reaction pathway: it is stable for a finite (if short) period. Unlike a transition state it is a minimum rather than a maximum on the reaction energy profile, and therefore has a finite existence—an inter mediate could, in principle, be isolated, and many have been (particularly at low temperature).

● Intermediates and transition states

• A transition state represents an energy maximum—any small displacement leads to a more stable product. It can never be isolated.

 • An intermediate is a molecule or ion that represents a localized energy minimum—an energy barrier must be overcome before the intermediate forms something more stable. An intermediate can in principle be isolated (although in practice its high energy can make this difficult).

مواضيع ذات صلة

Catalysis

Why some reactions are done at low temperature

The route from reactants to products: the transition state

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