More O'Ferrall–Jencks plot

More O’Ferrall to discuss mechanisms of β-eliminations[2] and later adopted by W. P. Jencks in an attempt to clarify the finer details involved in the general acid-base catalysis of reversible addition reactions to carbon electrophiles such as the hydration of carbonyls.

[1] Consider a generic example in which the initial transition state along a concerted pathway is represented by a black dot on a red diagonal (Figure 1).

Initially, More O’Ferrall introduced this type of analysis to discuss the continuity between concerted and step-wise β-elimination reaction mechanisms.

For example, the effects of introducing a better leaving group on a substrate that initially eliminates via an E2 mechanism are illustrated in Figure 2.

This can be explained by the fact that a better leaving group needs less assistance from a developing neighbouring negative charge in order to depart.

Gradually increasing the leaving group ability decreases the ρ-value until it becomes large and negative, indicating the development of positive charge in the transition state.

[7] Finally, this type of plot can readily be drawn to illustrate the effects of changing parameters in the acid-catalyzed nucleophilic addition to carbonyls.

Figure 1. A generic More O’Ferrall–Jencks plot. R, I(1), I(2) and P stand for reactant(s), intermediate(s) 1, intermediate(s) 2 and product(s) respectively. The thick arrows represent movement of the transition state (black dot) parallel and perpendicular to the diagonal (red line). The thin arrow is the vector sum of the thick arrows.
Figure 2. More O’Ferrall–Jencks plot of the competing β-elimination mechanisms: E2, E1 and E1cB. The arrows indicate the effects of increasing the leaving group ability on the position of the transition state.
Figure 3. More O’Ferrall–Jencks plot of competing nucleophilic aliphatic substitution mechanisms: S N 1 and S N 2. The arrows represent the effect of increasing the nucleophilicity of the nucleophile on the position of the transition state.
Figure 4. More O’Ferrall–Jencks plot of acid-catalyzed versus uncatalyzed nucleophilic addition to a carbonyl. The arrows represent the effects of increasing the strength of the acid on the position of the transition state.