# Why are some organic reactions run at elevated temperatures?

We are using a water bath at $70\ ^\circ\mathrm{C}$ for different reactions to identify oxygen-containing organic compounds, such as the Tollens reaction, the iodoform test and hydroxamic acid test.

What is the function of heat in these reactions? Does the heat act as a catalyst?

• Many if not most organic reactions are actually run under reduced temperatures. My personal lowest so far was the Masamune aldol which required $- 95~\mathrm{^\circ C}$. – Jan Nov 8 '16 at 17:52

## 2 Answers

Though they can produce a similar effect (faster reaction rate), heat doesn't act as a catalyst.

Reaction rate is dependent on how often reagents can overcome the activation energy of a reaction. Therefore, a lower activation energy, $E_\mathrm{a}$, means a faster reaction.

A catalyst lowers the activation energy of a reaction by producing lower energy transition states. This is how catalysts speed up reactions. A generic version of this is shown in the image below. Transition states occur at the local maxima of the graph in this image. Heat, on the other hand, increases the average kinetic energy of the molecules within a system (i.e. most molecules are moving more quickly). By doing this, the chances of two particles colliding and having enough energy to react are higher because the particles will move faster. Therefore, they will collide more often, and when they do, they will have more "momentum" (so to speak) to get over this energy hill. By making it so more of the reactant particles can overcome this activation energy, heat also speeds up reactions.

Take a look at either the Eyring equation or the Arrhenius equation. The Eyring equation is:

$$k = \frac{k_{\mathrm B}T}{h}\exp{\left(-\frac{\Delta G^{\ddagger}}{k_{\mathrm B}T}\right)}$$

Notice that for higher values of $T$, the rate constant increases, implying that the reaction occurs faster.