View source for More O'Ferrall–Jencks plot

'''More O’Ferrall–Jencks plots''' are two-dimensional representations of multiple [[Chemistry:Reaction coordinate|reaction coordinate]] [[Physics:Potential energy surface|potential energy surface]]s for [[Chemistry:Chemical reaction|chemical reaction]]s that involve simultaneous changes in two [[Chemistry:Chemical bond|bonds]]. As such, they are a useful tool to explain or predict how changes in the reactants or reaction conditions can affect the position and geometry of the [[Chemistry:Transition state|transition state]] of a reaction for which there are possible competing pathways.<ref name="Anslyn, E. V. 2006, pp. 407">Anslyn, E. V., and Dougherty, D. A. ''Modern Physical Organic Chemistry''. California: University Science Books, 2006, pp. 407–410.</ref>
==Brief history==
These plots were first introduced in a 1970 paper by R. A. More O’Ferrall to discuss [[Physics:Reaction mechanism|mechanisms]] of [[Chemistry:Elimination reaction|β-eliminations]]<ref name=MoreOFerrall1970>{{Cite journal| title = Relationships between E2 and E1cB mechanisms of beta-elimination | author = More O’Ferrall, R. A. | year = 1970 | journal= J. Chem. Soc. B |pages=274–277| doi=10.1039/J29700000274}}</ref> and later adopted by W. P. Jencks in an attempt to clarify the finer details involved in the general acid-base catalysis of reversible [[Chemistry:Nucleophilic addition|addition reactions]] to carbon electrophiles such as the hydration of carbonyls.<ref name=Jencks1972>{{cite journal|last=Jencks|first=William P.|title=General acid-base catalysis of complex reactions in water|journal=Chemical Reviews|year=1972|volume=72|issue=6|pages=705–718|doi=10.1021/cr60280a004}}</ref>

==Description==
thumb|right|'''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.In this type of plot (Figure 1), each axis represents a unique reaction coordinate, the corners represent local minima along the potential surface such as reactants, products or intermediates and the energy axis projects vertically out of the page. Changing a single reaction parameter can change the height of one or more of the corners of the plot. These changes are transmitted across the surface such that the position of the transition state (the saddle point) is altered.<ref name="Anslyn, E. V. 2006, pp. 407"/>

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). Changing the height of the corners can have two effects on the position of the transition state: it can move along the diagonal, reflecting a change in the [[Physics:Gibbs free energy|Gibbs free energy]] of the reaction (ΔG°), or perpendicular to it, reflecting a change in the energy of competing pathways. Thus, in accordance with the Hammond postulate, the transition state moves along the diagonal towards the corner that is raised in energy (a Hammond effect) and perpendicular to the diagonal towards the corner that is lowered (an anti-Hammond effect).<ref name="Anslyn, E. V. 2006, pp. 407"/><ref name="Thornton, E. R. 1967">Thornton, E. R. (1967) J. Am. Chem. Soc. 89, 2915</ref> In this example, R is raised in energy and I(2) is lowered in energy. The transition state moves accordingly and the vector sum of both movements gives the real change in its position.

==Applications==
===Elimination reactions===
[[Image:Elimination2.JPG|thumb|right|'''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.]]
Initially, More O’Ferrall introduced this type of analysis to discuss the continuity between concerted and step-wise β-elimination reaction mechanisms. The model also provided a framework within which to explain the effects of substituents and reaction conditions on the mechanism.<ref name=MoreOFerrall1970/> The appropriate lower energy species were placed at the corners of the two dimensional plot (Figure 2). These were the reactants (top left), the products (bottom right) and the intermediates of the two possible stepwise reactions: the [[Chemistry:Carbocation|carbocation]] for [[Chemistry:Elimination reaction|E1]] (bottom left) and the [[Chemistry:Carbanion|carbanion]] for E1cB (top-right). Thus, the horizontal axes represent the extent of deprotonation (C-H bond distance) and the vertical axes represent the extent of [[Chemistry:Leaving group|leaving group]] departure (C-LG distance). By applying the Hammond and anti-Hammond effects,<ref name="Thornton, E. R. 1967"/> he predicted the effects of various changes in the reactants or reaction conditions. 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. A better leaving group increases the energy of the reactants and of the carbanion intermediate. Thus, the transition state moves towards the reactants and away from the carbanion intermediate.

[[Image:substitution2.JPG|thumb|right|'''Figure 3.''' More O’Ferrall–Jencks plot of competing nucleophilic aliphatic substitution mechanisms: S<sub>N</sub>1 and S<sub>N</sub>2. The arrows represent the effect of increasing the nucleophilicity of the nucleophile on the position of the transition state.]]
The model does not predict any change in leaving group departure at the transition state. Instead the extent of deprotonation is expected to decrease. 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. The true change predicts more carbocation character at the transition state and a mechanism that is more E1-like. These observations can be correlated with Hammett ρ-values.<ref>Anslyn, E. V., and Dougherty, D. A. Modern Physical Organic Chemistry. California: University Science Books, 2006, pp. 586–588.</ref> Poor leaving groups correlate with large positive ρ-values. 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.

===Substitution reactions===
A similar analysis, done by J. M. Harris, has been applied to the competing S<sub>N</sub>1 and S<sub>N</sub>2 nucleophilic aliphatic substitution pathways.<ref name=Harris1979>{{cite journal|last=Harris|first=J. Milton|author2=Shafer, Steven G. |author3=Moffatt, John R. |author4= Becker, Allyn R. |title=Prediction of SN2 transition state variation by the use of More O'Ferrall plots|journal=J. Am. Chem. Soc.|year=1979|volume=101|issue=12|pages=3295–3300|doi=10.1021/ja00506a026}}</ref> The effects of increasing the nucleophilicity of the [[Chemistry:Nucleophile|nucleophile]] are shown as an example in Figure 3. An agreement with Hammet ρ-values is also apparent in this application.<ref>Anslyn, E. V., and Dougherty, D. A. Modern Physical Organic Chemistry. California: University Science Books, 2006, p. 650.</ref>

===Addition to carbonyls===
[[Image:addition.JPG|thumb|right|'''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.]]
Finally, this type of plot can readily be drawn to illustrate the effects of changing parameters in the acid-catalyzed nucleophilic addition to carbonyls. The example in Figure 4 demonstrates the effects of increasing the strength of the [[Chemistry:Acid|acid]]. In this case, the extent of protonation is the α-value in the [[Chemistry:Brønsted catalysis equation|Brønsted catalysis equation]]. The fact that the α-value remains unchanged explains the linearity of Brønsted plots for such a reaction.<ref>Anslyn, E. V., and Dougherty, D. A. Modern Physical Organic Chemistry. California: University Science Books, 2006, pp. 543–544.</ref>

Ultimately, the More O’Ferrall–Jencks plots have qualitative predictive and explanatory power regarding the effects of changing substituents and reaction conditions for a wide variety of reactions.

==See also==
* [[Physics:Potential energy surface|Potential energy surface]]
* [[Chemistry:Reaction coordinate|Reaction coordinate]]
* [[Chemistry:Transition state|Transition state]]

==References==
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