So far in this chapter you have seen that while heating a reaction can change the position of an equilibrium, the usual reason for heating a reaction is to speed it up by giving the reactants more energy to allow them to overcome the activation barrier. But as we said in the introduction, in a typical laboratory you will see many reactions being carried out at low temperatures. Why might a chemist want to slow a reaction down? Well, often molecules can react in several different ways. A good reaction will have a lower activation energy than these alternatives. But often there are other unavoidable reactions wait ing in the wings that will compete with the one that is wanted if the molecules have enough energy. The ideal situation is to give the starting materials enough energy to do the reaction we want, but not enough to do anything else: and that means keeping the reaction cold. A famous example of a reaction which must be kept cold is the diazotization of anilines to make diazonium salts. The reaction involves treating the amine with nitrous acid (HONO) made from NaNO₂ and HCl. You need not think about the mechanism at this stage—you will meet it in Chapter 22—but the key point is that the product is a rather unstable but very useful diazonium salt. The diazotization takes place readily at room temperature, but unfortunately so does the decomposition of the product to give a phenol. By lowering the temperature, we supply insufficient energy for the phenol formation, but the diazotization still works just fine.

The use of organolithiums (which you saw in Chapter 9) typically involves low temperatures, often as low as –78 °C. Organolithiums are very reactive, and the addition or deprotonation reactions they undergo have activation energies sufficiently low that they proceed even at such temperatures. However, they do also have a tendency to attack some of the solvents which they dissolve best in, such as THF. If lithiations are attempted at higher temperatures, THF also reacts with s-BuLi to give the surprising by-products discussed in Chapter 35.

مواضيع ذات صلة

Catalysis

Reaction intermediates

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