More O Ferrall Diagrams

The basic idea of this technique can be best demonstrated by reactions the course of which is governed by the variation of two nuclear coordinates Qi, Q2, representing, e.g., the lengths of two newly created bonds. As an example of such a process may serve, for example, the Diels-Alder reaction. 

---

More O Ferrall diagram for base-promoted, imine forming 1,2-elimination reactions. 

FIGURE 3. Structure-reactivity Jencks-More O Ferral diagram for the hydrolysis of l-X-2,4,6-tri-nitrobenzenes (a) vertical level line as a consequence of —dp/dpKbh = px = 0 (b) level line clockwise rotated from the horizontal as a consequence of —(—3/Sxh/ — dpKxh) - j>v negative ... 

The transition states for the stepwise (fej, Fig. 2.3) and concerted (fecon) reactions of (4-MeO,X)-3-Y lie at distinct well-separated positions on the More O Ferrall diagram and show different sensitivities to changes in solvent polarity, meta substituents X at the aromatic ring, and the leaving group Y. For example, in 50 50 (v/v) water/trifluoroethanol (4-MeO,H)-3-Cl reacts with azide ion exclusively by a stepwise mechanism through the primary carbocation intermediate (4-MeO,H)-3" with a selectivity for reaction with azide ion and solvent of feaz/ s = 25 However, two-thirds of the azide ion substitution product obtained from the reaction of (4-MeO,H)-3-Cl in the less polar solvent 80 20 acetone/water forms by concerted bimolecular substitution and only one-third forms by trapping of the carbocation intermediate (4-MeO,H)-3 with a selectivity of k z/h = 8 The preferred... 

The conceptual significance of the More O Ferrall diagram is that it divides substituent effects into two classes. Firstly substituent effects which... 

Fig. 1. Schematic representation of reaction paths in the More O Ferrall diagram...

Fig. 2. Schematic classification of reaction mechanisms in terms of partitioning of modified More O Ferrall diagrams. Line a represents an asynchronous but concerted process, line b a nonconcerted process...

The corresponding dissection of the More O Ferrall diagram is then given by the set of inequalities (17) ... 

This reaction is especially convenient for demonstration purposes since it displays both possible ways in which the More O Ferrall diagrams can be dissected, as schematically depicted in Figs. 4 and 5. 

Fig. 4. Schematic visualisation of the specific partitioning of the More O Ferrall diagram in the case of a reaction allowing classification of the reaction mechanism without a knowledge of the actual reaction path...

The basic idea of the above criterion is very simple and consists in the qualitative comparison of the relative ease of selected idealised reaction paths corresponding in More O Ferrall diagrams to purely synchronous (concerted) and purely... 

Fig. 6. The partitioning of a modified More O Ferrall diagram with the corresponding reaction path for thermally forbidden disrotatory cyclisation of butadiene to cyclobutene. The extent of electron reorganisation, measured by the value of the functional L, is — 0.48 along this reaction path...

If now the same analysis is applied to allowed conrotatory reaction then the corresponding More O Ferrall diagram (Fig. 7) is in this case compatible with both concerted and nonconcerted mechanisms and the discrimination between them requires a knowledge of the corresponding reaction path. 

Figure 1. More O Ferrall diagram of the Menschutkin-type reaction.

---