Reaction co-ordinate diagram

Figure 4. Free energy-reaction co-ordinate diagrams for acid-catalysed hydrolysis of simple acetals...

In any general reaction between A and B to give C and D, the reagent A must physically approach the substrate B so that the electrons within each molecule may interact this results in some bonds being broken and re-formed so as to produce the new molecules C and D. This process may be represented on a diagram called a reaction co-ordinate diagram. 

For a two-step reaction, the reaction co-ordinate diagram is as follows. In this case, there is an energy well, i.e. there is a peak either side of it. The species that is caught in the energy well is called an intermediate and may be isolated. The presence of the peaks on either side of the intermediate indicates that energy is required for both the forward or the reverse reaction to proceed from this intermediate species, i.e. there is an activation energy barrier that the intermediate must overcome before it may revert either to the starting materials or proceed to the final products. This is indicative of a real species, which can be isolated. This is in contrast to a transition state, which occupies the cusp of the... 

The rate of a reaction is related to the activation energy barrier that exists between the starting materials and the transition state. On the reaction co-ordinate diagram, this is represented by the difference between the starting materials and the peak of the reaction co-ordinate curve. If the activation barrier is large, how will this affect the rate of the reaction ... 

Transition state The transitory structure that exists at the mid-point in the reaction between the starting materials and the products. This corresponds to the structure that exists at the peak of the reaction co-ordinate diagram, which represents the configuration that has the highest energy along the reaction pathway. This hypothetical species cannot be isolated. 

Schematic diagram of energy versus distance along reaction co-ordinate for proton transfer.

The two geometric co-ordinates are naturally firstly the distance between the attacking nucleophile and the carbon atom, and secondly the distance between the carbon atom and its leaving group. It is convenient in plotting the diagram to normalize the distances with the distance found in either the reactants or the products. For the reaction (8) we define the normalized distances rx and ry by (9) and (10). Because we shall be concerned with observed rate constants and... 

The current understanding of the ion chemistry of the atmosphere has been achieved by co-ordinating the data obtained from in-situ ion composition measurements with the data obtained from appropriate laboratory experiments. This review has largely been concerned with the elementary ionic reaction processes involved in the overall chemistry and detailed chemical models of the ion chemistry of the atmosphere have been deliberately excluded since such have recently appeared in the literature8,73,74,, 47. However, it is appropriate here to summarise, through block diagrams, the chains of ionic reactions via which ions are formed, evolve and are finally lost from the atmosphere. To this end, it is convenient to consider separately three regions of the atmosphere ... 

The advantage of bidimensional representation is evident if four reactive components are involved, as in the class of reversible reactions A + B <-> C + D. This situation covers an important number of industrial applications, as the esterification of acids with alcohols. Selecting C as the reference, the transformed variables are XA = xA + xc, XB = xB + xc and XD = xD- xc since v, = 0. The transformed variables sums to one, but only two are used as co-ordinates. Accordingly, the pure components may be placed in the corner of a square diagram, reactants or products on the same diagonal. Figure A. 5 displays the reactive distillation map traced as before for the relative volatilities 4/2/6/1 and the equilibrium constant K t = 5. 

---